Nik Revenco's blog Zola 2025-09-18T00:00:00+00:00 https://nikrev.com/atom.xml 2025-09-18T00:00:00+00:00 2025-09-18T00:00:00+00:00 Nik Revenco https://nikrev.com/blog/multi-line-string-literals-in-rust/ <p>Writing multi-line strings in Rust is quite awkward:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-storage z-type z-rust">let</span> sql <span class="z-keyword z-operator z-assignment z-rust">=</span> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>create table student( </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust"> id int primary key, </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust"> name text </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust">)<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-terminator z-rust">;</span> </span></code></pre> <p>I like how it works in Zig, I think Zig’s multi-line string literal syntax is one of the best:</p> <pre data-lang="zig" class="language-zig z-code"><code class="language-zig" data-lang="zig"><span class="z-source z-zig"><span class="z-storage z-modifier z-zig">const</span> sql <span class="z-keyword z-operator z-assignment z-zig">=</span> </span><span class="z-source z-zig"> <span class="z-string z-quoted z-other z-zig">\\ create table student( </span></span><span class="z-source z-zig"> <span class="z-string z-quoted z-other z-zig">\\ id int primary key, </span></span><span class="z-source z-zig"> <span class="z-string z-quoted z-other z-zig">\\ name text </span></span><span class="z-source z-zig"> <span class="z-string z-quoted z-other z-zig">\\ ) </span></span><span class="z-source z-zig"><span class="z-punctuation z-terminator z-zig">;</span> </span></code></pre> <p>Comments in Zig use <code>//</code> and make content until end of the line part of the comment. Same with multi-line string literals, <code>\\</code> makes everything after it a part of the string. Incredible consistency.</p> <p>I’ve been missing this feature a lot in Rust. There are some solutions for this problem, like the <code>indoc</code> crate:</p> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-storage z-type z-rust">let</span> sql <span class="z-keyword z-operator z-assignment z-rust">=</span> <span class="z-support z-macro z-rust">indoc!</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span><span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-block z-rust"><span class="z-string z-quoted z-double z-rust"> create table student( </span></span></span><span class="z-source z-rust"><span class="z-meta z-block z-rust"><span class="z-string z-quoted z-double z-rust"> id int primary key, </span></span></span><span class="z-source z-rust"><span class="z-meta z-block z-rust"><span class="z-string z-quoted z-double z-rust"> name text </span></span></span><span class="z-source z-rust"><span class="z-meta z-block z-rust"><span class="z-string z-quoted z-double z-rust"> ) </span></span></span><span class="z-source z-rust"><span class="z-meta z-block z-rust"><span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span></span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span><span class="z-punctuation z-terminator z-rust">;</span> </span></code></pre> <p>Or you can do it using one of the following alternatives:</p> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-storage z-type z-rust">let</span> sql <span class="z-keyword z-operator z-assignment z-rust">=</span> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span><span class="z-punctuation z-separator z-continuation z-line z-rust">\</span> </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust">create table student( </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust"> id int primary key, </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust"> name text </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust">)<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-terminator z-rust">;</span> </span></code></pre> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-storage z-type z-rust">let</span> sql <span class="z-keyword z-operator z-assignment z-rust">=</span> <span class="z-support z-macro z-rust">concat!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>create table student<span class="z-constant z-character z-escape z-rust">\n</span><span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span> id int primary key,<span class="z-constant z-character z-escape z-rust">\n</span><span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span> name text<span class="z-constant z-character z-escape z-rust">\n</span><span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>)<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-terminator z-rust">;</span> </span></code></pre> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-storage z-type z-rust">let</span> sql <span class="z-keyword z-operator z-assignment z-rust">=</span> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span><span class="z-punctuation z-separator z-continuation z-line z-rust">\</span> </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust"> create table student(<span class="z-constant z-character z-escape z-rust">\n</span><span class="z-punctuation z-separator z-continuation z-line z-rust">\</span> </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust"> id int primary key,<span class="z-constant z-character z-escape z-rust">\n</span><span class="z-punctuation z-separator z-continuation z-line z-rust">\</span> </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust"> name text<span class="z-constant z-character z-escape z-rust">\n</span><span class="z-punctuation z-separator z-continuation z-line z-rust">\</span> </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust"> )<span class="z-punctuation z-separator z-continuation z-line z-rust">\</span> </span></span><span class="z-source z-rust"><span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-terminator z-rust">;</span> </span></code></pre> <p>But none of those are satsifying to me. The biggest selling point of multi-line string literals is that they would be automatically formatted by <code>rustfmt</code> to match the surrounding indentation.</p> <p>But wait… apparently Rust already has multi-line string literals? It’s just as elegant as Zig’s, and I’ve never seen anyone use it!</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-support z-function z-rust">macro_rules!</span> <span class="z-entity z-name z-macro z-rust">multi</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span></span></span><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> <span class="z-meta z-group z-macro-matcher z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> #[doc = $first:literal] </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> <span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust"><span class="z-keyword z-operator z-rust">$</span>(</span>#[doc = $rest:literal]<span class="z-punctuation z-section z-group z-end z-rust">)</span><span class="z-keyword z-operator z-rust">*</span></span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> <span class="z-punctuation z-section z-group z-end z-rust">)</span></span> <span class="z-keyword z-operator z-rust">=&gt;</span> <span class="z-meta z-block z-macro-body z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-block z-macro-body z-rust"> <span class="z-support z-macro z-rust">concat!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-variable z-other z-rust">$first</span><span class="z-punctuation z-separator z-rust">,</span> <span class="z-keyword z-operator z-rust">$</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span><span class="z-constant z-character z-escape z-rust">\n</span><span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-separator z-rust">,</span> <span class="z-variable z-other z-rust">$rest</span><span class="z-punctuation z-separator z-rust">,</span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-keyword z-operator z-arithmetic z-rust">*</span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-block z-macro-body z-rust"> <span class="z-punctuation z-section z-block z-end z-rust">}</span></span><span class="z-punctuation z-terminator z-macro-matcher z-rust">;</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"></span></span><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span> </span><span class="z-source z-rust"><span class="z-storage z-type z-rust">let</span> sql <span class="z-keyword z-operator z-assignment z-rust">=</span> <span class="z-support z-macro z-rust">multi!