rustc_parse_format: improve diagnostics for unsupported python numeric grouping

[Unique-Usman]

Detect Python-style numeric grouping syntax in format strings (e.g. {x:,})
and emit a clear diagnostic explaining that it is not supported in Rust.
This helps users coming from Python understand the error without exposing
the full set of valid Rust format specifiers.

[rustbot]

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[Unique-Usman]

r? @estebank

[Unique-Usman]

Hello as part of the ongoing effort in #145718

fn main() {
    println!("{x,}, world!",);
}

initially, the compiler gives the below error for the above

error: invalid format string: expected `}`, found `,`
 --> src/main.rs:3:14
  |
3 | println!("{x:,}");
  |           -  ^ expected `}` in format string
  |           |
  |           because of this opening brace
  |
  = note: if you intended to print `{`, you can escape it using `{{`

but after this changes, it gives a better error message suggesting the compiler understand that is a python-style numeric grouping, but it is not supported in rust.

error: invalid format string: expected `}`, found `,`
 --> ../test.rs:2:17
  |
2 |     println!("{x,}, world!",);
  |                 ^  python-style numeric grouping `,` is not supported in rust format strings in format string
  |
  = note: to print `{`, you can escape it using `{{`

error: aborting due to 1 previous error

[rust-bors]

☔ The latest upstream changes (presumably #150912) made this pull request unmergeable. Please resolve the merge conflicts.

[workingjubilee]

error: invalid format string: expected `}`, found `,`
 --> ../test.rs:2:17
  |
2 |     println!("{x,}, world!",);
  |                 ^  python-style numeric grouping `,` is not supported in rust format strings in format string
  |
  = note: to print `{`, you can escape it using `{{`

error: aborting due to 1 previous error

That seems like a very weird place for a very verbose error text to be. Things in that position are usually indicative of what they are pointing at, as opposed to commenting on whether it's correct or not and how. (or at least they are much more terse when they do)

[Unique-Usman]

Thank you very much for the review @workingjubilee

[workingjubilee]

I'm not aware of a digit grouping format string specifier, so it seems there is no meaningful suggestion to offer instead aside from recommending a random crate, and this seems an adequate improvement. Thank you.

@bors r+ rollup

[rust-bors]

📌 Commit 76ad528 has been approved by workingjubilee

It is now in the queue for this repository.

[Unique-Usman]

Thanks for the review @workingjubilee