Initialization by assignment not allowed for "drop-ful" types in constants.

[eddyb]

In the following example, FOO and ASSIGN are allowed but FOO_ASSIGN isn't:

pub struct Foo;
impl Drop for Foo {
    fn drop(&mut self) {}
}

pub const FOO: Foo = { let x = Foo; x };
pub const ASSIGN: i32 = { let x; x = 0; x };
pub const FOO_ASSIGN: Foo = { let x; x = Foo; x };

The reason is that x = Foo; is lowered to DropAndReplace in MIR, and that's not handled in const-checking as a Drop and an assignment (which it functionally is).

cc @davidtwco This might also break the [constant; N] promotion (while &T promotion would never encounter DropAndReplace because &T is always Copy).

EDIT: nevermind, promotion works based on temporaries, so it shouldn't be affected.

cc @oli-obk @RalfJung @Centril

[Centril]

I'm going to label this as a bug since it seems covered by #57175 from a T-Lang perspective.