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-comment z-line z-documentation z-rust"><span class="z-punctuation z-definition z-comment z-rust">///</span> create table student( </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-comment z-line z-documentation z-rust"><span class="z-punctuation z-definition z-comment z-rust">///</span> id int primary key, </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-comment z-line z-documentation z-rust"><span class="z-punctuation z-definition z-comment z-rust">///</span> name text </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-comment z-line z-documentation z-rust"><span class="z-punctuation z-definition z-comment z-rust">///</span> ); </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-terminator z-rust">;</span> </span></code></pre> <p>This works since <code>/// foo</code> is just syntax sugar for <code>#[doc = r"foo"]</code>.</p> <p>What I <em>really</em> like about this is that I can take the expression <code>multi!(...)</code> and copy-paste it wherever I feel like, <strong>and the indentation will be preserved!</strong>.</p> <p>And the best part, it is totally customizeable! You can make a cool new <code>format!</code> macro that makes use of that syntax:</p> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-support z-function z-rust">macro_rules!</span> <span class="z-entity z-name z-macro z-rust">multi_format</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span></span></span><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> <span class="z-meta z-group z-macro-matcher z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> #[doc = $first:literal] </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> <span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust"><span class="z-keyword z-operator z-rust">$</span>(</span>#[doc = $rest:literal]<span class="z-punctuation z-section z-group z-end z-rust">)</span><span class="z-keyword z-operator z-rust">*</span></span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> <span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust"><span class="z-keyword z-operator z-rust">$</span>(</span>, $(<span class="z-variable z-parameter z-macro z-rust">$tt</span><span class="z-punctuation z-separator z-rust">:</span><span class="z-storage z-type z-rust">tt</span><span class="z-punctuation z-section z-group z-end z-rust">)</span><span class="z-keyword z-operator z-rust">*</span></span><span class="z-punctuation z-section z-group z-end z-rust">)</span></span>? </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> ) =&gt; { </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> format!<span class="z-meta z-group z-macro-matcher z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span>concat!($first, <span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust"><span class="z-keyword z-operator z-rust">$</span>(</span>&quot;\n&quot;, $rest,<span class="z-punctuation z-section z-group z-end z-rust">)</span><span class="z-keyword z-operator z-rust">*</span></span><span class="z-punctuation z-section z-group z-end z-rust">)</span></span>$<span class="z-meta z-group z-macro-matcher z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span>, <span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust"><span class="z-keyword z-operator z-rust">$</span>(</span>$tt<span class="z-punctuation z-section z-group z-end z-rust">)</span><span class="z-keyword z-operator z-rust">*</span></span><span class="z-punctuation z-section z-group z-end z-rust">)</span></span>?) </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> </span></span><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span><span class="z-punctuation z-terminator z-rust">;</span> </span><span class="z-source z-rust">} </span><span class="z-source z-rust"> </span><span class="z-source z-rust"><span class="z-storage z-type z-rust">let</span> foo <span class="z-keyword z-operator z-assignment z-rust">=</span> <span class="z-support z-macro z-rust">multi_format!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-comment z-line z-documentation z-rust"><span class="z-punctuation z-definition z-comment z-rust">///</span> Hello, my name is {} </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-punctuation z-separator z-rust">,</span> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>bob<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-terminator z-rust">;</span> </span></code></pre> <p>It won’t work with interpolated variables, though:</p> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-storage z-type z-rust">let</span> age <span class="z-keyword z-operator z-assignment z-rust">=</span> <span class="z-constant z-numeric z-integer z-decimal z-rust">0</span><span class="z-punctuation z-terminator z-rust">;</span> </span><span class="z-source z-rust"><span class="z-storage z-type z-rust">let</span> bob <span class="z-keyword z-operator z-assignment z-rust">=</span> <span class="z-support z-macro z-rust">multi_format!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-comment z-line z-documentation z-rust"><span class="z-punctuation z-definition z-comment z-rust">///</span> Hello, my name is {} and I am {age} years old! </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-punctuation z-separator z-rust">,</span> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>bob<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-terminator z-rust">;</span> </span></code></pre> <div class="error"> <p>There is no argument named <code>age</code>.</p> <p>Did you intend to capture a variable <code>age</code> from the surrounding scope?</p> <p>To avoid ambiguity, <code>format_args!</code> cannot capture variables when the format string is expanded from a macro</p> </div> <p>That’s unfortunate, it takes away from the utility of a <em>naive</em> implementatio! That’s why I made a procedural macro <a href="https://github.com/nik-rev/docstr/tree/main"><code>docstr!</code></a>, which turns documentation into strings at compile-time:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-keyword z-other z-rust">use</span> <span class="z-meta z-path z-rust">docstr<span class="z-punctuation z-accessor z-rust">::</span></span>docstr<span class="z-punctuation z-terminator z-rust">;</span> </span><span class="z-source z-rust"> </span><span class="z-source z-rust"><span class="z-storage z-type z-rust">let</span> hello_world<span class="z-punctuation z-separator z-rust">:</span> <span class="z-keyword z-operator z-bitwise z-rust">&amp;</span><span class="z-storage z-modifier z-lifetime z-rust">&#39;static</span> <span class="z-storage z-type z-rust">str</span> <span class="z-keyword z-operator z-assignment z-rust">=</span> <span class="z-support z-macro z-rust">docstr!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span>format </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-comment z-line z-documentation z-rust"><span class="z-punctuation z-definition z-comment z-rust">///</span> fn main() { </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-comment z-line z-documentation z-rust"><span class="z-punctuation z-definition z-comment z-rust">///</span> println!(&quot;Hello, my name is {}&quot;); </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-comment z-line z-documentation z-rust"><span class="z-punctuation z-definition z-comment z-rust">///</span> } </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>Bob<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-terminator z-rust">;</span> </span><span class="z-source z-rust"> </span><span class="z-source z-rust"><span class="z-support z-macro z-rust">assert_eq!