Type mismatch mentioning trait as type

[lcnr]

trait DynEq {}

impl<'a> PartialEq for &'a (dyn DynEq + 'static) {
    fn eq(&self, _other: &Self) -> bool {
        true
    }
}

impl Eq for &dyn DynEq {}

Causes the following error:

error[E0308]: mismatched types
 --> src/lib.rs:9:6
  |
9 | impl Eq for &dyn DynEq {}
  |      ^^ lifetime mismatch
  |
  = note: expected type `std::cmp::PartialEq`
             found type `std::cmp::PartialEq`
note: the lifetime `'_` as defined on the impl at 9:13...
 --> src/lib.rs:9:13
  |
9 | impl Eq for &dyn DynEq {}
  |             ^
  = note: ...does not necessarily outlive the static lifetime

This is somewhat confusing, especially since std::cmp::PartialEq is not a type.

This issue has been assigned to @ayrtonm via this comment.

[rodrimati1992]

This looks similar to what I reported in #59865

[estebank]

Here we don't account for anything other than type mismatches

rust/src/librustc_infer/infer/error_reporting/mod.rs

Lines 1404 to 1433 in 2002eba

	let (expected_found, exp_found, is_simple_error) = match values {
	None => (None, None, false),
	Some(values) => {
	let (is_simple_error, exp_found) = match values {
	ValuePairs::Types(exp_found) => {
	let is_simple_err =
	exp_found.expected.is_simple_text() && exp_found.found.is_simple_text();
	OpaqueTypesVisitor::visit_expected_found(
	self.tcx,
	exp_found.expected,
	exp_found.found,
	span,
	)
	.report(diag);
	(is_simple_err, Some(exp_found))
	}
	_ => (false, None),
	};
	let vals = match self.values_str(&values) {
	Some((expected, found)) => Some((expected, found)),
	None => {
	// Derived error. Cancel the emitter.
	diag.cancel();
	return;
	}
	};
	(vals, exp_found, is_simple_error)
	}
	};

which means that later we use the generic "type" prefix

rust/src/librustc_infer/infer/error_reporting/mod.rs

Lines 1447 to 1448 in 2002eba

	let expected_label = exp_found.map_or("type".into(), |ef| ef.expected.prefix_string());
	let found_label = exp_found.map_or("type".into(), |ef| ef.found.prefix_string());

We need to account for ValuePairs::TraitRefs in the first snippet and ferry the prefix strings from there to the later (instead of what we do now).

[ayrtonm]

I'd like to work on this
@rustbot claim

[estebank]

@ayrtonm do not hesitate to reach out if you need help!

[ayrtonm]

Do we need to get the correct prefix strings from the contents of ValuePairs::TraitRefs or are they just "trait"?

[estebank]

I believe they can be just "trait".