rustdoc: list matching impls on type aliases

Remake of "List matching impls on type aliases"
* 4b1d13d
* 6f552c8
* 2ce7cd9

Partially reverts "Fix infinite loop when retrieving impls for
type alias", but keeps the test case.

This version of the PR avoids the infinite loop by structurally
matching types instead of using full unification. This version
does not support type alias trait bounds, but the compiler does
not enforce those anyway
(rust-lang#21903).

Author: notriddle

src/librustdoc/html/render/mod.rs

@@ -54,6 +54,7 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir::def_id::{DefId, DefIdSet};
 use rustc_hir::Mutability;
 use rustc_middle::middle::stability;
+use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_middle::ty::TyCtxt;
 use rustc_span::{
     symbol::{sym, Symbol},
@@ -62,6 +63,7 @@ use rustc_span::{
 use serde::ser::{SerializeMap, SerializeSeq};
 use serde::{Serialize, Serializer};
 
+use crate::clean::types::TypeAliasItem;
 use crate::clean::{self, ItemId, RenderedLink, SelfTy};
 use crate::error::Error;
 use crate::formats::cache::Cache;
@@ -1139,8 +1141,40 @@ fn render_assoc_items_inner(
     info!("Documenting associated items of {:?}", containing_item.name);
     let shared = Rc::clone(&cx.shared);
     let cache = &shared.cache;
-    let Some(v) = cache.impls.get(&it) else { return };
-    let (non_trait, traits): (Vec<_>, _) = v.iter().partition(|i| i.inner_impl().trait_.is_none());
+    let tcx = cx.tcx();
+    let av = if let TypeAliasItem(ait) = &*containing_item.kind &&
+        let aliased_clean_type = ait.item_type.as_ref().unwrap_or(&ait.type_) &&
+        let Some(aliased_type_defid) = aliased_clean_type.def_id(cache) &&
+        let Some(mut av) = cache.impls.get(&aliased_type_defid).cloned() &&
+        let Some(alias_def_id) = containing_item.item_id.as_def_id()
+    {
+        // This branch of the compiler compares types structually, but does
+        // not check trait bounds. That's probably fine, since type aliases
+        // don't normally constrain on them anyway.
+        // https://github.com/rust-lang/rust/issues/21903
+        //
+        // If that changes, then this will need to check them with type
+        // unification.
+        let aliased_ty = tcx.type_of(alias_def_id).skip_binder();
+        let reject_cx = DeepRejectCtxt {
+            treat_obligation_params: TreatParams::AsCandidateKey,
+        };
+        av.retain(|impl_| {
+            if let Some(impl_def_id) = impl_.impl_item.item_id.as_def_id() {
+                reject_cx.types_may_unify(aliased_ty, tcx.type_of(impl_def_id).skip_binder())
+            } else {
+                false
+            }
+        });
+        av
+    } else {
+        Vec::new()
+    };
+    let blank = Vec::new();
+    let v = cache.impls.get(&it).unwrap_or(&blank);
+    let (non_trait, traits): (Vec<_>, _) =
+        v.iter().chain(&av[..]).partition(|i| i.inner_impl().trait_.is_none());
+    let mut saw_impls = FxHashSet::default();
     if !non_trait.is_empty() {
         let mut tmp_buf = Buffer::html();
         let (render_mode, id, class_html) = match what {
@@ -1169,6 +1203,9 @@ fn render_assoc_items_inner(
         };
         let mut impls_buf = Buffer::html();
         for i in &non_trait {
+            if !saw_impls.insert(i.def_id()) {
+                continue;
+            }
             render_impl(
                 &mut impls_buf,
                 cx,
@@ -1214,8 +1251,10 @@ fn render_assoc_items_inner(
 
         let (synthetic, concrete): (Vec<&Impl>, Vec<&Impl>) =
             traits.into_iter().partition(|t| t.inner_impl().kind.is_auto());
-        let (blanket_impl, concrete): (Vec<&Impl>, _) =
-            concrete.into_iter().partition(|t| t.inner_impl().kind.is_blanket());
+        let (blanket_impl, concrete): (Vec<&Impl>, _) = concrete
+            .into_iter()
+            .filter(|t| saw_impls.insert(t.def_id()))
+            .partition(|t| t.inner_impl().kind.is_blanket());
 
         render_all_impls(w, cx, containing_item, &concrete, &synthetic, &blanket_impl);
     }

tests/rustdoc/issue-32077-type-alias-impls.rs

@@ -0,0 +1,59 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/32077>.
+
+#![crate_name = "foo"]
+
+pub struct GenericStruct<T>(T);
+
+impl<T> GenericStruct<T> {
+    pub fn on_gen(arg: T) {}
+}
+
+impl GenericStruct<u32> {
+    pub fn on_u32(arg: u32) {}
+}
+
+pub trait Foo {}
+pub trait Bar {}
+
+impl<T> Foo for GenericStruct<T> {}
+impl Bar for GenericStruct<u32> {}
+
+// @has 'foo/type.TypedefStruct.html'
+// We check that we have the implementation of the type alias itself.
+// @has - '//*[@id="impl-TypedefStruct"]/h3' 'impl TypedefStruct'
+// @has - '//*[@id="method.on_alias"]/h4' 'pub fn on_alias()'
+// @has - '//*[@id="impl-GenericStruct%3CT%3E"]/h3' 'impl<T> GenericStruct<T>'
+// @has - '//*[@id="method.on_gen"]/h4' 'pub fn on_gen(arg: T)'
+// @has - '//*[@id="impl-Foo-for-GenericStruct%3CT%3E"]/h3' 'impl<T> Foo for GenericStruct<T>'
+// This trait implementation doesn't match the type alias parameters so shouldn't appear in docs.
+// @!has - '//h3' 'impl Bar for GenericStruct<u32> {}'
+// Same goes for the `Deref` impl.
+// @!has - '//h2' 'Methods from Deref<Target = u32>'
+pub type TypedefStruct = GenericStruct<u8>;
+
+impl TypedefStruct {
+    pub fn on_alias() {}
+}
+
+impl std::ops::Deref for GenericStruct<u32> {
+    type Target = u32;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+pub struct Wrap<T>(GenericStruct<T>);
+
+// @has 'foo/type.Alias.html'
+// @has - '//h2' 'Methods from Deref<Target = u32>'
+// @has - '//*[@id="impl-Deref-for-Wrap%3CT%3E"]/h3' 'impl<T> Deref for Wrap<T>'
+pub type Alias = Wrap<u32>;
+
+impl<T> std::ops::Deref for Wrap<T> {
+    type Target = GenericStruct<T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}