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span>hello_world_in_c<span class="z-punctuation z-separator z-rust">,</span> <span class="z-string z-quoted z-double z-raw z-rust"><span class="z-storage z-type z-string z-rust">r</span><span class="z-punctuation z-definition z-string z-begin z-rust">#&quot;</span>fn main() { </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-string z-quoted z-double z-raw z-rust"> println!(&quot;Hello, my name is Bob&quot;); </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-string z-quoted z-double z-raw z-rust">}<span class="z-punctuation z-definition z-string z-end z-rust">&quot;#</span></span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></code></pre> <p>Expands to this:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-support z-macro z-rust">format!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span></span><span class="z-meta z-group z-rust"><span class="z-string z-quoted z-double z-raw z-rust"><span class="z-storage z-type z-string z-rust">r</span><span class="z-punctuation z-definition z-string z-begin z-rust">#</span>&quot;fn main() { </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-string z-quoted z-double z-raw z-rust"> println!(&quot;Hello, my name is <span class="z-constant z-other z-placeholder z-rust">{}</span>&quot;); </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-string z-quoted z-double z-raw z-rust">}<span class="z-punctuation z-definition z-string z-end z-rust">&quot;#</span></span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-separator z-rust">,</span> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>Bob<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></code></pre> 2025-05-16T00:00:00+00:00 2025-05-16T00:00:00+00:00 Nik Revenco https://nikrev.com/blog/a-curious-way-to-combine-module-paths-in-rust-declarative-macros/ <p>Today, I ran into an interesting problem with a not-so-obvious solution: How do you concatenate path segments in Rust’s declarative macros?</p> <h2 id="why-i-need-to-do-this"><a class="zola-anchor" href="#why-i-need-to-do-this" aria-label="Anchor link for: why-i-need-to-do-this">Why I need to do this</a></h2> <p>In my project <a href="https://github.com/nik-rev/ferrishot">ferrishot</a> I have two <em>very</em> similar enums:</p> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-storage z-modifier z-rust">pub</span> <span class="z-storage z-type z-enum z-rust">enum</span> <span class="z-entity z-name z-enum z-rust">Command</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> ImageUpload<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-keyword z-other z-rust">crate</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">image<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">action<span class="z-punctuation z-accessor z-rust">::</span></span>Command</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> App<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">app<span class="z-punctuation z-accessor z-rust">::</span></span>Command</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> DebugOverlay<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">debug_overlay<span class="z-punctuation z-accessor z-rust">::</span></span>Command</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> KeybindingsCheatsheet<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">keybindings_cheatsheet<span class="z-punctuation z-accessor z-rust">::</span></span>Command</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> Letters<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">popup<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">letters<span class="z-punctuation z-accessor z-rust">::</span></span>Command</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> Selection<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">selection<span class="z-punctuation z-accessor z-rust">::</span></span>Command</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"></span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span> </span><span class="z-source z-rust"> </span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-storage z-modifier z-rust">pub</span> <span class="z-storage z-type z-enum z-rust">enum</span> <span class="z-entity z-name z-enum z-rust">KeymappableCommand</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> ImageUpload<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-keyword z-other z-rust">crate</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">image<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">action<span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> App<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">app<span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> DebugOverlay<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">debug_overlay<span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> KeybindingsCheatsheet<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">keybindings_cheatsheet<span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> Letters<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">popup<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">letters<span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> Selection<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">selection<span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"></span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span> </span></code></pre> <p>Each <code>Command</code> and <code>KeymappableCommand</code> pair is created by a macro in their specific module.</p> <hr /> <p><code>KeymappableCommand</code> is almost the same as <code>Command</code>, but it has additional fields representing the keystrokes required to invoke this command.</p> <p>This enum is used to parse the <a href="https://kdl.dev/">KDL</a> config file using <a href="https://github.com/TheLostLambda/knus">knus</a>:</p> <pre data-lang="json" class="language-json z-code"><code class="language-json" data-lang="json"><span class="z-source z-json">keys <span class="z-meta z-mapping z-json"><span class="z-punctuation z-section z-mapping z-begin z-json">{</span> </span></span><span class="z-source z-json"><span class="z-meta z-mapping z-json"> <span class="z-invalid z-illegal z-expected-mapping-key z-json">e</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">x</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">i</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">t</span> <span class="z-invalid z-illegal z-expected-mapping-key z-json">w</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">a</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">i</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">t</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">=</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">1</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">0</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">s</span> <span class="z-invalid z-illegal z-expected-mapping-key z-json">k</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">e</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">y</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">=</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">&lt;</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">e</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">s</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">c</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">&gt;</span> </span></span><span class="z-source z-json"><span class="z-meta z-mapping z-json"> <span class="z-invalid z-illegal z-expected-mapping-key