rustdoc: add support for incoherent impls on structs and traits

Fixes #103170

src/librustdoc/clean/inline.rs

@@ -323,6 +323,21 @@ pub(crate) fn build_impls(
     for &did in tcx.inherent_impls(did).iter() {
         build_impl(cx, parent_module, did, attrs, ret);
     }
+
+    // This pretty much exists expressly for `dyn Error` traits that exist in the `alloc` crate.
+    // See also:
+    //
+    // * https://github.com/rust-lang/rust/issues/103170 — where it didn't used to get documented
+    // * https://github.com/rust-lang/rust/pull/99917 — where the feature got used
+    // * https://github.com/rust-lang/rust/issues/53487 — overall tracking issue for Error
+    if tcx.has_attr(did, sym::rustc_has_incoherent_inherent_impls) {
+        use rustc_middle::ty::fast_reject::SimplifiedTypeGen::*;
+        let type_ =
+            if tcx.is_trait(did) { TraitSimplifiedType(did) } else { AdtSimplifiedType(did) };
+        for &did in tcx.incoherent_impls(type_) {
+            build_impl(cx, parent_module, did, attrs, ret);
+        }
+    }
 }
 
 /// `parent_module` refers to the parent of the re-export, not the original item

src/test/rustdoc/auxiliary/incoherent-impl-types.rs

@@ -0,0 +1,7 @@
+#![feature(rustc_attrs)]
+
+#[rustc_has_incoherent_inherent_impls]
+pub trait FooTrait {}
+
+#[rustc_has_incoherent_inherent_impls]
+pub struct FooStruct;

src/test/rustdoc/rustc-incoherent-impls.rs

@@ -0,0 +1,28 @@
+// aux-build:incoherent-impl-types.rs
+// build-aux-docs
+
+#![crate_name = "foo"]
+#![feature(rustc_attrs)]
+
+extern crate incoherent_impl_types;
+
+// The only way this actually shows up is if the type gets inlined.
+#[doc(inline)]
+pub use incoherent_impl_types::FooTrait;
+
+// @has foo/trait.FooTrait.html
+// @count - '//section[@id="method.do_something"]' 1
+impl dyn FooTrait {
+    #[rustc_allow_incoherent_impl]
+    pub fn do_something() {}
+}
+
+#[doc(inline)]
+pub use incoherent_impl_types::FooStruct;
+
+// @has foo/struct.FooStruct.html
+// @count - '//section[@id="method.do_something"]' 1
+impl FooStruct {
+    #[rustc_allow_incoherent_impl]
+    pub fn do_something() {}
+}