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

[Unique-Usman]

@estebank thanks for the review. I made the necessary changes.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.