NLL accepts higher-ranked subtype that non-NLL rejects

[matthewjasper]

The following code example (playground)

// #![feature(nll)]

trait X {
    type G;
    fn make_g() -> Self::G;
}

impl<'a> X for fn(&'a ()) {
    type G = &'a ();

    fn make_g() -> Self::G {
        &()
    }
}

trait Y {
    type F;
    fn make_f() -> Self::F;
}

impl<T> Y for fn(T) {
    type F = fn(T);
    
    fn make_f() -> Self::F {
        |_| {}
    }
}

// Always an error, but the message is bad
fn higher_ranked_region_has_lost_its_binder() {
    let x = <fn (&())>::make_g();
}

// Works, but only with feature(nll)
fn magical() {
    let x = <fn (&())>::make_f();
}

produces the output

Details

error[E0308]: mismatched types
  --> src/lib.rs:31:13
   |
31 |     let x = <fn (&())>::make_g();
   |             ^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected type `X`
              found type `X`

error[E0308]: mismatched types
  --> src/lib.rs:36:13
   |
36 |     let x = <fn (&())>::make_f();
   |             ^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected type `Y`
              found type `Y`

The second example also unexpectedly compiles in full NLL mode.

[nikomatsakis]

I renamed the title to focus on the part I would most like to investigate, which is why this function works but only with feature-nll:

// Works, but only with feature(nll)
fn magical() {
    let x = <fn (&())>::make_f();
}

That seems like a blocker to disabling migration mode.

[pnkfelix]

cc #58781

[pnkfelix]

marking P-medium to reflect that this is solely about #![feature(nll)], not migration mode (#58781). When the latter becomes higher priority, this will likewise be raised up.

[matthewjasper]

Update: this is only an issue now with -Zno-leak-check

[pnkfelix]

cc blocks #59490