z-json">c</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">o</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">p</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">y</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">-</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">t</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">o</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">-</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">c</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">l</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">i</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">p</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">b</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">o</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">a</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">r</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">d</span> <span class="z-invalid z-illegal z-expected-mapping-key z-json">k</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">e</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">y</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">=</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">&lt;</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">e</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">n</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">t</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">e</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">r</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">&gt;</span> </span></span><span class="z-source z-json"><span class="z-meta z-mapping z-json"> <span class="z-invalid z-illegal z-expected-mapping-key z-json">s</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">a</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">v</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">e</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">-</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">s</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">c</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">r</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">e</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">e</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">n</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">s</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">h</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">o</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">t</span> <span class="z-invalid z-illegal z-expected-mapping-key z-json">m</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">o</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">d</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">=</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">c</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">t</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">r</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">l</span> <span class="z-invalid z-illegal z-expected-mapping-key z-json">k</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">e</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">y</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">=</span><span class="z-invalid z-illegal z-expected-mapping-key z-json">s</span> </span></span><span class="z-source z-json"><span class="z-meta z-mapping z-json"><span class="z-punctuation z-section z-mapping z-end z-json">}</span></span> </span></code></pre> <p>By parsing that we will have something like this:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-keyword z-other z-rust">use</span> <span class="z-keyword z-other z-rust">crate</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">image<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">action<span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand<span class="z-punctuation z-terminator z-rust">;</span> </span><span class="z-source z-rust"> </span><span class="z-source z-rust"><span class="z-support z-macro z-rust">vec!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">[</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-meta z-path z-rust">KeymappableCommand<span class="z-punctuation z-accessor z-rust">::</span></span>Exit <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-meta z-block z-rust"> wait<span class="z-punctuation z-separator z-rust">:</span> <span class="z-meta z-path z-rust">Duration<span class="z-punctuation z-accessor z-rust">::</span></span>from_seconds<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-constant z-numeric z-integer z-decimal z-rust">10</span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-meta z-block z-rust"> key<span class="z-punctuation z-separator z-rust">:</span> <span class="z-meta z-path z-rust">Key<span class="z-punctuation z-accessor z-rust">::</span></span>Escape<span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-meta z-block z-rust"> mods<span class="z-punctuation z-separator z-rust">:</span> <span class="z-support z-type z-rust">None</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-meta z-block z-rust"> </span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-meta z-path z-rust">KeymappableCommand<span class="z-punctuation z-accessor z-rust">::</span></span>CopyToClipboard <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-meta z-block z-rust"> key<span class="z-punctuation z-separator z-rust">:</span> <span class="z-meta z-path z-rust">Key<span class="z-punctuation z-accessor z-rust">::</span></span>Enter<span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-meta z-block z-rust"> mods<span class="z-punctuation z-separator z-rust">:</span> <span class="z-support z-type z-rust">None</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-meta z-block z-rust"> </span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-meta z-path z-rust">KeymappableCommand<span class="z-punctuation z-accessor z-rust">::</span></span>SaveScreenshot <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-meta z-block z-rust"> key<span class="z-punctuation z-separator z-rust">:</span> <span class="z-meta z-path z-rust">Key<span class="z-punctuation z-accessor z-rust">::</span></span>Character<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-string z-quoted z-single z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&#39;</span>s<span class="z-punctuation z-definition z-string z-end z-rust">&#39;</span></span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-meta z-block z-rust"> mods<span class="z-punctuation z-separator z-rust">:</span> <span class="z-support z-type z-rust">Some</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">Modifier<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-constant z-other z-rust">CTRL</span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-meta z-block z-rust"> </span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">]</span></span> </span></code></pre> <p>What we actually store is a <code>HashMap&lt;KEYS, Command&gt;</code> where <code>Command</code> is a version of <code>KeymappableCommand</code> that excludes the <code>key</code> and <code>mods</code> field, where <code>KEYS</code> is a tuple of said <code>(key, mods)</code>.</p> <p>This allows us to retrieve the correct <code>Command</code> for any given key sequence in <code>O(1)</code> time.</p> <p>Each module has its own pair of <code>(KeymappableCommand, Command)</code>, and then there are 2 separate enums at the root of the crate: <code>crate::KeymappableCommand</code> and <code>crate::Command</code> which <em>compose</em> all of the other <code>KeymappableCommand</code>s and <code>Command</code>s from other module.