Direct coercion from closures to `unsafe fn` pointers

[Centril]

While you can coerce closures to fn pointers, it is currently not possible to coerce closures to unsafe fn pointers since that requires a transitive coercion from fn pointers to unsafe fn pointers. We do not allow transitive coercions. Example:

static A: fn() = || {}; // OK
static B: unsafe fn() = A; // OK
static C: unsafe fn() = || {}; // ERROR; would require the two coercions above transitively.

This behavior of current Rust was unexpected in rust-lang/rfcs#2592 (comment).

@cramertj had the idea to implement an immediately attainable direct coercion between closures and unsafe fn pointers rather than go the more complex and contentious route of transitive coercions (which may have unforeseen consequences). This idea seems reasonable to me.

cc @nikomatsakis @arielb1

PS: should we fcp the PR once made?

[cramertj]

should we fcp

Traditionally we've just accepted PRs for transitive coercions, since they already have a standing RFC. One of these was actually one of my first rustc changes: rust-lang/rfcs#1762

[arielb1]

Looks like just a bug, and should be implemented directly.