Closure-to-fn coercion does not work for `unsafe fn`

[RalfJung]

Given that non-capturing closures coerce to fn, and fn coerces to unsafe fn, I would expect the transitive step to also happen -- but it does not:

fn coerce(_x: fn()) {}
fn coerce_unsafe(_x: unsafe fn()) {}

fn foo() {}

fn main() {
    coerce(|| panic!()); // works
    coerce_unsafe(foo); // works
    coerce_unsafe(|| panic!()); // fails
}

error[E0308]: mismatched types
 --> src/main.rs:9:19
  |
9 |     coerce_unsafe(|| panic!());
  |                   ^^^^^^^^^^^ expected unsafe fn, found normal fn
  |
  = note: expected type `unsafe fn()`
             found type `[closure@src/main.rs:9:19: 9:30]`

This is particularly annoying when instantiating RawWakerVTable, because it means one cannot just use closure syntax there.

[hellow554]

https://doc.rust-lang.org/stable/reference/type-coercions.html#coercion-types

Coercion is allowed between the following types:

• T_1 to T_3 where T_1 coerces to T_2 and T_2 coerces to T_3 (transitive case)
  Note that this is not fully supported yet

I guess this is one of the not supported cases? I ask myself why this is generally not supported? What's missing here?

[Centril]

Duplicate of #57883.