Fix ICE caused by missing span in a region error

[gurry]

Fixes #130012

The ICE occurs on line 634 in this error handling code:

rust/compiler/rustc_trait_selection/src/error_reporting/infer/region.rs

Lines 617 to 637 in 085744b

	infer::Subtype(box ref trace)
	if matches!(
	&trace.cause.code().peel_derives(),
	ObligationCauseCode::WhereClause(..)
	| ObligationCauseCode::WhereClauseInExpr(..)
	) =>
	{
	// Hack to get around the borrow checker because trace.cause has an `Rc`.
	if let ObligationCauseCode::WhereClause(_, span)
	| ObligationCauseCode::WhereClauseInExpr(_, span, ..) =
	&trace.cause.code().peel_derives()
	&& !span.is_dummy()
	{
	let span = *span;
	self.report_concrete_failure(generic_param_scope, placeholder_origin, sub, sup)
	.with_span_note(span, "the lifetime requirement is introduced here")
	} else {
	unreachable!(
	"control flow ensures we have a `BindingObligation` or `WhereClauseInExpr` here..."
	)
	}

It is caused by the span being a dummy span and !span.is_dummy() on line 628 evaluating to false.

A dummy span, however, is expected here thanks to the Self: Trait predicate from predicates_of (see line 61):

rust/compiler/rustc_hir_analysis/src/collect/predicates_of.rs

Lines 61 to 69 in 085744b

	let span = DUMMY_SP;
	result.predicates = tcx.arena.alloc_from_iter(
	result
	.predicates
	.iter()
	.copied()
	.chain(std::iter::once((ty::TraitRef::identity(tcx, def_id).upcast(tcx), span))),
	);

This PR changes the error handling code to omit the note which needed the span instead of ICE'ing in the presence of a dummy span.

[rustbot]

r? @cjgillot

rustbot has assigned @cjgillot.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[cjgillot]

@bors r+ rollup

[bors]

📌 Commit 0f8efb3 has been approved by cjgillot

It is now in the queue for this repository.