`offset_of_slice` exposes whether a custom DST has a private slice field

The following program compiles:

#![feature(offset_of_slice)]

mod foo {
    pub struct Inner {
        foo: u8,
        tail: [u8],
        // tail: dyn core::fmt::Debug,
    }
    pub struct Outer {
        pub dst: Inner,
    }
}

fn main() {
    assert_eq!(core::mem::offset_of!(foo::Outer, dst), 0);
}

It does not compile if the type of tail is changed to a dyn type. I believe neither version should compile, because tail is a private field, and therefore information about its type should not leak to places where it is not visible.

rustc version: 1.87.0-nightly (2025-03-09 3ea711f17e3946ac3f4d)

@rustbot label F-offset_of_slice C-bug

No source file or test is named. Start by reproducing the provided program on the referenced nightly compiler, then trace the compiler's handling of offset_of! and privacy for custom DST fields. Done means both the private slice and private dyn-tail variants are rejected, with a regression test covering the behavior.