Confusing lifetime diagnostics for trait impl

[eduardosm]

Code

trait Tr {
    fn func(x: &u8, y: &u16) -> !;
}

impl Tr for () {
    fn func(_x: &u8, _y: &'static u16) -> ! {
        unimplemented!();
    }
}

Current output

error[E0308]: method not compatible with trait
 --> lib.rs:6:5
  |
6 |     fn func(_x: &u8, _y: &'static u16) -> ! {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
  |
  = note: expected signature `fn(&u8, &u16) -> _`
             found signature `fn(&u8, &'static u16) -> _`
note: the anonymous lifetime as defined here...
 --> lib.rs:6:17
  |
6 |     fn func(_x: &u8, _y: &'static u16) -> ! {
  |                 ^
  = note: ...does not necessarily outlive the static lifetime

Desired output

No response

Rationale and extra context

The lifetime mismatch occurs because for y, the lifetime of the trait (anonymous) will not outlive the lifetime of the impl ('static). However, the diagnosis is making reference to (the lifetime of?) x, which does not have anything to do with the mismatch.

If I define an explicit lifetime for x in the impl fn:

note: the anonymous lifetime as defined here...
 --> lib.rs:6:5
  |
6 |     fn func<'a>(_x: &'a u8, _y: &'static u16) -> ! {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: ...does not necessarily outlive the static lifetime

It refers to some anonymouse lifetime using the span of the whole impl function signature.

Something similar happens if I remove the x argument (from both trait and impl):

note: the anonymous lifetime as defined here...
 --> lib.rs:6:5
  |
6 |     fn func(_y: &'static u16) -> ! {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: ...does not necessarily outlive the static lifetime

Other cases

No response

Anything else?

I think the issue might be in the msg_span_from_named_region function from compiler/rustc_infer/src/infer/error_reporting/mod.rs:

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

Lines 230 to 244 in 341ef15

	ty::BoundRegionKind::BrNamed(_, name) => {
	let span = if let Some(param) =
	tcx.hir().get_generics(scope).and_then(|generics| generics.get_named(name))
	{
	param.span
	} else {
	tcx.def_span(scope)
	};
	let text = if name == kw::UnderscoreLifetime {
	"the anonymous lifetime as defined here".to_string()
	} else {
	format!("the lifetime `{name}` as defined here")
	};
	(text, Some(span))
	}

In this case, name is kw::UnderscoreLifetime and generics.get_named(name) will find the span of the first anonymous lifetime. If there is none, it will fallback to the span of scope (the whole function signature).

[eopb]

@rustbot claim

[eopb]

@rustbot release-assignment

[veera-sivarajan]

Output on rustc 1.79.0:

error[E0308]: method not compatible with trait
 --> <source>:6:5
  |
6 |     fn func(_x: &u8, _y: &'static u16) -> ! {
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
  |
  = note: expected signature `fn(&_, &_) -> _`
             found signature `fn(&_, &'static _) -> _`
note: the anonymous lifetime defined here...
 --> <source>:2:24
  |
2 |     fn func(x: &u8, y: &u16) -> !;
  |                        ^^^^
  = note: ...does not necessarily outlive the static lifetime

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0308`.