doc test that should not compile still fails compile_fail test

[tspiteri]

The below code (in cf/lib.rs) has three doc tests:

1. The first succeeds as expected.
2. The second test fails as expected: the constant M would be 6 - 8 and there is an attempt to subtract with overflow.
3. The third test should succeed as it has the exact same code as the second test but is marked as compile_fail. However the test fails with the message “Test compiled successfully, but it's marked compile_fail.”

use std::marker::PhantomData;
use std::mem;

/// This test succeeds as expected:
///
/// ```rust
/// let works = cf::S::<u32>::default();
/// works.foo();
/// ```
///
/// This test fails as expected, as it fails to compile:
///
/// ```rust
/// let should_fail = cf::S::<u64>::default();
/// should_fail.foo();
/// ```
///
/// This test should succeed, but fails with “Test compiled
/// successfully, but it's marked `compile_fail`.”:
///
/// ```compile_fail
/// let should_fail = cf::S::<u64>::default();
/// should_fail.foo();
/// ```
#[derive(Default)]
pub struct S<T>(PhantomData<T>);

impl<T> S<T> {
    const M: usize = 6 - mem::size_of::<T>();
    pub fn foo(&self) {
        let _ = Self::M;
    }
}

[ehuss]

This is a consequence of how compile_fail works. compile_fail (and no_run) does the equivalent of --emit=rmeta (or the same as cargo check), which does not perform all steps of compilation (like codegen and linking). One of the issues tracking this problem is #49292.

I'm not sure why rustdoc does not perform the full compilation steps for compile_fail and no_run.