Do not list impl when trait has doc(hidden)

src/librustdoc/clean/inline.rs

@@ -1,5 +1,6 @@
 //! Support for inlining external documentation into the current AST.
 
+use std::collections::VecDeque;
 use std::iter::once;
 use std::sync::Arc;
 
@@ -15,7 +16,9 @@ use rustc_span::hygiene::MacroKind;
 use rustc_span::symbol::{kw, sym, Symbol};
 use rustc_span::Span;
 
-use crate::clean::{self, Attributes, AttributesExt, FakeDefId, GetDefId, ToSource};
+use crate::clean::{
+    self, Attributes, AttributesExt, FakeDefId, GetDefId, NestedAttributesExt, ToSource, Type,
+};
 use crate::core::DocContext;
 use crate::formats::item_type::ItemType;
 
@@ -420,6 +423,20 @@ crate fn build_impl(
     if trait_.def_id() == tcx.lang_items().deref_trait() {
         super::build_deref_target_impls(cx, &trait_items, ret);
     }
+
+    // Return if the trait itself or any types of the generic parameters are doc(hidden).
+    let mut deque: VecDeque<&Type> = trait_.iter().collect();
+    while let Some(ty) = deque.pop_back() {
+        if let Some(did) = ty.def_id() {
+            if cx.tcx.get_attrs(did).lists(sym::doc).has_word(sym::hidden) {
+                return;
+            }
+        }
+        if let Some(generics) = ty.generics() {
+            deque.extend(generics);
+        }
+    }
+
     if let Some(trait_did) = trait_.def_id() {
         record_extern_trait(cx, trait_did);
     }

src/test/rustdoc/auxiliary/cross-crate-hidden.rs

@@ -0,0 +1,2 @@
+#[doc(hidden)]
+pub enum HiddenType {}

src/test/rustdoc/cross-crate-hidden.rs

@@ -0,0 +1,23 @@
+// Issue #86448: test for cross-crate `doc(hidden)`
+#![crate_name = "foo"]
+
+// aux-build:cross-crate-hidden.rs
+extern crate cross_crate_hidden;
+
+pub use ::cross_crate_hidden::HiddenType; // OK, not re-exported
+
+pub enum MyLibType {}
+
+// @!has foo/enum.MyLibType.html '//*[@id="impl-From%3CHiddenType%3E"]' 'impl From<HiddenType> for MyLibType'
+impl From<HiddenType> for MyLibType {
+    fn from(it: HiddenType) -> MyLibType {
+        match it {}
+    }
+}
+
+// @!has foo/enum.MyLibType.html '//*[@id="impl-From%3COption%3COption%3COption%3COption%3CHiddenType%3E%3E%3E%3E%3E"]' 'impl From<Option<Option<Option<Option<HiddenType>>>>> for MyLibType'
+impl From<Option<Option<Option<Option<HiddenType>>>>> for MyLibType {
+    fn from(it: Option<Option<Option<Option<HiddenType>>>>) -> MyLibType {
+        todo!()
+    }
+}