fix: Calculate completions after type anchors

crates/hir/src/semantics.rs

@@ -324,6 +324,10 @@ impl<'db, DB: HirDatabase> Semantics<'db, DB> {
         self.imp.resolve_type(ty)
     }
 
+    pub fn resolve_trait(&self, trait_: &ast::Path) -> Option<Trait> {
+        self.imp.resolve_trait(trait_)
+    }
+
     // FIXME: Figure out a nice interface to inspect adjustments
     pub fn is_implicit_reborrow(&self, expr: &ast::Expr) -> Option<Mutability> {
         self.imp.is_implicit_reborrow(expr)
@@ -1014,6 +1018,20 @@ impl<'db> SemanticsImpl<'db> {
         Some(Type::new_with_resolver(self.db, &analyze.resolver, ty))
     }
 
+    fn resolve_trait(&self, path: &ast::Path) -> Option<Trait> {
+        let analyze = self.analyze(path.syntax())?;
+        let hygiene = hir_expand::hygiene::Hygiene::new(self.db.upcast(), analyze.file_id);
+        let ctx = body::LowerCtx::with_hygiene(self.db.upcast(), &hygiene);
+        let hir_path = Path::from_src(path.clone(), &ctx)?;
+        match analyze
+            .resolver
+            .resolve_path_in_type_ns_fully(self.db.upcast(), hir_path.mod_path())?
+        {
+            TypeNs::TraitId(id) => Some(Trait { id }),
+            _ => None,
+        }
+    }
+
     fn is_implicit_reborrow(&self, expr: &ast::Expr) -> Option<Mutability> {
         self.analyze(expr.syntax())?.is_implicit_reborrow(self.db, expr)
     }

crates/ide-completion/src/completions/attribute.rs

@@ -115,7 +115,7 @@ pub(crate) fn complete_attribute_path(
             });
             acc.add_nameref_keywords_with_colon(ctx);
         }
-        Qualified::Infer | Qualified::With { .. } => {}
+        Qualified::TypeAnchor { .. } | Qualified::With { .. } => {}
     }
 
     let attributes = annotated_item_kind.and_then(|kind| {

crates/ide-completion/src/completions/attribute/derive.rs

@@ -97,7 +97,7 @@ pub(crate) fn complete_derive_path(
             });
             acc.add_nameref_keywords_with_colon(ctx);
         }
-        Qualified::Infer | Qualified::With { .. } => {}
+        Qualified::TypeAnchor { .. } | Qualified::With { .. } => {}
     }
 }
 

crates/ide-completion/src/completions/expr.rs

@@ -46,11 +46,32 @@ pub(crate) fn complete_expr_path(
     };
 
     match qualified {
-        Qualified::Infer => ctx
+        Qualified::TypeAnchor { ty: None, trait_: None } => ctx
             .traits_in_scope()
             .iter()
             .flat_map(|&it| hir::Trait::from(it).items(ctx.sema.db))
             .for_each(|item| add_assoc_item(acc, item)),
+        Qualified::TypeAnchor { trait_: Some(trait_), .. } => {
+            trait_.items(ctx.sema.db).into_iter().for_each(|item| add_assoc_item(acc, item))
+        }
+        Qualified::TypeAnchor { ty: Some(ty), trait_: None } => {
+            if let Some(hir::Adt::Enum(e)) = ty.as_adt() {
+                cov_mark::hit!(completes_variant_through_alias);
+                acc.add_enum_variants(ctx, path_ctx, e);
+            }
+
+            ctx.iterate_path_candidates(&ty, |item| {
+                add_assoc_item(acc, item);
+            });
+
+            // Iterate assoc types separately
+            ty.iterate_assoc_items(ctx.db, ctx.krate, |item| {
+                if let hir::AssocItem::TypeAlias(ty) = item {
+                    acc.add_type_alias(ctx, ty)
+                }
+                None::<()>
+            });
+        }
         Qualified::With { resolution: None, .. } => {}
         Qualified::With { resolution: Some(resolution), .. } => {
             // Add associated types on type parameters and `Self`.

