MIR borrowck: error does not know about free/named lifetimes

[arielb1]

e.g.

fn bar<'a>() -> &'a mut u32 {
    &mut 4
}

Gives the following errors for AST and MIR borrowck - the MIR error should be more like the AST error

error[E0597]: borrowed value does not live long enough (Ast)
 --> bar.rs:2:10
  |
2 |     &mut 4
  |          ^ does not live long enough
3 | }
  | - temporary value only lives until here
  |
note: borrowed value must be valid for the lifetime 'a as defined on the function body at 1:1...
 --> bar.rs:1:1
  |
1 | / fn bar<'a>() -> &'a mut u32 {
2 | |     &mut 4
3 | | }
  | |_^

error[E0597]: borrowed value does not live long enough (Mir)
 --> bar.rs:3:2
  |
2 |     &mut 4
  |          - temporary value created here
3 | }
  | -^ temporary value dropped here while still borrowed
  | |
  | temporary value needs to live until here
  |
  = note: consider using a `let` binding to increase its lifetime

error: aborting due to 2 previous errors

[nikomatsakis]

Some mentoring notes:

The error is issued here:

rust/src/librustc_mir/borrow_check/mod.rs

Lines 2374 to 2382 in 7b637b7

	let mut err = self.tcx
	.path_does_not_live_long_enough(span, "borrowed value", Origin::Mir);
	err.span_label(proper_span, "temporary value created here");
	err.span_label(span, "temporary value dropped here while still borrowed");
	err.note("consider using a `let` binding to increase its lifetime");
	if let Some(end) = end_span {
	err.span_label(end, "temporary value needs to live until here");
	}

Note in particular the call to span_label at the end:

rust/src/librustc_mir/borrow_check/mod.rs

Line 2381 in 7b637b7

err.span_label(end, "temporary value needs to live until here");

I think the first thing to add a new parameter to report_borrowed_value_does_not_live_long_enough:

rust/src/librustc_mir/borrow_check/mod.rs

Lines 2363 to 2368 in 7b637b7

	fn report_borrowed_value_does_not_live_long_enough(
	&mut self,
	_: Context,
	(place, span): (&Place<'tcx>, Span),
	end_span: Option<Span>,
	) {

It's going to need access to the borrow that does not live long enough. The callers ought to have a variable borrow: &BorrowData in scope, so we can pass that in.

In that case, we can match on borrow.region. If we have a RegionKind::Free or RegionKind::EarlyBound, that corresponds to a named lifetime on the fn, so we don't probably want to emit that call to span_label I mentioned earlier. Instead, we want to invoke err.note. It might be worth ripgrep'ing through librustc_borrowck to find the code that it uses to emit its message.

[Yoric]

I'm interested in handling this.

[Yoric]

Oh, closed? Ok, moving to something else.