rustc_parse_format: improve error for missing : before ? in format args

[Unique-Usman]

Detect the {ident?} pattern where ? is immediately followed by } and emit a clearer diagnostic explaining that : is required for Debug formatting. This avoids falling back to a generic “invalid format string” error and adds a targeted UI test for the case.

[rustbot]

r? @SparrowLii

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[Unique-Usman]

r? @estebank

[Unique-Usman]

This pr serves as an efforts toward issues 145718.

Following up with our conversation @estebank, I was working on the issue but, I saw this and I thought on working on it while i can keep working on other part of the issue.

If you have a

fn main() {
    let x = 4;
    println!("{x?}, world!",);
}

Someone will get this error

error: invalid format string: expected `}`, found `?`
 --> testing.rs:3:17
  |
3 |     println!("{x?}, world!",);
  |               - ^ expected `}` in format string
  |               |
  |               because of this opening brace
  |
  = note: if you intended to print `{`, you can escape it using `{{`

error: aborting due to 1 previous error

But, I believe, missing colon : is a better diagnostic here.

My proposed improvement was to sort of combine the current diagnostic error with the missing colon. After this pr, this is what will be output which I believe is a better diagnostic.

error: invalid format string: expected `}`, found `?` or missing `:` before `?`
 --> ../testing.rs:3:17
  |
3 |     println!("{x?}, world!",);
  |                 ^ expected `}` in format string or missing `:` before `?` in format string
  |
  = note: to print `{`, you can escape it using `{{`, to format with `Debug`, use `:?`

error: aborting due to 1 previous error

I am also thinking having only missing colon is okay here, what do you think ? We can also improve the above diagnostic message here.

[Unique-Usman]

Thanks for the review @estebank, made the requested change.

[estebank]

@bors r+

[rust-bors]

📌 Commit b0e65da has been approved by estebank

It is now in the queue for this repository.