Duplicated implementation is not general enough error on function pointers

[madsmtm]

Given the following code (playground link):

trait Foo {
    const ASSOC: bool = true;
}
impl<T> Foo for fn(T) {}

fn foo(_x: i32) {}

fn impls_foo<T: Foo>(_x: T) {}

fn main() {
    // Works, but if the below code is present, it is shown as failed
    impls_foo(foo as fn(i32));

    // Fails as expected, see https://github.com/rust-lang/rust/issues/56105
    <fn(&u8) as Foo>::ASSOC;
}

The current (nightly ec55c61 2022-06-10, regressed in the past week) output is:

   Compiling playground v0.0.1 (/playground)
error: implementation of `Foo` is not general enough
  --> src/main.rs:12:5
   |
12 |     impls_foo(foo as fn(i32));
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
   |
   = note: `Foo` would have to be implemented for the type `for<'r> fn(&'r u8)`
   = note: ...but `Foo` is actually implemented for the type `fn(&'0 u8)`, for some specific lifetime `'0`

error: implementation of `Foo` is not general enough
  --> src/main.rs:15:5
   |
15 |     <fn(&u8) as Foo>::ASSOC;
   |     ^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
   |
   = note: `Foo` would have to be implemented for the type `for<'r> fn(&'r u8)`
   = note: ...but `Foo` is actually implemented for the type `fn(&'0 u8)`, for some specific lifetime `'0`

error: could not compile `playground` due to 2 previous errors

Ideally the output should look like:

   Compiling playground v0.0.1 (/playground)
error: implementation of `Foo` is not general enough
  --> src/main.rs:15:5
   |
15 |     <fn(&u8) as Foo>::ASSOC;
   |     ^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
   |
   = note: `Foo` would have to be implemented for the type `for<'r> fn(&'r u8)`
   = note: ...but `Foo` is actually implemented for the type `fn(&'0 u8)`, for some specific lifetime `'0`

error: could not compile `playground` due to previous error

The desired result can be achieved by swapping the order of the two statements <fn(&u8) as Foo>::ASSOC; and impls_foo(foo as fn(i32));.

[compiler-errors]

searched nightlies: from nightly-2022-06-07 to nightly-2022-06-08
regressed nightly: nightly-2022-06-08
searched commit range: 50b0025...5435ed6
regressed commit: bb55bd4

bisected with cargo-bisect-rustc v0.6.3

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start=2022-06-07 --end=2022-06-08 --prompt 

@TaKO8Ki, are you working on this? I'll probably be investigating this due to it being related to the recent removal of the borrowck migration mode.

[TaKO8Ki]

@compiler-errors
I've been working on it a bit, but please feel free to take over this issue!

[compiler-errors]

This actually looks like a particularly difficult issue.

I think it's either a bug in RegionInferenceContext::find_constraint_paths_between_regions, or something weird with the RegionInferenceContext.constraint_graph that is convincing us that the correct source for this region error is through the TypeAscription on line 12 of the example.

It may also be notable that we end up going thru a code path that is commented as "unusual" when emitting this error.

This issue seems to be out of my area of expertise, so I won't claim it. @TaKO8Ki, if you can't find a solution then it may be best to hand off to someone with mir-borrowck expertise.

[compiler-errors]

Actually, what I said was totally wrong. This is definitely because we're passing in the wrong Location when visiting consts here. I think I have a fix for this, though I probably want to eliminate other dummy locations we're passing around mir borrowck...

cc @TaKO8Ki sorry for all the pings, but don't keep wasting time on this unless you've made significant progress 😛

@rustbot claim