Remove Parser::missing_fragment_specifier.

[nnethercote]

r? @ghost

[nnethercote]

@petrochenkov: this is my attempt to implement what you suggested, but there are problems. Consider the program I mentioned previously:

#![allow(unused_macros)]
macro_rules! m_used { ( $id ) => {} }
macro_rules! m_unused { ( $id ) => {} }
fn main() { m_used!(x); 

Currently, the first macro results in a hard error, the second in a lint error (which is ignorable).

With this change, both macros result in a lint error (which is ignorable). If they are ignored, I get an ICE somewhere in codegen, getting a Layout:

error: internal compiler error: compiler/rustc_codegen_llvm/src/context.rs:956:13: failed to get layout for `[type error]`: the type `[type error]` has an unknown layout
 --> y.rs:6:1
  |
6 | fn main() { m_used!(x); }
  | ^^^^^^^^^

thread 'rustc' panicked at 'Box<dyn Any>', /home/njn/dev/rust2/compiler/rustc_errors/src/lib.rs:1289:9

I suspect this is related to the fact that an invalid macro will get an empty MatcherLoc vec, though I'm not certain. There are also some test failures, but I'm focused on this ICE first.

I'm not sure if this is possible. The current mix of hard errors and lint errors seems hard to adjust. Or am I overlooking something?

[rust-log-analyzer]

The job x86_64-gnu-llvm-12 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

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---
---- [ui] ui/macros/macro-match-nonterminal.rs stdout ----
diff of stderr:

3    |
4 LL |     ($a, $b) => {
+    |
+    |
+    = note: `#[deny(missing_fragment_specifier)]` on by default
+    = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>
6 
7 error: missing fragment specifier
8   --> $DIR/macro-match-nonterminal.rs:2:10
8   --> $DIR/macro-match-nonterminal.rs:2:10

10 LL |     ($a, $b) => {
12    |
12    |
-    = note: `#[deny(missing_fragment_specifier)]` on by default
15    = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>
16 



The actual stderr differed from the expected stderr.
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/macros/macro-match-nonterminal/macro-match-nonterminal.stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args macros/macro-match-nonterminal.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/macros/macro-match-nonterminal.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/macros/macro-match-nonterminal" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/macros/macro-match-nonterminal/auxiliary"
stdout: none
--- stderr -------------------------------
error: missing fragment specifier
   |
   |
LL |     ($a, $b) => {
   |
   |
   = note: `#[deny(missing_fragment_specifier)]` on by default
Some tests failed in compiletest suite=ui mode=ui host=x86_64-unknown-linux-gnu target=x86_64-unknown-linux-gnu
   = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>

error: missing fragment specifier
  --> /checkout/src/test/ui/macros/macro-match-nonterminal.rs:2:10
  --> /checkout/src/test/ui/macros/macro-match-nonterminal.rs:2:10
   |
LL |     ($a, $b) => {
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>

---
---- [ui] ui/macros/macro-missing-fragment.rs stdout ----
diff of stderr:

3    |
4 LL |     ( $( any_token $field_rust_type )* ) => {};
+    |
+    |
+    = note: `#[deny(missing_fragment_specifier)]` on by default
+    = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>
6 
7 error: aborting due to previous error
8 
---
To only update this specific test, also pass `--test-args macros/macro-missing-fragment.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/macros/macro-missing-fragment.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/macros/macro-missing-fragment" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/macros/macro-missing-fragment/auxiliary"
stdout: none
--- stderr -------------------------------
error: missing fragment specifier
   |
   |
LL |     ( $( any_token $field_rust_type )* ) => {}; //~ ERROR missing fragment
   |
   |
   = note: `#[deny(missing_fragment_specifier)]` on by default
   = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>

error: aborting due to previous error
------------------------------------------
------------------------------------------


---- [ui] ui/parser/macro/issue-33569.rs stdout ----
diff of stderr:

15    |
16 LL |     { $+ } => {
+    |
+    |
+    = note: `#[deny(missing_fragment_specifier)]` on by default
+    = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>
18 
19 error: aborting due to 3 previous errors
20 
---
To only update this specific test, also pass `--test-args parser/macro/issue-33569.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/parser/macro/issue-33569.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/parser/macro/issue-33569" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/parser/macro/issue-33569/auxiliary"
stdout: none
--- stderr -------------------------------
error: expected identifier, found `+`
   |
   |
LL |     { $+ } => { //~ ERROR expected identifier, found `+`


error: expected one of: `*`, `+`, or `?`
   |
   |
LL |         $(x)(y) //~ ERROR expected one of: `*`, `+`, or `?`

error: missing fragment specifier
  --> /checkout/src/test/ui/parser/macro/issue-33569.rs:2:8
   |
   |
LL |     { $+ } => { //~ ERROR expected identifier, found `+`
   |
   |
   = note: `#[deny(missing_fragment_specifier)]` on by default
   = note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>

error: aborting due to 3 previous errors
------------------------------------------

[nnethercote]

Thinking this through some more...

• In quoted::parse() we detect the missing fragment specifier, and record it in Parser::missing_fragment_specifier.
• When we first hit a missing fragment specifier during matching, we remove it from Parser::missing_fragment_specifier and issue a hard error. The removal prevents us from issuing duplicate errors on subsequent matching attempts.
• If a match succeeds, we have to look for more missing fragment specifiers before accepting the success, because there might be a missing fragment specifier within a sequence that wasn't looked at during matching (e.g. see ui/macros/macro-missing-fragment.rs). If so, we issue a hard error.
• After matching is finished, we look in Parser::missing_fragment_specifier for any remaining cases. These are ones that weren't hit during matching. For these we issue a lint error.

In the past, attempts were made to change the lint errors to hard errors, but this broke too much existing code.

This PR is about changing the hard errors to lint errors. But then, if we ignore what were previously hard errors, we can't continue compilation sensibly.

So I can't see how to make this work.

[petrochenkov]

I missed the fact that they are reported as hard errors on actual match.
I'll try to experiment with this myself today.

The removal prevents us from issuing duplicate errors on subsequent matching attempts.

This is the less valuable part of this logic given that the compiler deduplicates identical error messages anyway, and given that it requires a piece of global mutable state.

[petrochenkov]

Closing in favor of #95808.