Rust complains about mismatched types, but the types are identical

[jimblandy]

When I compile the following code:

use std::future::Future;

pub async fn async_magic<F, R>(f: F) -> Vec<u8>
where
    F: FnOnce(&mut Vec<u8>) -> R,
    R: Future,
{
    todo!()
}

async fn push_zero(v: &mut Vec<u8>) {
    todo!()
}

pub async fn test_async_magic() {
    async_magic(push_zero).await;
}

async fn sleep() {}

I get the error message:

error[E0308]: mismatched types
  --> src/lib.rs:16:5
   |
16 |     async_magic(push_zero).await;
   |     ^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected associated type `<for<'_> fn(&mut Vec<u8>) -> impl for<'_> Future {push_zero} as FnOnce<(&mut Vec<u8>,)>>::Output`
              found associated type `<for<'_> fn(&mut Vec<u8>) -> impl for<'_> Future {push_zero} as FnOnce<(&mut Vec<u8>,)>>::Output`
   = note: the required lifetime does not necessarily outlive the empty lifetime
note: the lifetime requirement is introduced here
  --> src/lib.rs:5:32
   |
5  |     F: FnOnce(&mut Vec<u8>) -> R,
   |                                ^

I'm sure the 'expected' and 'found' types are indeed different somehow, but there's nothing in the output that would allow us to distinguish them.

EDIT: Output from rustc -vV:

binary: rustc
commit-hash: c8dfcfe046a7680554bf4eb612bad840e7631c4b
commit-date: 2021-09-06
host: x86_64-unknown-linux-gnu
release: 1.55.0
LLVM version: 12.0.1

[klensy]

rustc -vV ?

[mbartlett21]

The difference is that it is expecting R: Future not to be related to the lifetime of the &mut Vec<u8>, while it is. There doesn't seem to be any way to actually state that in bounds :(.

[estebank]

Triage: output hasn't changed

error[E0308]: mismatched types
  --> f.rs:16:5
   |
16 |     async_magic(push_zero).await;
   |     ^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected trait `for<'a> <for<'a> fn(&'a mut Vec<u8>) -> impl Future<Output = ()> {push_zero} as FnOnce<(&'a mut Vec<u8>,)>>`
              found trait `for<'a> <for<'a> fn(&'a mut Vec<u8>) -> impl Future<Output = ()> {push_zero} as FnOnce<(&'a mut Vec<u8>,)>>`
note: the lifetime requirement is introduced here
  --> f.rs:5:32
   |
5  |     F: FnOnce(&mut Vec<u8>) -> R,
   |                                ^

Running with -Zverbose gives us a clue of what's happening

error[E0308]: mismatched types
  --> f.rs:16:5
   |
16 |     async_magic(push_zero).await;
   |     ^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected trait `FnOnce<(&RePlaceholder(Placeholder { universe: U1, name: BrNamed(DefId(0:14 ~ f[29b3]::async_magic::'_), '_) }) mut Vec<u8>,)>`
              found trait `for<Region(BrNamed(DefId(0:16 ~ f[29b3]::push_zero::'_), '_))> FnOnce<(&ReLateBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:16 ~ f[29b3]::push_zero::'_), '_) }) mut Vec<u8>,)>`
note: the lifetime requirement is introduced here
  --> f.rs:5:32
   |
5  |     F: FnOnce(&mut Vec<u8>) -> R,
   |                                ^

[estebank]

Current output:

error[E0308]: mismatched types
  --> src/lib.rs:16:5
   |
16 |     async_magic(push_zero).await;
   |     ^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected opaque type `impl for<'a> Future<Output = ()>`
              found opaque type `impl Future<Output = ()>`
   = help: consider `await`ing on both `Future`s
   = note: distinct uses of `impl Trait` result in different opaque types
note: the lifetime requirement is introduced here
  --> src/lib.rs:5:32
   |
5  |     F: FnOnce(&mut Vec<u8>) -> R,
   |                                ^