rustdoc: Improve calculation of "Impls on Foreign Types"

src/librustdoc/formats/mod.rs

@@ -7,6 +7,7 @@ use rustc_hir::def_id::DefId;
 crate use renderer::{run_format, FormatRenderer};
 
 use crate::clean::{self, ItemId};
+use cache::Cache;
 
 /// Specifies whether rendering directly implemented trait items or ones from a certain Deref
 /// impl.
@@ -60,4 +61,28 @@ impl Impl {
             }
         }
     }
+
+    // Returns true if this is an implementation on a "local" type, meaning:
+    // the type is in the current crate, or the type and the trait are both
+    // re-exported by the current crate.
+    pub(crate) fn is_on_local_type(&self, cache: &Cache) -> bool {
+        let for_type = &self.inner_impl().for_;
+        if let Some(for_type_did) = for_type.def_id(cache) {
+            // The "for" type is local if it's in the paths for the current crate.
+            if cache.paths.contains_key(&for_type_did) {
+                return true;
+            }
+            if let Some(trait_did) = self.trait_did() {
+                // The "for" type and the trait are from the same crate. That could
+                // be different from the current crate, for instance when both were
+                // re-exported from some other crate. But they are local with respect to
+                // each other.
+                if for_type_did.krate == trait_did.krate {
+                    return true;
+                }
+            }
+            return false;
+        };
+        true
+    }
 }

src/librustdoc/html/render/mod.rs

@@ -2287,9 +2287,7 @@ fn sidebar_trait(cx: &Context<'_>, buf: &mut Buffer, it: &clean::Item, t: &clean
     if let Some(implementors) = cache.implementors.get(&it.item_id.expect_def_id()) {
         let mut res = implementors
             .iter()
-            .filter(|i| {
-                i.inner_impl().for_.def_id(cache).map_or(false, |d| !cache.paths.contains_key(&d))
-            })
+            .filter(|i| !i.is_on_local_type(cache))
             .filter_map(|i| extract_for_impl_name(&i.impl_item, cx))
             .collect::<Vec<_>>();
 

src/librustdoc/html/render/print_item.rs

@@ -818,9 +818,8 @@ fn item_trait(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clean::Tra
             }
         }
 
-        let (local, foreign) = implementors.iter().partition::<Vec<_>, _>(|i| {
-            i.inner_impl().for_.def_id(cache).map_or(true, |d| cache.paths.contains_key(&d))
-        });
+        let (local, foreign) =
+            implementors.iter().partition::<Vec<_>, _>(|i| i.is_on_local_type(cache));
 
         let (mut synthetic, mut concrete): (Vec<&&Impl>, Vec<&&Impl>) =
             local.iter().partition(|i| i.inner_impl().kind.is_auto());

src/test/rustdoc/issue-75588.rs

@@ -13,5 +13,5 @@ extern crate real_gimli;
 // @!has foo/trait.Deref.html '//*[@id="impl-Deref-for-EndianSlice"]//h3[@class="code-header in-band"]' 'impl Deref for EndianSlice'
 pub use realcore::Deref;
 
-// @has foo/trait.Join.html '//*[@id="impl-Join-for-Foo"]//h3[@class="code-header in-band"]' 'impl Join for Foo'
+// @has foo/trait.Join.html '//*[@id="impl-Join"]//h3[@class="code-header in-band"]' 'impl Join for Foo'
 pub use realcore::Join;