Types in `#[doc(hidden)]` module show up under trait implementations

Code

Example in this PR: https://github.com/rust-lang/rust/pull/145335#issuecomment-3182304585

The wtf8 module is hidden, but the linkchecker shows that the implementations of traits for its types are not hidden, leading to dead links.

Reproduction Steps

1. In non-hidden module, define non-hidden trait
2. In hidden module, define a type and implement the non-hidden trait
3. When generating documentation, the implementation of the hidden type will show up on the non-hidden trait, with a dead link

Expected Outcome

Types in hidden modules should be hidden in all cases, including trait implementations.

Workaround

Adding #[doc(hidden)] directly to the types inside the hidden module correctly hides the implementations under the trait. However, this should not be necessary, since the hidden module does not generate documentation for its items.

Version

rustdoc 1.90.0-nightly (430d6eddf 2025-07-25)

Start by reproducing the reported case with rustdoc or cargo doc: a non-hidden trait implemented for a type in a #[doc(hidden)] module. Inspect the generated trait documentation and linkchecker results. Done means the hidden type's trait implementation no longer appears publicly or produces a dead link, without requiring #[doc(hidden)] on the type itself.