</p> <h2 id="the-naive-approach"><a class="zola-anchor" href="#the-naive-approach" aria-label="Anchor link for: the-naive-approach">The naive approach</a></h2> <p>In order to create a macro that lets me define the following:</p> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-storage z-modifier z-rust">pub</span> <span class="z-storage z-type z-enum z-rust">enum</span> <span class="z-entity z-name z-enum z-rust">Command</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> ImageUpload<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-keyword z-other z-rust">crate</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">image<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">action<span class="z-punctuation z-accessor z-rust">::</span></span>Command</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> App<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">app<span class="z-punctuation z-accessor z-rust">::</span></span>Command</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> Letters<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">popup<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">letters<span class="z-punctuation z-accessor z-rust">::</span></span>Command</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"></span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span> </span><span class="z-source z-rust"> </span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-storage z-modifier z-rust">pub</span> <span class="z-storage z-type z-enum z-rust">enum</span> <span class="z-entity z-name z-enum z-rust">KeymappableCommand</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> ImageUpload<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-keyword z-other z-rust">crate</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">image<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">action<span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> App<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">app<span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> Letters<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">popup<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">letters<span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"></span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span> </span></code></pre> <p>As follows, for instance:</p> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-support z-macro z-rust">declare_commands!</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span><span class="z-source z-rust"><span class="z-meta z-block z-rust"> ImageUpload<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-keyword z-other z-rust">crate</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">image<span class="z-punctuation z-accessor z-rust">::</span></span>action</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span><span class="z-source z-rust"><span class="z-meta z-block z-rust"> App<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span>app</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span> </span></span><span class="z-source z-rust"><span class="z-meta z-block z-rust"> Letters<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-meta z-path z-rust">ui<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">popup<span class="z-punctuation z-accessor z-rust">::</span></span>letters</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span><span class="z-source z-rust"><span class="z-meta z-block z-rust"></span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span> </span></code></pre> <p>I need to just write the path once, then concatenate it with the item I want to use. I initially tried this:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-comment z-line z-double-slash z-rust"><span class="z-punctuation z-definition z-comment z-rust">//</span> surely, this will work right? </span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-support z-function z-rust">macro_rules!</span> <span class="z-entity z-name z-macro z-rust">declare_commands</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span></span></span><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> <span class="z-meta z-group z-macro-matcher z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> <span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust"><span class="z-keyword z-operator z-rust">$</span>(</span> </span></span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"><span class="z-meta z-group z-rust"> <span class="z-variable z-parameter z-macro z-rust">$Variant</span><span class="z-punctuation z-separator z-rust">:</span><span class="z-storage z-type z-rust">ident</span>(<span class="z-variable z-parameter z-macro z-rust">$path</span><span class="z-punctuation z-separator z-rust">:</span><span class="z-storage z-type z-rust">path</span><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> <span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-terminator z-macro-matcher z-rust">,</span>* </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> ) =&gt; { </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> pub enum Command { </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> $<span class="z-meta z-group z-macro-matcher z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> $Variant($path::Command<span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> ),* </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> </span></span><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span> </span><span class="z-source z-rust"> </span><span class="z-source z-rust"> <span class="z-meta z-enum z-rust"><span class="z-storage z-modifier z-rust">pub</span> <span class="z-storage z-type z-enum z-rust">enum</span> <span class="z-entity z-name z-enum z-rust">KeymappableCommand</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> <span class="z-keyword z-operator z-rust">$</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-rust"> <span class="z-variable z-other z-rust">$Variant</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-variable z-other z-rust">$path</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-rust"> </span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span><span class="z-keyword z-operator z-arithmetic z-rust">*</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> </span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span> </span><span class="z-source z-rust"> } </span><span class="z-source z-rust">} </span></code></pre> <p>It feels like it should work. But specifically, this part is not valid Rust:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-variable z-other z-rust">$path</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span>Command </span><span class="z-source z-rust"><span class="z-variable z-other z-rust">$path</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand </span></code></pre> <p>But an attempt to compile the above results in an error:</p> <div class="error"> <pre class="z-code"><code><span class="z-text z-plain">error: missing angle brackets in associated item path </span><span class="z-text z-plain"> --&gt; src/main.rs:30:18 </span><span class="z-text z-plain"> | </span><span class="z-text z-plain">30 | $Variant($path::Command) </span><span class="z-text z-plain"> | ^^^^^ </span><span class="z-text z-plain">... </span><span class="z-text z-plain">42 | / declare_commands! { </span><span class="z-text z-plain">43 | | ImageUpload(crate::image::action), </span><span class="z-text z-plain">44 | | App(ui::app), </span><span class="z-text z-plain">45 | | Letters(ui::popup::letters) </span><span class="z-text z-plain">46 | | } </span><span class="z-text z-plain"> | |_- in this macro invocation </span><span class="z-text z-plain"> | </span><span class="z-text z-plain"> = note: this error originates in the macro `declare_commands` (in Nightly builds, run with -Z macro-backtrace for more info) </span><span class="z-text z-plain">help: types that don&#39;t start with an identifier need to be surrounded with angle brackets in qualified paths </span><span class="z-text z-plain"> | </span><span class="z-text z-plain">30 | $Variant(&lt;$path&gt;::Command) </span><span class="z-text z-plain"> | + + </span><span class="z-text z-plain"> </span><span class="z-text z-plain">error: missing angle brackets in associated item path </span><span class="z-text z-plain"> --&gt; src/main.rs:36:18 </span><span class="z-text z-plain"> | </span><span class="z-text z-plain">36 | $Variant($path::KeymappableCommand) </span><span class="z-text z-plain"> | ^^^^^ </span><span class="z-text z-plain">... </span><span class="z-text z-plain">42 | / declare_commands! { </span><span class="z-text z-plain">43 | | ImageUpload(crate::image::action), </span><span class="z-text z-plain">44 | | App(ui::app), </span><span class="z-text z-plain">45 | | Letters(ui::popup::letters) </span><span class="z-text z-plain">46 | | } </span><span class="z-text z-plain"> | |_- in this macro invocation </span></code></pre> </div> <p>The suggestion to add <code>&lt; &gt;</code> is incorrect here, as that is not valid syntax for referencing items in modules.