cross-crate `#[doc(hidden)]` is ignored when listing trait implementations

[danielhenrymantilla]

Context

https://users.rust-lang.org/t/hidden-trait-implementation-in-online-docs/61256/4

Repro

set -euo pipefail
cd $(mktemp -d)

cargo new -q --lib dependency
cat> dependency/src/lib.rs <<'EOF'
    #[doc(hidden)]
    pub enum HiddenType {}
    
    pub enum MyLibType {}
    impl From<HiddenType> for MyLibType {
        fn from (it: HiddenType)
          -> MyLibType
        {
            match it {}
        }
    }
EOF
(cd dependency
    set -x
    cargo doc -q --no-deps
    # No matches
    ! grep -o HiddenType target/doc/dependency/index.html
)

cargo init -q --lib --name mylib
cat> src/lib.rs <<'EOF'
    pub use ::dependency::HiddenType; // OK, not re-exported
    
    pub enum MyLibType {}
    impl From<HiddenType> for MyLibType {
        fn from (it: HiddenType)
          -> MyLibType
        {
            match it {}
        }
    }
EOF

cat>> Cargo.toml <<'EOF'
    dependency.path = 'dependency'
EOF

set -x
cargo doc -q --no-deps
# No matches
! grep -o HiddenType target/doc/mylib/index.html
# There are matches
grep -o HiddenType target/doc/mylib/enum.MyLibType.html

• Last line yields several occurrences of HiddenType in the generated .html file.

  Expected behavior

  No occurrences of HiddenType anywhere since it was #[doc(hidden)]

• Playground

@rustbot label C-bug T-rustdoc

cc @GuillaumeGomez

[jyn514]

I expect the fix is to check for doc(hidden) just after

rust/src/librustdoc/clean/inline.rs

Line 370 in e82b650

let for_ = match &impl_item {

.

[fee1-dead]

@rustbot claim

[fee1-dead]

cc @jyn514

I checked for doc(hidden) like this:

    if let Some(did) = for_.def_id() {
        if cx.tcx.get_attrs(did).lists(sym::doc).has_word(sym::hidden) {
            return
        }
    }

But it doesn't seem to work, as my test would fail. I also tried using the same way to check for trait_ but it also failed. I noticed that the issue involves From<HiddenType> so I might need to iterate over generic parameters, I cannot find any function that does that in TyCtxt, so can someone point me in the right direction? Thanks :)

[danielhenrymantilla]

You can look at how the strip-hidden-items pass works:

rust/src/librustdoc/passes/stripper.rs

Lines 142 to 143 in 0aa2844

	if let Some(generics) = imp.trait_.as_ref().and_then(|t| t.generics()) {
	for typaram in generics {

[jyn514]

@danielhenrymantilla am I right in understanding this only happens for cross-crate traits, and impls for hidden traits in the same crate are still hidden? In other words, #86513 just makes the cross-crate case consistent?

[danielhenrymantilla]

I believe so, yes: the strip-hidden-items pass was already taking care of such cases for local #[doc(hidden)] types. FWIW, the thing that motivated this discussion was noticing a From<Unique<T>> for ptr::NonNull<T> leaking on the docs for ::std, but not on the ones for ::core