rustc_parse_format: improve diagnostics for unsupported debug = syntax

[Unique-Usman]

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]

r? @JonathanBrouwer

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[Unique-Usman]

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

140bb59

[rust-bors]

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

[rustbot]

Some changes occurred in src/tools/cargo

cc @ehuss

[Unique-Usman]

r? @estebank

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

fmt: checked 6649 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [style (compiler)]: /checkout/compiler/rustc_builtin_macros/src/format.rs:346: `dbg!` macro is intended as a debugging tool. It should not be in version control.
tidy [style (compiler)]: /checkout/compiler/rustc_builtin_macros/src/format.rs:361: `dbg!` macro is intended as a debugging tool. It should not be in version control.
tidy [style (compiler)]: FAIL
tidy: The following check failed: style (compiler)
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 yarn` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1357:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:00:50
  local time: Wed Jan 14 19:23:47 UTC 2026
  network time: Wed, 14 Jan 2026 19:23:47 GMT
##[error]Process completed with exit code 1.