`fn(&u8) -> &u8` doesn't implement `FnOnce(T) -> T`: ReInfer(ReSkolemized(.. error message

[japaric]

Small variation of #19126

STR

#![feature(unboxed_closures)]

fn foo<T, F: FnOnce(T) -> T>(f: F) {}
fn id<'a>(input: &'a u8) -> &'a u8 { input }

fn main() {
    foo(id);
}

Output

reskolemized.rs:7:5: 7:8 error: cannot infer an appropriate lifetime due to conflicting requirements
reskolemized.rs:7     foo(id);
                      ^~~
note: first, the lifetime cannot outlive lifetime ReInfer(ReSkolemized(1, BrNamed(DefId { krate: 0, node: 31 }, "'a"(71))))...
reskolemized.rs:7:5: 7:8 note: ...so that trait type parameters matches those specified on the impl (expected `core::ops::Fn<(&u8,), &u8>`, found `core::ops::Fn<(&u8,), &u8>`)
reskolemized.rs:7     foo(id);
                      ^~~
reskolemized.rs:7:5: 7:8 note: but, the lifetime must be valid for the expression at 7:4...
reskolemized.rs:7     foo(id);
                      ^~~
reskolemized.rs:7:5: 7:8 note: ...so that type parameter instantiated with `&u8`, will meet its declared lifetime bounds.
reskolemized.rs:7     foo(id);
                      ^~~
error: aborting due to previous error

Version

#19113 + #18868 on top of bfaa7bc

Not sure if this should work or not, but the error message could be improved.

cc @nikomatsakis