Support ZSTs in DispatchFromDyn

[TimDiekmann]

Allows to use ZSTs with 1 byte alignment in DispatchFromDyn implementation. This is required for Box<T, A: Alloc>

cc #58457

[rust-highfive]

r? @eddyb

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[Centril]

This possibly needs an FCP or some discussion so I've marked it as such tentatively.

[kennytm]

Please ensure that the ZST has alignment ≤ a pointer, because we should not DispatchFromDyn those:

#[repr(align(64))]
struct A;

struct P<'a, T: ?Sized + 'a> {
    p: &'a T,
    a: A,
}

fn main() {
    use std::mem::size_of;
    dbg!(size_of::<(P<'static, u8>, *const (/*vtable*/))>());
    dbg!(size_of::<P<'static, dyn Sync>>());
}

[eddyb]

@kennytm I'd prefer we check alignment to be 1, to be more conservative.

[eddyb]

@bors r+

[bors]

📌 Commit 1b679e7 has been approved by eddyb