</p> <p>If we try the advice, we get another error:</p> <div class="error"> <pre class="z-code"><code><span class="z-text z-plain">error[E0573]: expected type, found module `crate::image::action` </span><span class="z-text z-plain"> --&gt; src/main.rs:43:16 </span><span class="z-text z-plain"> | </span><span class="z-text z-plain">43 | ImageUpload(crate::image::action), </span><span class="z-text z-plain"> | ^^^^^^^^^^^^^^^^^^^^ not a type </span><span class="z-text z-plain"> </span><span class="z-text z-plain">error[E0573]: expected type, found module `ui::app` </span><span class="z-text z-plain"> --&gt; src/main.rs:44:8 </span><span class="z-text z-plain"> | </span><span class="z-text z-plain">44 | App(ui::app), </span><span class="z-text z-plain"> | ^^^^^^^ not a type </span><span class="z-text z-plain"> </span><span class="z-text z-plain">error[E0412]: cannot find type `letters` in module `ui::popup` </span><span class="z-text z-plain"> --&gt; src/main.rs:45:23 </span><span class="z-text z-plain"> | </span><span class="z-text z-plain">45 | Letters(ui::popup::letters) </span><span class="z-text z-plain"> | ^^^^^^^ not found in `ui::popup` </span><span class="z-text z-plain"> </span><span class="z-text z-plain">Some errors have detailed explanations: E0412, E0573. </span><span class="z-text z-plain">For more information about an error, try `rustc --explain E0412`. </span><span class="z-text z-plain">error: could not compile `macro_example` (bin &quot;macro_example&quot;) due to 3 previous errors </span></code></pre> </div> <p>Composing path segments like this does not have an obvious solution. I initially tried the <a href="https://crates.io/crates/paste">paste</a> crate which lets you compose identifiers in this manner:</p> <h3 id="tried-paste"><a class="zola-anchor" href="#tried-paste" aria-label="Anchor link for: tried-paste">Tried <code>paste!</code></a></h3> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-support z-function z-rust">macro_rules!</span> <span class="z-entity z-name z-macro z-rust">declare_commands</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span></span></span><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> <span class="z-meta z-group z-macro-matcher z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> <span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust"><span class="z-keyword z-operator z-rust">$</span>(</span> </span></span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"><span class="z-meta z-group z-rust"> <span class="z-variable z-parameter z-macro z-rust">$Variant</span><span class="z-punctuation z-separator z-rust">:</span><span class="z-storage z-type z-rust">ident</span>(<span class="z-variable z-parameter z-macro z-rust">$path</span><span class="z-punctuation z-separator z-rust">:</span><span class="z-storage z-type z-rust">path</span><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> <span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-terminator z-macro-matcher z-rust">,</span>* </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> ) =&gt; { </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> pub enum Command { </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> $<span class="z-meta z-group z-macro-matcher z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> $Variant(paste! {[&lt;$path::Command&gt;]}<span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> ),* </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> </span></span><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span> </span><span class="z-source z-rust"> </span><span class="z-source z-rust"> <span class="z-meta z-enum z-rust"><span class="z-storage z-modifier z-rust">pub</span> <span class="z-storage z-type z-enum z-rust">enum</span> <span class="z-entity z-name z-enum z-rust">KeymappableCommand</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> <span class="z-keyword z-operator z-rust">$</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-rust"> <span class="z-variable z-other z-rust">$Variant</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-support z-macro z-rust">paste!</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">[</span><span class="z-keyword z-operator z-comparison z-rust">&lt;</span><span class="z-variable z-other z-rust">$path</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand<span class="z-keyword z-operator z-comparison z-rust">&gt;</span><span class="z-punctuation z-section z-group z-end z-rust">]</span></span></span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-rust"> </span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span><span class="z-keyword z-operator z-arithmetic z-rust">*</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> </span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span> </span><span class="z-source z-rust"> } </span><span class="z-source z-rust">} </span></code></pre> <p>But <code>paste!</code> does not work for composing paths like that.</p> <div class="error"> <pre class="z-code"><code><span class="z-text z-plain">error: expected identifier after `:` </span><span class="z-text z-plain"> --&gt; src/main.rs:44:22 </span><span class="z-text z-plain"> | </span><span class="z-text z-plain">44 | ImageUpload(crate::image::action), </span><span class="z-text z-plain"> | ^ </span><span class="z-text z-plain"> </span><span class="z-text z-plain">error: expected identifier after `:` </span><span class="z-text z-plain"> --&gt; src/main.rs:45:11 </span><span class="z-text z-plain"> | </span><span class="z-text z-plain">45 | App(ui::app), </span><span class="z-text z-plain"> | ^ </span><span class="z-text z-plain"> </span><span class="z-text z-plain">error: expected identifier after `:` </span><span class="z-text z-plain"> --&gt; src/main.rs:46:15 </span><span class="z-text z-plain"> | </span><span class="z-text z-plain">46 | Letters(ui::popup::letters) </span><span class="z-text z-plain"> | ^ </span><span class="z-text z-plain"> </span><span class="z-text z-plain">error: could not compile `macro_example` (bin &quot;macro_example&quot;) due to 6 previous errors </span></code></pre> </div> <h2 id="the-solution"><a class="zola-anchor" href="#the-solution" aria-label="Anchor link for: the-solution">The Solution</a></h2> <p>If we look at what a path is, it’s just identifiers (<code>ident</code>) separated by double-colon <code>::</code>:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-meta z-path z-rust">path<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">to<span class="z-punctuation z-accessor z-rust">::</span></span>some_module </span></code></pre> <p>You can represent the above using the following macro specifier:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-keyword z-operator z-rust">$</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"> <span class="z-variable z-other z-rust">$segment</span><span class="z-punctuation z-separator z-rust">:</span>ident </span></span><span class="z-source z-rust"><span class="z-meta z-group z-rust"></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">:</span><span class="z-punctuation z-separator z-rust">:</span><span class="z-keyword z-operator z-arithmetic z-rust">*</span> </span></code></pre> <ul> <li><code>$( ... )*</code> denotes a repetition</li> <li><code>$( ... )SEPARATOR*</code> denotes a repetition, but we have to insert the <code>SEPARATOR</code> in-between each repetition.