region error messages involving impls are confusing

[nikomatsakis]

#![feature(nll)]
#![feature(in_band_lifetimes)]
#![feature(impl_header_lifetime_elision)]

struct Foo<'a, 'b> {
    x: &'a u32,
    y: &'b u32,
}

struct Bar<'b> {
    z: &'b u32
}

impl Foo<'_, '_> {
    fn take_bar(&mut self, b: Bar<'_>) {
        self.y = b.z
    }
}

fn main() { }

produces this confusing error:

error[E0621]: explicit lifetime required in the type of `b`
  --> src/main.rs:15:9
   |
14 |     fn take_bar(&mut self, b: Bar<'_>) {
   |                            - consider changing the type of `b` to `Bar<'_>`
15 |         self.y = b.z
   |         ^^^^^^^^^^^^ lifetime `'_` required

I think the problem here is that the "nice region error" code is actually getting in the way of the NLL errors, which are better.

[nikomatsakis]

Pulling out some comments from the Zulip thread. It is an interesting question what the output should look like.

Here are two possibilities:

error: unsatisfied lifetime constraints
  --> $DIR/issue-52742.rs:25:9
   |
LL |     fn take_bar(&mut self, b: Bar<'_>) {
   |                 ---------         -- let's call this `'1`
   |                 |
   |                 has type `&mut Foo<'1, '_>`
LL |         self.y = b.z
   |         ^^^^^^^^^^^^ requires that `'1` must outlive `'2`

error: unsatisfied lifetime constraints
  --> $DIR/issue-52742.rs:25:9
   |
LL |  impl Foo<'_, '_> {
  |            -- let's call this `'2`
LL |     fn take_bar(&mut self, b: Bar<'_>) {
   |                                   -- let's call this `'1`
LL |         self.y = b.z
   |         ^^^^^^^^^^^^ requires that `'1` must outlive `'2`

It's also interesting to consider the case where we have a named, in-band lifetime:

error: unsatisfied lifetime constraints
  --> $DIR/issue-52742.rs:25:9
   |
LL |  impl Foo<'tcx, '_> {
  |            ----`'tcx` declared here
LL |     fn take_bar(&mut self, b: Bar<'_>) {
   |                                   -- let's call this `'1`
LL |         self.y = b.z
   |         ^^^^^^^^^^^^ requires that `'1` must outlive `'tcx`

[nikomatsakis]

I suppose you could imagine a hybrid:

error: unsatisfied lifetime constraints
  --> $DIR/issue-52742.rs:25:9
   |
LL |  impl Foo<'_, '_> {
  |            -- let's call this `'2`
LL |     fn take_bar(&mut self, b: Bar<'_>) {
   |                 ---------         -- let's call this `'1`
   |                 |
   |                 has type `&mut Foo<'2, '_>`
LL |         self.y = b.z
   |         ^^^^^^^^^^^^ requires that `'1` must outlive `'2`

[nikomatsakis]

We now get:

error: unsatisfied lifetime constraints
  --> src/main.rs:16:9
   |
15 |     fn take_bar(&mut self, b: Bar<'_>) {
   |                 ---------         -- let's call this `'1`
   |                 |
   |                 has type `&mut Foo<'_, '2>`
16 |         self.y = b.z
   |         ^^^^^^^^^^^^ requires that `'1` must outlive `'2`

This seems pretty reasonable. Going to mark as NLL-deferred for now.