std::mem::zeroed() panics on nightly with ash::Device, but not on stable

[aloucks]

As stated in the title, the behavior of mem::zeroed() seems to have changed on the nightly channel and now panics when called with ash::Device as the type parameter. This is causing vk_mem::AllocatorCreateInfo::default() to panic in the vk_mem crate.

[dependencies]
ash = "0.31"

#[test]
fn zeroed() {
    let _device = unsafe { std::mem::zeroed::<ash::Device>() }; 
}

The last nightly to behave the same as stable

info: latest update on 2020-09-27, rust version 1.48.0-nightly (623fb90b5 2020-09-26)

$ cargo +nightly-2020-09-27 test
...
running 1 test
test zeroed ... ok

And started failing with

info: latest update on 2020-09-28, rust version 1.48.0-nightly (7f7a1cbfd 2020-09-27)

$ cargo +nightly-2020-09-28 test
...
running 1 test
test zeroed ... FAILED

failures:

---- zeroed stdout ----
thread 'zeroed' panicked at 'attempted to zero-initialize type `ash::Device`, which is invalid', /rustc/7f7a1cbfd3b55daee191247770627afab09eece2\library\core\src\mem\mod.rs:622:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[jyn514]

I don't think this is a bug, it's a soundness fix. What's the definition of ash::Device? Not all types are valid as zeroed, for example mem::zeroed<Box<usize>>() is immediate UB.

[aloucks]

@jyn514 It contains function pointers. I suspected the same, but wanted to confirm and report anyway as it will cause a breaking change when it reaches the stable channel.

[jonas-schievink]

Yeah, this was UB and is now being diagnosed, so closing as expected behavior.