UB with ThinVec::new

[Cyborus04]

The following code is unsound, yet compiles:

use thin_vec::ThinVec;

fn main() {
    // this can be any type, not just `u8`
    let mut vec_a = ThinVec::<u8>::new();
    let mut vec_b = ThinVec::<u8>::new();

    let slice_a = vec_a.as_mut_slice();
    let slice_b = vec_b.as_mut_slice();

    assert!(std::ptr::eq(slice_a, slice_b));
}

This triggers 2 forms of undefined behavior:
1. slice_a and slice_b are aliased mutable references. nevermind, see below
2. slice_a and slice_b are mutable references created from a shared reference. ThinVec::new() turns a &Header into a *mut Header, then ThinVec::as_mut_slice creates a &mut [T] from this pointer

[Cyborus04]

Wait, do the aliasing rules apply to ZSTs?
Either way, 2 is still an issue

[Cyborus04]

The same example still compiles and doesn't panic if you use Vec, so I assume that's alright

Edit:
Yes, this is okay

Aliasing only occurs when memory regions, within which the objects are stored, overlap. Since the memory regions occupied by ZSTs are, obviously, zero-sized, there can never be any overlap, even for pointers with the same address.

from rust-lang/rust-memory-model#44 (comment)

[Gankra]

PS: if you think something is UB, consider running it under miri. It's not perfect but if miri's happy, that's a pretty good sign you're doing something fine.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=775af409fd3ce3283507d11e9082c380

(See Tools > Miri in the top right)

[Cyborus04]

Now I feel silly, sorry about this