Fix rustdoc duplicated blanket impls

src/librustdoc/formats/cache.rs

@@ -127,6 +127,8 @@ crate struct Cache {
 /// This struct is used to wrap the `cache` and `tcx` in order to run `DocFolder`.
 struct CacheBuilder<'a, 'tcx> {
     cache: &'a mut Cache,
+    /// This field is used to prevent duplicated impl blocks.
+    impl_ids: FxHashMap<DefId, FxHashSet<DefId>>,
     tcx: TyCtxt<'tcx>,
 }
 
@@ -170,12 +172,19 @@ impl Cache {
                 .insert(def_id, (vec![crate_name, prim.as_sym()], ItemType::Primitive));
         }
 
-        krate = CacheBuilder { tcx, cache: &mut cx.cache }.fold_crate(krate);
+        let (krate, mut impl_ids) = {
+            let mut cache_builder =
+                CacheBuilder { tcx, cache: &mut cx.cache, impl_ids: FxHashMap::default() };
+            krate = cache_builder.fold_crate(krate);
+            (krate, cache_builder.impl_ids)
+        };
 
         for (trait_did, dids, impl_) in cx.cache.orphan_trait_impls.drain(..) {
             if cx.cache.traits.contains_key(&trait_did) {
                 for did in dids {
-                    cx.cache.impls.entry(did).or_default().push(impl_.clone());
+                    if impl_ids.entry(did).or_default().insert(impl_.def_id()) {
+                        cx.cache.impls.entry(did).or_default().push(impl_.clone());
+                    }
                 }
             }
         }
@@ -467,7 +476,13 @@ impl<'a, 'tcx> DocFolder for CacheBuilder<'a, 'tcx> {
             let impl_item = Impl { impl_item: item };
             if impl_item.trait_did().map_or(true, |d| self.cache.traits.contains_key(&d)) {
                 for did in dids {
-                    self.cache.impls.entry(did).or_insert_with(Vec::new).push(impl_item.clone());
+                    if self.impl_ids.entry(did).or_default().insert(impl_item.def_id()) {
+                        self.cache
+                            .impls
+                            .entry(did)
+                            .or_insert_with(Vec::new)
+                            .push(impl_item.clone());
+                    }
                 }
             } else {
                 let trait_did = impl_item.trait_did().expect("no trait did");

src/librustdoc/formats/mod.rs

@@ -6,7 +6,7 @@ use rustc_hir::def_id::DefId;
 
 crate use renderer::{run_format, FormatRenderer};
 
-use crate::clean;
+use crate::clean::{self, ItemId};
 
 /// Specifies whether rendering directly implemented trait items or ones from a certain Deref
 /// impl.
@@ -40,4 +40,24 @@ impl Impl {
     crate fn trait_did(&self) -> Option<DefId> {
         self.inner_impl().trait_.as_ref().map(|t| t.def_id())
     }
+
+    /// This function is used to extract a `DefId` to be used as a key for the `Cache::impls` field.
+    ///
+    /// It allows to prevent having duplicated implementations showing up (the biggest issue was
+    /// with blanket impls).
+    ///
+    /// It panics if `self` is a `ItemId::Primitive`.
+    crate fn def_id(&self) -> DefId {
+        match self.impl_item.item_id {
+            ItemId::Blanket { impl_id, .. } => impl_id,
+            ItemId::Auto { trait_, .. } => trait_,
+            ItemId::DefId(def_id) => def_id,
+            ItemId::Primitive(_, _) => {
+                panic!(
+                    "Unexpected ItemId::Primitive in expect_def_id: {:?}",
+                    self.impl_item.item_id
+                )
+            }
+        }
+    }
 }

src/test/rustdoc/duplicated_impl.rs

@@ -0,0 +1,14 @@
+// This test ensures that the same implementation doesn't show more than once.
+// It's a regression test for https://github.com/rust-lang/rust/issues/96036.
+
+#![crate_name = "foo"]
+
+// We check that there is only one "impl<T> Something<Whatever> for T" listed in the
+// blanket implementations.
+
+// @has 'foo/struct.Whatever.html'
+// @count - '//*[@id="blanket-implementations-list"]/section[@class="impl has-srclink"]' 1
+
+pub trait Something<T> { }
+pub struct Whatever;
+impl<T> Something<Whatever> for T {}