[NLL] Use span of the closure args in free region errors

[matthewjasper]

Also adds a note when one of the free regions is BrEnv.
In a future PR I'll change these messages to say "return requires", which should improve them a bit more.

r? @nikomatsakis

[nikomatsakis]

I think it would be nice to say "in return type of this closure" -- this confused me for quite a bit

[nikomatsakis]

We can do better here, too. mir.span is pretty horrific in all cases. Maybe we leave that for another issue though. We would want to extract (from the HIR) the span of the return type, I suppose. Really, we should do the same "return type is &'1 u32" sort of thing we do for arguments, right?

I wonder if, for closures, we can highlight just the final |?

So it would look like:

error: unsatisfied lifetime constraints
  --> $DIR/E0621-does-not-trigger-for-closures.rs:25:26
   |
LL |     invoke(&x, |a, b| if a > b { a } else { b }); //~ ERROR E0495
   |                    --    ^^^^^ requires that `'1` must outlive `'2`
   |                    ||
   |                    |return type of closure is `&'2 i32`
   |                    has type `&'1 i32`

[nikomatsakis]

This is the code to extract the type name:

rust/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/region_name.rs

Lines 278 to 280 in 4b8089d

	let type_name = with_highlight_region(needle_fr, *counter, || {
	infcx.extract_type_name(&argument_ty)
	});

[nikomatsakis]

To get the final character you can use the end_point method

[matthewjasper]

Comments addressed

[nikomatsakis]

I have some ideas for future improvement, but they should not block this PR I think. I wonder where the best place to note them is.

[nikomatsakis]

so nice

[nikomatsakis]

also so nice — I do wonder if we can "re-orient" the message around this note though. Something like this:

error: captured variable cannot escape a `FnMut` closure
  --> $DIR/issue-40510-1.rs:18:9
   |
LL |     || {
   |     -- closure implements `FnMut`, so references to captured variables can't escape the closure
LL |         &mut x
   |         ^^^^^^ returns a reference that is local to the closure

error: aborting due to previous error

maybe we should shelve this for a later improvement, though -- in particular, I would really like to do a bit more analysis and connect the message to the variable x. Something like this:

error: captured variable cannot escape a `FnMut` closure
  --> $DIR/issue-40510-1.rs:18:9
   |
LL |     || {
   |     -- closure implements `FnMut`, so references to captured variables can't escape the closure
LL |         &mut x
   |              ^ references to `x` cannot escape the closure body

error: aborting due to previous error

[nikomatsakis]

another place we should take a note for future improvement: rather than printing the return type here, I'd rather we found some way to describe the role that '2 plays (i.e., the reference to x). But this is a start.

[nikomatsakis]

@bors r+

[bors]

📌 Commit b13e3f8 has been approved by nikomatsakis

[bors]

⌛ Testing commit b13e3f8 with merge 63fe441...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 63fe441 to master...