crates/ide-completion/src/completions/item_list.rs

@@ -66,7 +66,7 @@ pub(crate) fn complete_item_list(
             });
             acc.add_nameref_keywords_with_colon(ctx);
         }
-        Qualified::Infer | Qualified::No | Qualified::With { .. } => {}
+        Qualified::TypeAnchor { .. } | Qualified::No | Qualified::With { .. } => {}
     }
 }
 

crates/ide-completion/src/completions/pattern.rs

@@ -180,6 +180,6 @@ pub(crate) fn complete_pattern_path(
 
             acc.add_nameref_keywords_with_colon(ctx);
         }
-        Qualified::Infer | Qualified::With { .. } => {}
+        Qualified::TypeAnchor { .. } | Qualified::With { .. } => {}
     }
 }

crates/ide-completion/src/completions/type.rs

@@ -49,11 +49,27 @@ pub(crate) fn complete_type_path(
     };
 
     match qualified {
-        Qualified::Infer => ctx
+        Qualified::TypeAnchor { ty: None, trait_: None } => ctx
             .traits_in_scope()
             .iter()
             .flat_map(|&it| hir::Trait::from(it).items(ctx.sema.db))
             .for_each(|item| add_assoc_item(acc, item)),
+        Qualified::TypeAnchor { trait_: Some(trait_), .. } => {
+            trait_.items(ctx.sema.db).into_iter().for_each(|item| add_assoc_item(acc, item))
+        }
+        Qualified::TypeAnchor { ty: Some(ty), trait_: None } => {
+            ctx.iterate_path_candidates(&ty, |item| {
+                add_assoc_item(acc, item);
+            });
+
+            // Iterate assoc types separately
+            ty.iterate_assoc_items(ctx.db, ctx.krate, |item| {
+                if let hir::AssocItem::TypeAlias(ty) = item {
+                    acc.add_type_alias(ctx, ty)
+                }
+                None::<()>
+            });
+        }
         Qualified::With { resolution: None, .. } => {}
         Qualified::With { resolution: Some(resolution), .. } => {
             // Add associated types on type parameters and `Self`.

crates/ide-completion/src/completions/use_.rs

@@ -115,6 +115,6 @@ pub(crate) fn complete_use_path(
             });
             acc.add_nameref_keywords_with_colon(ctx);
         }
-        Qualified::Infer | Qualified::With { resolution: None, .. } => {}
+        Qualified::TypeAnchor { .. } | Qualified::With { resolution: None, .. } => {}
     }
 }

crates/ide-completion/src/completions/vis.rs

@@ -29,7 +29,7 @@ pub(crate) fn complete_vis_path(
 
             acc.add_super_keyword(ctx, *super_chain_len);
         }
-        Qualified::Absolute | Qualified::Infer | Qualified::With { .. } => {}
+        Qualified::Absolute | Qualified::TypeAnchor { .. } | Qualified::With { .. } => {}
         Qualified::No => {
             if !has_in_token {
                 cov_mark::hit!(kw_completion_in);

crates/ide-completion/src/context.rs

@@ -193,7 +193,10 @@ pub(super) enum Qualified {
         super_chain_len: Option<usize>,
     },
     /// <_>::
-    Infer,
+    TypeAnchor {
+        ty: Option<hir::Type>,
+        trait_: Option<hir::Trait>,
+    },
     /// Whether the path is an absolute path
     Absolute,
 }

crates/ide-completion/src/context/analysis.rs

@@ -920,49 +920,53 @@ impl<'a> CompletionContext<'a> {
         path_ctx.has_type_args = segment.generic_arg_list().is_some();
 
