Inference failure with mutable reference to closure

[eefriedman]

fn foo<'a, X:'static>(mut cb: &mut (FnMut(&u64) -> X + 'a)) -> X {
    let mut cb = move |expr1| {
        cb(expr1)
    };
    let cb = &mut cb;
    foo(cb)
}

// Alternate version which works
fn id<X, T: FnMut(&u64) -> X>(t: T) -> T { t }
fn foo2<'a, X:'static>(mut cb: &mut (FnMut(&u64) -> X + 'a)) -> X {
    let mut cb = id(move |expr1| {
        cb(expr1)
    });
    let cb = &mut cb;
    foo2(cb)
}

Gives:

error: type mismatch: the type `[closure@<anon>:2:18: 4:6 cb:_]` implements the trait `std::ops::FnMut<(&u64,)>`, but the trait `for<'r> std::ops::FnMut<(&'r u64,)>` is required (expected concrete lifetime, found bound lifetime parameter ) [--explain E0281]
 --> <anon>:6:9
  |>
6 |>     foo(cb)
  |>         ^^
note: required for the cast to the object type `for<'r> std::ops::FnMut(&'r u64) -> X`

error: type mismatch resolving `for<'r> <[closure@<anon>:2:18: 4:6 cb:_] as std::ops::FnOnce<(&'r u64,)>>::Output == X`:
 expected bound lifetime parameter ,
    found concrete lifetime [--explain E0271]
 --> <anon>:6:9
  |>
6 |>     foo(cb)
  |>         ^^
note: required for the cast to the object type `for<'r> std::ops::FnMut(&'r u64) -> X`

error: aborting due to 2 previous errors

[arielb1]

Yeah. Higher-ranked inference is an undecidable mess.

[DemiMarie]

Can the compiler be made to loop this way?

[Mark-Simulacrum]

Closing in favor of #41078.