rustc_parse_format: improve diagnostics for unsupported debug = syntax #150897

Unique-Usman · 2026-01-09T21:30:14Z

Detect Python-style f-string debug syntax in format strings and emit a
clear diagnostic explaining that it is not supported in Rust. When the
intended operation can be inferred, suggest the corresponding Rust
alternative (e.g. dbg! for {x=}).

rustbot · 2026-01-09T21:30:18Z

r? @JonathanBrouwer

rustbot has assigned @JonathanBrouwer.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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Unique-Usman · 2026-01-09T21:33:06Z

@estebank, kindly help take a look. I got stuck in implementing the suggestion part.

140bb59

rust-bors · 2026-01-12T20:24:57Z

☔ The latest upstream changes (presumably #151003) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2026-01-14T19:10:47Z

Some changes occurred in src/tools/cargo

cc @ehuss

Unique-Usman · 2026-01-14T19:21:25Z

r? @estebank

Detect Python-style f-string debug syntax in format strings and emit a clear diagnostic explaining that it is not supported in Rust. When the intended operation can be inferred, suggest the corresponding Rust alternative (e.g. `dbg!` for `{x=}`). Signed-off-by: Usman Akinyemi <[email protected]>

rust-log-analyzer · 2026-01-26T13:21:13Z

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

test [ui] tests/ui/extern/issue-64655-extern-rust-must-allow-unwind.rs#fat2 ... ok
test [ui] tests/ui/extern/issue-64655-extern-rust-must-allow-unwind.rs#fat3 ... ok
test [ui] tests/ui/extern/issue-80074.rs ... ok
test [ui] tests/ui/extern/issue-95829.rs ... ok
test [ui] tests/ui/extern/lgamma-linkage.rs ... ok
test [ui] tests/ui/extern/no-mangle-associated-fn.rs ... ok
test [ui] tests/ui/extern/not-in-block.rs ... ok
test [ui] tests/ui/extern/unsized-extern-derefmove.rs ... ok
test [ui] tests/ui/extern/windows-tcb-trash-13259.rs ... ok
test [ui] tests/ui/feature-gates/allow-features-empty.rs ... ok
---
test [ui] tests/ui/imports/ambiguous-6.rs ... ok
test [ui] tests/ui/imports/ambiguous-4.rs ... ok
test [ui] tests/ui/imports/ambiguous-9.rs ... ok
test [ui] tests/ui/imports/ambiguous-7.rs ... ok
test [ui] tests/ui/imports/ambiguous-panic-globvsglob.rs ... ok
test [ui] tests/ui/imports/ambiguous-panic-glob-vs-multiouter.rs ... ok
test [ui] tests/ui/imports/ambiguous-8.rs ... ok
test [ui] tests/ui/imports/ambiguous-panic-no-implicit-prelude.rs ... ok
test [ui] tests/ui/imports/ambiguous-panic-non-prelude-core-glob.rs ... ok
test [ui] tests/ui/imports/ambiguous-glob-vs-expanded-extern.rs ... ok
test [ui] tests/ui/imports/ambiguous-panic-non-prelude-std-glob.rs ... ok
---
failures:

---- [ui] tests/ui/fmt/format-string-error-3.rs stdout ----

error: 1 diagnostics reported in JSON output but not expected in test file
tests/ui/fmt/format-string-error-3.rs:3:17: ERROR: invalid format string: python's f-string debug `=` is not supported in rust, use `dbg(x)` instead

thread '[ui] tests/ui/fmt/format-string-error-3.rs' panicked at src/tools/compiletest/src/runtest.rs:925:13:
errors differ from expected
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/fmt/format-string-error-3.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/fmt/format-string-error-3" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"

stack backtrace:
   8: __rustc::rust_begin_unwind
             at /rustc/9b1f8ff42d110b0ca138116745be921df5dc97e7/library/std/src/panicking.rs:689:5
   9: core::panicking::panic_fmt

estebank · 2026-01-26T16:36:40Z

tests/ui/fmt/format-string-error-3.rs

@@ -0,0 +1,4 @@
+//@ run-rustfix
+fn main() {
+    println!("{x=}");


Suggested change

println!("{x=}");

println!("{x=}"); //~ ERROR invalid format string: python's f-string debug

Let's not address this one for now, given that it needs further changes.

estebank · 2026-02-02T16:37:36Z

compiler/rustc_builtin_macros/src/format.rs

+                if args.all_args().len() == 1 {
+                    let arg = &args.all_args()[0];


This can be

Suggested change

if args.all_args().len() == 1 {

let arg = &args.all_args()[0];

if [arg] = args.all_args() {

rustbot assigned JonathanBrouwer Jan 9, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 9, 2026