Don't ICE on unconstrained anonymous lifetimes inside associated types.

[smmalis37]

Fixes #62200. The change here is inspired (copied) by how this case is handled on bare fns at

rust/src/librustc_typeck/astconv.rs

Lines 3083 to 3106 in e8b55a4

	for br in late_bound_in_ret.difference(&late_bound_in_args) {
	let lifetime_name = match *br {
	ty::BrNamed(_, name) => format!("lifetime `{}`,", name),
	ty::BrAnon(_) | ty::BrEnv => "an anonymous lifetime".to_string(),
	};
	let mut err = struct_span_err!(
	tcx.sess,
	decl.output.span(),
	E0581,
	"return type references {} which is not constrained by the fn input types",
	lifetime_name
	);
	if let ty::BrAnon(_) = *br {
	// The only way for an anonymous lifetime to wind up
	// in the return type but **also** be unconstrained is
	// if it only appears in "associated types" in the
	// input. See #47511 for an example. In this case,
	// though we can easily give a hint that ought to be
	// relevant.
	err.note(
	"lifetimes appearing in an associated type are not considered constrained",
	);
	}
	err.emit();

.

[rust-highfive]

r? @davidtwco

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[davidtwco]

@bors r+

[bors]

📌 Commit bbaab63 has been approved by davidtwco

[bors]

⌛ Testing commit bbaab63 with merge e9d67a3cd88b7a8350112e69507668a7c71ac50d...

[rust-log-analyzer]

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[bors]

💔 Test failed - checks-actions

[davidtwco]

@rust-lang/infra @rust-log-analyzer's snippet is empty in the above comment - the error seems spurious too.

[ehuss]

@bors retry
rust-lang/cargo#8517