Invalid span when emitting a diagnostic note on the top-level module

[jakubadamw]

While I was investigating a fix for #63064, I noticed one problematic error case, where, as it turns out, the compiler emits a broken span.

fn main() {}

trait Foo<X = Box<dyn Foo>> {}

This is the error I'm getting with the nightly:

error[E0391]: cycle detected when processing `Foo::X`
 --> ./src/test/ui/cycle-trait/cycle-trait-default-type-trait-oneliner.rs:3:23
  |
3 | trait Foo<X = Box<dyn Foo>> {}
  |                       ^^^
  |
  = note: ...which again requires processing `Foo::X`, completing the cycle
note: cycle used when collecting item types in top-level module
 --> ./src/test/ui/cycle-trait/cycle-trait-default-type-trait-oneliner.rs:1:1
  |
1 | fn main() {}
  | ^^^^^^^^^

The second span should extend to the whole top-level module but instead terminates at the first {. This is due to how SourceMap::def_span() is defined:

rust/src/libsyntax/source_map.rs

Lines 689 to 691 in 023525d

	pub fn def_span(&self, sp: Span) -> Span {
	self.span_until_char(sp, '{')
	}

This works kind of okay for all item definitions, except for top-level modules. I did try to come up with a way to fix it up in libsyntax, but there's no clean way of doing it as it only deals with Spans. I figured the place to look at would be:

rust/src/librustc/ty/query/plumbing.rs

Lines 313 to 315 in 023525d

	if let Some((span, query)) = usage {
	err.span_note(fix_span(span, &query),
	&format!("cycle used when {}", query.describe(self)));

but I am not at all familiar with the new query system, so decided to file this issue.

[jakubadamw]

@eddyb, there's a relevant-looking FIXME with your name in that file:

rust/src/librustc/ty/query/plumbing.rs

Lines 900 to 909 in 023525d

	// FIXME(eddyb) Get more valid Span's on queries.
	pub fn default_span(&self, tcx: TyCtxt<$tcx>, span: Span) -> Span {
	if !span.is_dummy() {
	return span;
	}
	// The def_span query is used to calculate default_span,
	// so exit to avoid infinite recursion
	if let Query::def_span(..) = *self {
	return span
	}

Would you have an idea on a good fix for this? Seemed like an easy one for me at first but then I noticed that it would involve meddling with this plumbing.

[eddyb]

That def_span is pretty weird, cc @petrochenkov

[estebank]

Current output:

error[[E0391]](https://doc.rust-lang.org/stable/error_codes/E0391.html): cycle detected when computing type of `Foo::X`
 --> src/main.rs:3:23
  |
3 | trait Foo<X = Box<dyn Foo>> {}
  |                       ^^^
  |
  = note: ...which immediately requires computing type of `Foo::X` again
note: cycle used when collecting item types in top-level module
 --> src/main.rs:1:1
  |
1 | / fn main() {}
2 | |
3 | | trait Foo<X = Box<dyn Foo>> {}
  | |______________________________^