</li> </ul> <p>In <code>path::to::some_module</code>, it would parse as follows:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-variable z-other z-rust">$segment</span> <span class="z-comment z-line z-double-slash z-rust"><span class="z-punctuation z-definition z-comment z-rust">//</span> `path` </span></span><span class="z-source z-rust"><span class="z-punctuation z-separator z-rust">:</span><span class="z-punctuation z-separator z-rust">:</span> <span class="z-comment z-line z-double-slash z-rust"><span class="z-punctuation z-definition z-comment z-rust">//</span> `SEPARATOR` </span></span><span class="z-source z-rust"><span class="z-variable z-other z-rust">$segment</span> <span class="z-comment z-line z-double-slash z-rust"><span class="z-punctuation z-definition z-comment z-rust">//</span> `to` </span></span><span class="z-source z-rust"><span class="z-punctuation z-separator z-rust">:</span><span class="z-punctuation z-separator z-rust">:</span> <span class="z-comment z-line z-double-slash z-rust"><span class="z-punctuation z-definition z-comment z-rust">//</span> `SEPARATOR` </span></span><span class="z-source z-rust"><span class="z-variable z-other z-rust">$segment</span> <span class="z-comment z-line z-double-slash z-rust"><span class="z-punctuation z-definition z-comment z-rust">//</span> `some_module` </span></span></code></pre> <p>Then, using <code>$($segment)::*</code> in a similar fashion <em>expands</em> this repetition to the path:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust">path <span class="z-comment z-line z-double-slash z-rust"><span class="z-punctuation z-definition z-comment z-rust">//</span> `<span class="z-variable z-other z-rust">$segment</span>` </span></span><span class="z-source z-rust"><span class="z-punctuation z-separator z-rust">:</span><span class="z-punctuation z-separator z-rust">:</span> <span class="z-comment z-line z-double-slash z-rust"><span class="z-punctuation z-definition z-comment z-rust">//</span> `SEPARATOR` </span></span><span class="z-source z-rust">to <span class="z-comment z-line z-double-slash z-rust"><span class="z-punctuation z-definition z-comment z-rust">//</span> `<span class="z-variable z-other z-rust">$segment</span>` </span></span><span class="z-source z-rust"><span class="z-punctuation z-separator z-rust">:</span><span class="z-punctuation z-separator z-rust">:</span> <span class="z-comment z-line z-double-slash z-rust"><span class="z-punctuation z-definition z-comment z-rust">//</span> `SEPARATOR` </span></span><span class="z-source z-rust">some_module <span class="z-comment z-line z-double-slash z-rust"><span class="z-punctuation z-definition z-comment z-rust">//</span> `<span class="z-variable z-other z-rust">$segment</span>` </span></span></code></pre> <p>And what’s interesting is that we can compose <em>this</em> with another double-colon <code>::</code> followed by an identifier (<code>ident</code>) like this:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-keyword z-operator z-rust">$</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-variable z-other z-rust">$segment</span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">:</span><span class="z-punctuation z-separator z-rust">:</span><span class="z-keyword z-operator z-arithmetic z-rust">*</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span>some_identifier </span></code></pre> <p>The above (with <code>path::to::some_module</code> as <code>$($segment)::*</code>) will expand to this:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-meta z-path z-rust">path<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">to<span class="z-punctuation z-accessor z-rust">::</span></span><span class="z-meta z-path z-rust">some_module<span class="z-punctuation z-accessor z-rust">::</span></span>some_identifier </span></code></pre> <p>I have looked for a long time and haven’t found anyone talk about this. So the correct version of the macro I tried to make is this:</p> <pre data-lang="rust" class="language-rust z-code"><code class="language-rust" data-lang="rust"><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-support z-function z-rust">macro_rules!</span> <span class="z-entity z-name z-macro z-rust">declare_commands</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span></span></span><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> <span class="z-meta z-group z-macro-matcher z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> <span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust"><span class="z-keyword z-operator z-rust">$</span>(</span> </span></span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"><span class="z-meta z-group z-rust"> <span class="z-variable z-parameter z-macro z-rust">$Variant</span><span class="z-punctuation z-separator z-rust">:</span><span class="z-storage z-type z-rust">ident</span>($(<span class="z-variable z-parameter z-macro z-rust">$segment</span><span class="z-punctuation z-separator z-rust">:</span><span class="z-storage z-type z-rust">ident</span><span class="z-punctuation z-section z-group z-end z-rust">)</span></span>::*<span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> ),* </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> ) =&gt; { </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> pub enum Command { </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> $<span class="z-meta z-group z-macro-matcher z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-macro-matcher z-rust"> $Variant(<span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust"><span class="z-keyword z-operator z-rust">$</span>(</span>$segment<span class="z-punctuation z-section z-group z-end z-rust">)</span></span>::*::Command<span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> ),* </span></span></span><span class="z-source z-rust"><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"> </span></span><span class="z-meta z-macro z-rust"><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span> </span><span class="z-source z-rust"> </span><span class="z-source z-rust"> <span class="z-meta z-enum z-rust"><span class="z-storage z-modifier z-rust">pub</span> <span class="z-storage z-type z-enum z-rust">enum</span> <span class="z-entity z-name z-enum z-rust">KeymappableCommand</span> <span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-begin z-rust">{</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> <span class="z-keyword z-operator z-rust">$</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-rust"> <span class="z-variable z-other z-rust">$Variant</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-keyword z-operator z-rust">$</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-variable z-other z-rust">$segment</span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">:</span><span class="z-punctuation z-separator z-rust">:</span><span class="z-keyword z-operator z-arithmetic z-rust">*</span><span class="z-meta z-path z-rust"><span class="z-punctuation z-accessor z-rust">::</span></span>KeymappableCommand</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"><span class="z-meta z-group z-rust"> </span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span><span class="z-punctuation z-separator z-rust">,</span><span class="z-keyword z-operator z-arithmetic z-rust">*</span> </span></span></span><span class="z-source z-rust"><span class="z-meta z-enum z-rust"><span class="z-meta z-block z-rust"> </span><span class="z-meta z-block z-rust"><span class="z-punctuation z-section z-block z-end z-rust">}</span></span></span> </span><span class="z-source z-rust"> } </span><span class="z-source z-rust">} </span></code></pre> <p><a href="https://nikrev.com/blog/a-curious-way-to-combine-module-paths-in-rust-declarative-macros/main.rs">See the full code</a></p> 2025-04-07T00:00:00+00:00 2025-04-07T00:00:00+00:00 Nik Revenco https://nikrev.com/blog/what-will-rust-keywords-do/ <p>I found a <a href="https://doc.rust-lang.org/reference/keywords.html">page in the Rust reference</a> that lists all of the currently reserved keywords:</p> <ul> <li><code>priv</code></li> <li><code>become</code></li> <li><code>abstract</code></li> <li><code>override</code></li> <li><code>final</code></li> <li><code>box</code></li> <li><code>do</code></li> <li><code>macro</code></li> <li><code>typeof</code></li> <li><code>unsized</code></li> <li><code>virtual</code></li> <li><code>yield</code></li> <li><code>try</code></li> </ul> <p>I am really curious what they will do. I did some research, compiling a list of RFCs which use these keywords with my own speculations mixed in:</p> <ul> <li><code>priv</code>: Everything in Rust is already private by default<sup class="footnote-reference" id="fr-1-1"><a href="#fn-1">1</a></sup> except trait items, enum variants, and fields in enum variants. Perhaps at some point these can be made opt-in private.