Suggest examples of format specifiers in error messages

[sunjay]

This is real code that someone I am helping learn Rust wrote today. It's a problem you could easily stare at for hours and never figure it out. Especially if you're new to using format strings. The code actually looks correct (from a distance).

fn main() {
    println!(":#?", vec![123]);
}

Here's the error message we produce:

error: argument never used
 --> src/main.rs:2:21
  |
2 |     println!(":#?", vec![123]);
  |              -----  ^^^^^^^^^ argument never used
  |              |
  |              formatting specifier missing

The person I was helping assumed that some formatting trait hadn't been implemented. It didn't occur to them that the format specifier was incorrect even though it said "formatting specifier missing".

I think we could help them by adding a hint or something that shows an example of a specifier whenever we say "formatting specifier missing".

Something like:

hint: format specifiers usually look like `{}` or `{:?}`

This would then help them compare against what they have and find the problem sooner.

From @estebank: (source)

We already handle C-style formatting strings and provide an appropriate suggestion, so I think this would also be reasonable to handle this.

fn main() {
    println!("%d", 42);
}

Adding logic to here should be relatively straightforward.

Also, this case should also be slightly tweaked to point at the formatting
specifiers that were found:

fn main() {
    println!("{} :#?", 1, 42);
}

[kungfukennyg]

Would this be good for someone relatively new to contributing? I’d like to take it if so.

[estebank]

Hi @kennethbgoodin! I believe this is a great first issue!

For the suggestion

format specifiers usually look like `{}` or `{:?}`

you will want to modify

rust/src/librustc_builtin_macros/format.rs

Lines 1222 to 1224 in d8dcb63

	if !found_foreign && errs_len == 1 {
	diag.span_label(cx.fmtsp, "formatting specifier missing");
	}

to have something like diag.note("format specifiers..");.

For the println!("{} :#?", 1, 42); case, you'll want to look at

rust/src/librustc_builtin_macros/format.rs

Lines 1137 to 1154 in d8dcb63

	let mut diag = {
	if errs_len == 1 {
	let (sp, msg) = errs.into_iter().next().unwrap();
	let mut diag = cx.ecx.struct_span_err(sp, msg);
	diag.span_label(sp, msg);
	diag
	} else {
	let mut diag = cx.ecx.struct_span_err(
	errs.iter().map(|&(sp, _)| sp).collect::<Vec<Span>>(),
	"multiple unused formatting arguments",
	);
	diag.span_label(cx.fmtsp, "multiple missing formatting specifiers");
	for (sp, msg) in errs {
	diag.span_label(sp, msg);
	}
	diag
	}
	};

and add a span label for each found parameter, IIRC you can get those from cx.fmtsp.

The idea of accepting :#? as a "valid foreign" formatting string seems a bit too heavy handed, the other two improvements should be enough. If this is your first time hacking on rustc, take a look at https://rust-lang.github.io/rustc-guide/, it should be able to guide you through setting your environment and trying things out in no time.

[macdonaldo]

r? @estebank
Hi, I'm new to rust and contributing to rust-lang. Suggestions and improvements to this pull request #76443 are very welcome. Thank you.

[estebank]

Fixed by #146123.