Incorrect std::mem::size_of_var for extern types

[OpatrilPeter]

Hello, following code on nightly playground returns zero, indicating extern type behaving like ZST, while in comparison, size_of fails to compile. According to the RFC, panicking or compilation failure should happen, so this behavior seem incorrect.

#![feature(extern_types)]

extern {
    type Foo;
}

pub fn test() -> usize {
    let ptr: *mut Foo = std::ptr::null_mut();
    unsafe{std::mem::size_of_val::<Foo>(&*ptr)}
}

[clubby789]

@rustbot label +F-extern_types

[krtab]

I think this issue is known and discussed here:
#43467 (comment)

[tmiasko]

#118534 turned this into a panic:

thread 'main' panicked at library/core/src/panicking.rs:221:5:
attempted to compute the size or alignment of extern type `Foo`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread caused non-unwinding panic. aborting.
Aborted