trying to get pointer to extern typed field causes runtime panic

I tried this code:

#![feature(extern_types)]
extern {
    type Opaque;
}

struct Thing {
    _pad: u8,
    o: Opaque,
}

fn main() {
    let v = b"a";
    let x: &Thing = unsafe { std::mem::transmute(&v) };
    let field_ptr = &x.o as *const Opaque;
    println!("{:X}", field_ptr as usize)
}

I expected to see this happen: printing a stack address

Instead, this happened: panic: attempted to compute the size or alignment of extern type

Context

one of the main usecases for extern types is representing trailing fields of variable size for C ffi. this bug completely makes this impossible. it also panics during a piece of safe code that should be infallible. this panic does not occur if the extern type is the only field.

Meta

rustc --version --verbose:

rustc 1.80.0-nightly (debd22da6 2024-05-29)
binary: rustc
commit-hash: debd22da66cfa97c74040ebf68e420672ac8560e
commit-date: 2024-05-29
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.6

Compiling playground v0.0.1 (/playground)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.48s
     Running `target/debug/playground`
thread 'main' panicked at library/core/src/panicking.rs:221:5:
attempted to compute the size or alignment of extern type `Opaque`
stack backtrace:
   0: rust_begin_unwind
             at /rustc/684b3553f70148ded97a80371c2405984d4f6aa7/library/std/src/panicking.rs:661:5
   1: core::panicking::panic_nounwind_fmt::runtime
             at /rustc/684b3553f70148ded97a80371c2405984d4f6aa7/library/core/src/panicking.rs:112:18
   2: core::panicking::panic_nounwind_fmt
             at /rustc/684b3553f70148ded97a80371c2405984d4f6aa7/library/core/src/panicking.rs:122:5
   3: core::panicking::panic_nounwind
             at /rustc/684b3553f70148ded97a80371c2405984d4f6aa7/library/core/src/panicking.rs:221:5
   4: playground::main
             at ./[src/main.rs:14](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#):21
   5: core::ops::function::FnOnce::call_once
             at /rustc/684b3553f70148ded97a80371c2405984d4f6aa7/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
thread caused non-unwinding panic. aborting.

Start by compiling and running the minimal extern-types example from the issue, confirming the panic and its backtrace in library/core/src/panicking.rs. Trace how taking a reference to the extern-typed field computes size or alignment. Done means the example no longer panics and prints the field pointer address as expected, with regression coverage for this case.