</li> <li><code>become</code>: <a href="https://github.com/rust-lang/rfcs/pull/3407">Tail call optimization</a> for recursive functions.</li> <li><code>abstract</code>, <code>override</code>, <code>final</code>: <a href="https://rust-lang.github.io/rfcs/0342-keywords.html">Was reserved for a possibility of adding “more efficient inheritance”</a><sup class="footnote-reference" id="fr-2-1"><a href="#fn-2">2</a></sup></li> <li><code>box</code>: <a href="https://github.com/rust-lang/rust/issues/29641">Box patterns</a>.</li> <li><code>do</code>: In the old days, <code>do</code> was a way to invoke a function that takes a closure as a parameter such as <code>do task::spawn { println!("Hello, world!"); }</code>.<sup class="footnote-reference" id="fr-2-2"><a href="#fn-2">2</a></sup></li> <li><code>macro</code>: <a href="https://github.com/rust-lang/rust/issues/39412">Declarative macros 2.0</a>, which will bring many improvements over the current system – such as improved hygiene</li> <li><code>typeof</code>: Obtain the type of an expression so you can do something like: <code>let x: typeof("a") = "hello"</code>.</li> <li><code>unsized</code>: Could be syntax sugar for <code>!Sized</code>.</li> <li><code>virtual</code>: Rust used to have “virtual structs” which were implemented as a test, this lead to the <code>virtual</code> keyword. <a href="https://rust-lang.github.io/rfcs/0341-remove-virtual-structs.html">They were later removed, but the keyword was kept.</a></li> <li><code>yield</code> and <code>gen</code>: <a href="https://github.com/rust-lang/rust/issues/117078">Generator functions and blocks</a>.</li> <li><code>try</code>: <a href="https://github.com/rust-lang/rust/issues/31436">Local blocks</a> which the <code>?</code> operator can return from without returning from the function.</li> </ul> <p><code>unsized</code> only makes sense as sugar for <code>!Sized</code> but is this really necessary? Rust tries hard not to special case the standard library and just adding syntax for a built-in item seems like isn’t necessary. I didn’t find an RFC for it though so it could be used for more than that.</p> <p><code>do</code> is used for the <code>do yeet</code> in https://github.com/rust-lang/rust/issues/96373 but the syntax will change when its stabilized so I don’t count it.</p> <footer class="footnotes"> <ol class="footnotes-list"> <li id="fn-1"> <p>This wasn’t always the case, <a href="https://github.com/rust-lang/rust/issues/8122">there was a time when not everything was private by default!</a>. <a href="#fr-1-1">↩</a></p> </li> <li id="fn-2"> <p>Thank you <a href="https://www.reddit.com/user/steveklabnik1/"><code>u/steveklabnik1</code></a> for the info. <a href="#fr-2-1">↩</a> <a href="#fr-2-2">↩2</a></p> </li> </ol> </footer> 2025-02-27T00:00:00+00:00 2025-02-27T00:00:00+00:00 Nik Revenco https://nikrev.com/blog/lit-str-concat-macro/ <p>You can use the <code>concat!</code> macro to pass string literals into macros:</p> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-support z-macro z-rust">env!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>ab<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span><span class="z-source z-rust"><span class="z-support z-macro z-rust">env!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-support z-macro z-rust">concat!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>a<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-separator z-rust">,</span> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>b<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></code></pre> <p>Both of the above calls will <strong>become</strong> <code>env!("ab")</code>.</p> <p>However, a macro created using the <a href="https://crates.io/crates/syn"><code>syn</code></a> crate that expects a literal string <code>syn::LitStr</code> as its input, such as Dioxus’s <a href="https://docs.rs/dioxus/0.6.3/dioxus/prelude/macro.asset.html"><code>asset!</code></a> macro:</p> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-support z-macro z-rust">asset!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>/assets/icons/github.svg<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></code></pre> <p>Will <em>not</em> be able to receive <code>concat!</code>.</p> <p>This does not compile.</p> <pre data-lang="rs" class="language-rs z-code"><code class="language-rs" data-lang="rs"><span class="z-source z-rust"><span class="z-support z-macro z-rust">asset!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-support z-macro z-rust">concat!</span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-begin z-rust">(</span><span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>/assets/icons/<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-separator z-rust">,</span> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>github<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span><span class="z-punctuation z-separator z-rust">,</span> <span class="z-string z-quoted z-double z-rust"><span class="z-punctuation z-definition z-string z-begin z-rust">&quot;</span>.svg<span class="z-punctuation z-definition z-string z-end z-rust">&quot;</span></span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span></span><span class="z-meta z-group z-rust"><span class="z-punctuation z-section z-group z-end z-rust">)</span></span> </span></code></pre> <div class="error"> <pre class="z-code"><code><span class="z-text z-plain">ERROR: Expected a string literal </span></code></pre> </div> <p>I assume this is because the <code>asset!</code> macro expects a string literal. But <em>is</em> <code>concat!("...", "...")</code> a string literal?</p> <p>Because <code>concat!(...)</code> itself only accepts string literals, and you can use it inside of itself: <code>concat!(concat!(""))</code>, it <strong>must</strong> be, right? For this to work, the macro has to be evaluated in this order.</p> <ol> <li>Macro expands</li> <li>The expanded form is passed inside the <code>concat!</code></li> </ol> <p>When you try to write your own macro that can do the above, you’ll notice it isn’t possible.</p> <p>Rust’s macros such as <code>include_str!</code> and <code>env!</code> have a special “superpower” where they can expand the <code>concat!</code> macro <em>before</em> parsing. However, user-created declarative macros using the <code>syn</code> crate are unable to.</p> <p>These macros get the raw <code>TokenStream</code> corresponding to the tokens in <code>"concat!("/assets/icons/", "github", ".svg"))"</code></p> <h2 id="solution"><a class="zola-anchor" href="#solution" aria-label="Anchor link for: solution">Solution</a></h2> <p>Thankfully, the amazing <a href="https://github.com/dtolnay">David Tolnay</a> shared a pretty cool crate called <a href="https://crates.io/crates/macro-string"><code>macro-string</code></a> which allows creating a derive macro which eagerly evaluates the following <code>std</code> macros, at the time of writing:</p> <ul> <li><code>concat!</code></li> <li><code>env!</code></li> <li><code>include!</code></li> <li><code>include_str!</code></li> <li><code>stringify!</code></li> </ul> <p>Meaning you’ll be able to pass them into macros that expect a string literal, and they’ll get expanded <em>before</em> being passed to the macro!</p>