         // calculate the qualifier context
-        if let Some((path, use_tree_parent)) = path_or_use_tree_qualifier(&path) {
+        if let Some((qualifier, use_tree_parent)) = path_or_use_tree_qualifier(&path) {
             path_ctx.use_tree_parent = use_tree_parent;
             if !use_tree_parent && segment.coloncolon_token().is_some() {
                 path_ctx.qualified = Qualified::Absolute;
             } else {
-                let path = path
+                let qualifier = qualifier
                     .segment()
                     .and_then(|it| find_node_in_file(original_file, &it))
                     .map(|it| it.parent_path());
-                if let Some(path) = path {
-                    // `<_>::$0`
-                    let is_infer_qualifier = path.qualifier().is_none()
-                        && matches!(
-                            path.segment().and_then(|it| it.kind()),
-                            Some(ast::PathSegmentKind::Type {
-                                type_ref: Some(ast::Type::InferType(_)),
-                                trait_ref: None,
-                            })
-                        );
+                if let Some(qualifier) = qualifier {
+                    let type_anchor = match qualifier.segment().and_then(|it| it.kind()) {
+                        Some(ast::PathSegmentKind::Type {
+                            type_ref: Some(type_ref),
+                            trait_ref,
+                        }) if qualifier.qualifier().is_none() => Some((type_ref, trait_ref)),
+                        _ => None,
+                    };
 
-                    path_ctx.qualified = if is_infer_qualifier {
-                        Qualified::Infer
+                    path_ctx.qualified = if let Some((ty, trait_ref)) = type_anchor {
+                        let ty = match ty {
+                            ast::Type::InferType(_) => None,
+                            ty => sema.resolve_type(&ty),
+                        };
+                        let trait_ = trait_ref.and_then(|it| sema.resolve_trait(&it.path()?));
+                        Qualified::TypeAnchor { ty, trait_ }
                     } else {
-                        let res = sema.resolve_path(&path);
+                        let res = sema.resolve_path(&qualifier);
 
                         // For understanding how and why super_chain_len is calculated the way it
                         // is check the documentation at it's definition
                         let mut segment_count = 0;
-                        let super_count = iter::successors(Some(path.clone()), |p| p.qualifier())
-                            .take_while(|p| {
-                                p.segment()
-                                    .and_then(|s| {
-                                        segment_count += 1;
-                                        s.super_token()
-                                    })
-                                    .is_some()
-                            })
-                            .count();
+                        let super_count =
+                            iter::successors(Some(qualifier.clone()), |p| p.qualifier())
+                                .take_while(|p| {
+                                    p.segment()
+                                        .and_then(|s| {
+                                            segment_count += 1;
+                                            s.super_token()
+                                        })
+                                        .is_some()
+                                })
+                                .count();
 
                         let super_chain_len =
                             if segment_count > super_count { None } else { Some(super_count) };
 
-                        Qualified::With { path, resolution: res, super_chain_len }
+                        Qualified::With { path: qualifier, resolution: res, super_chain_len }
                     }
                 };
             }

crates/ide-completion/src/tests/special.rs

@@ -674,7 +674,60 @@ fn bar() -> Bar {
         expect![[r#"
                 fn foo() (as Foo) fn() -> Self
             "#]],
-    )
+    );
+}
+
+#[test]
+fn type_anchor_type() {
+    check(
+        r#"
+trait Foo {
+    fn foo() -> Self;
+}
+struct Bar;
+impl Bar {
+    fn bar() {}
+}
+impl Foo for Bar {
+    fn foo() -> {
+        Bar
+    }
+}
+fn bar() -> Bar {
+    <Bar>::$0
+}
+"#,
+        expect![[r#"
+            fn bar()          fn()
+            fn foo() (as Foo) fn() -> Self
+        "#]],
+    );
+}
+
+#[test]
+fn type_anchor_type_trait() {
+    check(
+        r#"
+trait Foo {
+    fn foo() -> Self;
+}
+struct Bar;
+impl Bar {
+    fn bar() {}
+}
+impl Foo for Bar {
+    fn foo() -> {
+        Bar
+    }
+}
+fn bar() -> Bar {
+    <Bar as Foo>::$0
+}
+"#,
+        expect![[r#"
+            fn foo() (as Foo) fn() -> Self
+        "#]],
+    );
 }
 
 #[test]