"cannot relate bound region" in a concrete lifetime vs for<'a> mismatch.

[eddyb]

#![feature(fn_traits, unboxed_closures)]

fn test<F: for<'x> FnOnce<(&'x str,)>>(_: F) {}

struct Compose<F,G>(F,G);
impl<T,F,G> FnOnce<(T,)> for Compose<F,G>
where F: FnOnce<(T,)>, G: FnOnce<(F::Output,)> {
    type Output = G::Output;
    extern "rust-call" fn call_once(self, (x,): (T,)) -> G::Output {
        (self.1)((self.0)(x))
    }
}

fn bad<T>(f: fn(&'static str) -> T) {
    // internal compiler error: cannot relate bound region:
    //   ReLateBound(DebruijnIndex { depth: 2 },
    //     BrNamed(DefId { krate: 0, node: DefIndex(6) => test::'x }, 'x(65)))
    //<= ReSkolemized(0,
    //     BrNamed(DefId { krate: 0, node: DefIndex(6) => test::'x }, 'x(65)))
    test(Compose(f, |_| {}));
}

Usage of FnOnce<(T,)> is to avoid proving an explicit type parameter for the Output associated type, as an workaround for #30867.

Originally submitted as part of #30904.

[eddyb]

Must be caused by the built-in implementation of FnOnce for fn types, the following testcase (run on playpen) gives a regular error instead of an ICE:

fn test<F: for<'x> Lambda<&'x str>>(_: F) {}

trait Lambda<T> {
    type Output;
}

impl<T, U> Lambda<T> for fn(T) -> U {
    type Output = U;
}

struct Compose<F,G>(F,G);
impl<T,F,G> Lambda<T> for Compose<F,G>
where F: Lambda<T>, G: Lambda<F::Output> {
    type Output = G::Output;
}

fn bad<T>(f: fn(&'static str) -> T) {
    test(Compose(f, std::mem::drop as fn(T)));
}

[shepmaster]

@eddyb does this look the same or should I open a new issue:

trait SomeTrait {}

fn foo<P, Q, F>(func: F)
where
    for<'a> F: Fake<'a, P, Q>,
    // just for an impl
    P: Default,
    Q: std::fmt::Debug,
{
    let p = P::default();
    let q = func(&p);
    println!("{:?}", q);
}

trait Fake<'a, P: 'a, Q>: Fn(&'a P) -> Q
where
    Self::Output: SomeTrait + 'a,
{
}

impl<'a, P: 'a, Q, F> Fake<'a, P, Q> for F
where
    F: Fn(&'a P) -> Q,
    F::Output: SomeTrait + 'a,
{
}

#[derive(Debug)]
struct Wrapper<'a>(&'a u8);

impl<'a> SomeTrait for Wrapper<'a> {}

fn main() {
    foo(Wrapper);
}

error: internal compiler error: /checkout/src/librustc/infer/region_constraints/mod.rs:685: cannot relate bound region: ReLateBound(DebruijnIndex { depth: 2 }, BrNamed(CrateNum(0):DefIndex(1:12), 'a(91))) <= ReSkolemized(0, BrNamed(CrateNum(0):DefIndex(1:12), 'a(91)))
  --> src/main.rs:34:5
   |
34 |     foo(Wrapper);
   |     ^^^

[estebank]

No longer ICEs:

error: implementation of `std::ops::FnOnce` is not general enough
  --> src/lib.rs:20:5
   |
20 |     test(Compose(f, |_| {}));
   |     ^^^^
   |
   = note: Due to a where-clause on `test`,
   = note: `Compose<fn(&str) -> T, [closure@src/lib.rs:20:21: 20:27]>` must implement `std::ops::FnOnce<(&'0 str,)>`, for any lifetime `'0`
   = note: but `Compose<fn(&str) -> T, [closure@src/lib.rs:20:21: 20:27]>` actually implements `std::ops::FnOnce<(&'1 str,)>`, for some specific lifetime `'1`

error: implementation of `Lambda` is not general enough
  --> src/lib.rs:18:5
   |
18 |     test(Compose(f, std::mem::drop as fn(T)));
   |     ^^^^
   |
   = note: Due to a where-clause on `test`,
   = note: `Compose<fn(&str) -> T, fn(T)>` must implement `Lambda<&'0 str>`, for any lifetime `'0`
   = note: but `Compose<fn(&str) -> T, fn(T)>` actually implements `Lambda<&'1 str>`, for some specific lifetime `'1`

error: implementation of `Fake` is not general enough
  --> src/main.rs:34:5
   |
34 |     foo(Wrapper);
   |     ^^^
   |
   = note: Due to a where-clause on `foo`,
   = note: `fn(&u8) -> Wrapper<'_> {Wrapper<'_>}` must implement `Fake<'0, u8, Wrapper<'_>>`, for any lifetime `'0`
   = note: but `fn(&u8) -> Wrapper<'_> {Wrapper<'_>}` actually implements `Fake<'1, u8, Wrapper<'_>>`, for some specific lifetime `'1`

[estebank]

Current output:

error[E0308]: mismatched types
  --> src/lib.rs:20:5
   |
20 |     test(Compose(f, |_| {}));
   |     ^^^^ one type is more general than the other
   |
   = note: expected type `std::ops::FnOnce<(&'x str,)>`
              found type `std::ops::FnOnce<(&str,)>`

error: implementation of `Lambda` is not general enough
  --> src/lib.rs:18:5
   |
18 |     test(Compose(f, std::mem::drop as fn(T)));
   |     ^^^^
   |
   = note: `Lambda<&'0 str>` would have to be implemented for the type `fn(&str) -> T`, for any lifetime `'0`
   = note: but `Lambda<&'1 str>` is actually implemented for the type `fn(&'1 str) -> T`, for some specific lifetime `'1`

error: implementation of `Fake` is not general enough
  --> src/main.rs:34:5
   |
34 |     foo(Wrapper);
   |     ^^^
   |
   = note: Due to a where-clause on `foo`,
   = note: `fn(&u8) -> Wrapper<'_> {Wrapper<'_>}` must implement `Fake<'0, u8, Wrapper<'_>>`, for any lifetime `'0`
   = note: but `fn(&u8) -> Wrapper<'_> {Wrapper<'_>}` actually implements `Fake<'1, u8, Wrapper<'_>>`, for some specific lifetime `'1`

[eddyb]

#30904 (comment) should've been posted here - this is what's been fixed, not #30904.