Add format specifier suggestion

[macdonaldo]

When format specifier is missing from the format string:

• Found format specifiers are pointed at
• Suggest format specifier if none is present

Fix #68293

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[jyn514]

This is really good!

r? @estebank

[estebank]

Thank you for your contribution!

I am slightly concerned by making the used formatting specifiers part of the primary multi span, as it semantically implies that there's something problematic about them. Moving that to a note makes it easier to read and I think should help with clarity.

[estebank]

I think that ideally this would look like

error: argument never used
  --> $DIR/ifmt-bad-arg.rs:33:22
   |
LL |     format!("{}", 1, 2);
   |             ----     ^ argument never used
   |             |
   |             formatting specifier missing
note: the following existing formatting specifiers are used
   |
LL |     format!("{}", 1, 2);
   |              ^^      - used by positional formatting specifier `0`
   |              |
   |              this is positional formatting specifier `0`

In order to accomplish that we would need to

• keep track of the spans for each formatting specifier and a way to refer to them ("positional specifier 0", "named specifier foo", etc.)
• look at the mapping for which formatting specifier is used by which argument
• we'll need to account for named and positional specifiers that are not used

For the proposed output we would need to do something like (simplified):

let note_span: MultiSpan = cx.arg_spans.into();
for span in cx.arg_spans {
    note_span.push_span_label(span, "used argument");
}
for span in errs.iter().map(|&(sp, _)| sp) {
  note_span.push_span_label(span, "argument used here");
}
err.span_note(note_span, "the following existing formatting specifiers are used");

Would you be ok with taking on trying to accomplish this?

[macdonaldo]

yup, I'll work on making these changes. Thanks for the feedback!

[JohnCSimon]

Ping from triage:
@macdonaldo can you post your status on this PR?

[macdonaldo]

@JohnCSimon I made some progress on this but I'm a bit stuck at the moment, so I asked for help on zulip
@estebank Could you help me out here ?

[macdonaldo]

Sorry folks, I don't have much time to commit to this at the moment