bad suggestion for type lifetime bounds with anonymous lifetimes

[aliemjay]

Given this code:

type Lt<'lt> = *mut &'lt u8;

fn outlives<'lt, T: 'lt>(_: T, _: Lt<'lt>) {}

pub fn test<A>(arg: A, lt: Lt<'_>, _: &str) {
    outlives(arg, lt);
    //~^ ERROR the parameter type `A` may not live long enough
}

The current error message is this: (note the invalid suggestion)

error[[E0311]](https://doc.rust-lang.org/nightly/error_codes/E0311.html): the parameter type `A` may not live long enough
 --> src/lib.rs:6:5
  |
6 |     outlives(arg, lt);
  |     ^^^^^^^^^^^^^^^^^
  |
note: the parameter type `A` must be valid for the anonymous lifetime defined here...
 --> src/lib.rs:5:28
  |
5 | pub fn test<A>(arg: A, lt: Lt<'_>, _: &str) {
  |                            ^^^^^^
note: ...so that the type `A` will meet its required lifetime bounds
 --> src/lib.rs:6:5
  |
6 |     outlives(arg, lt);
  |     ^^^^^^^^^^^^^^^^^
help: consider adding an explicit lifetime bound...
  |
5 | pub fn test<'a, A: 'a>(arg: A, lt: Lt<'_<'a>>, _: &'a str) {
  |             +++  ++++                   ++++       ++

For more information about this error, try `rustc --explain E0311`.

Two problems with this suggestion:

• the last argument of type &str should not be touched by the suggestion.
• the anonymous lifetime '_ should be correctly replaced by the named lifetime 'a.

The fixed suggestion should be:

help: consider adding an explicit lifetime bound...
  |
5 | pub fn test<'a, A: 'a>(arg: A, lt: Lt<'a>, _: &str) {
  |             +++  ++++                 ++

[atsuzaki]

the suggestions for lt: Lt<'_> and _: &str are added by this code:

rust/compiler/rustc_infer/src/infer/error_reporting/mod.rs

Lines 2408 to 2437 in 2f5df8a

	if let Some(ast_generics) = ast_generics {
	for p in ast_generics.params {
	if p.is_elided_lifetime() {
	if self
	.tcx
	.sess
	.source_map()
	.span_to_prev_source(p.span.shrink_to_hi())
	.ok()
	.is_some_and(|s| *s.as_bytes().last().unwrap() == b'&')
	{
	add_lt_suggs
	.push(Some(
	(
	p.span.shrink_to_hi(),
	if let Ok(snip) = self.tcx.sess.source_map().span_to_next_source(p.span)
	&& snip.starts_with(' ')
	{
	new_lt.to_string()
	} else {
	format!("{new_lt} ")
	}
	)
	));
	} else {
	add_lt_suggs.push(Some((p.span.shrink_to_hi(), format!("<{new_lt}>"))));
	}
	}
	}
	}