fix variant resolve for type alias

Author: Austaras

crates/hir-ty/src/infer.rs

@@ -1152,20 +1152,14 @@ impl<'a> InferenceContext<'a> {
                 (ty, variant)
             }
             TypeNs::TypeAliasId(it) => {
-                let container = it.lookup(self.db.upcast()).container;
-                let parent_subst = match container {
-                    ItemContainerId::TraitId(id) => {
-                        let subst = TyBuilder::subst_for_def(self.db, id, None)
-                            .fill_with_inference_vars(&mut self.table)
-                            .build();
-                        Some(subst)
-                    }
-                    // Type aliases do not exist in impls.
-                    _ => None,
+                let resolved_seg = match unresolved {
+                    None => path.segments().last().unwrap(),
+                    Some(n) => path.segments().get(path.segments().len() - n - 1).unwrap(),
                 };
-                let ty = TyBuilder::def_ty(self.db, it.into(), parent_subst)
-                    .fill_with_inference_vars(&mut self.table)
-                    .build();
+                let substs = ctx.substs_from_path_segment(resolved_seg, Some(it.into()), true, None);
+                let ty = self.db.ty(it.into());
+                let ty = self.insert_type_vars(ty.substitute(Interner, &substs));
+
                 self.resolve_variant_on_alias(ty, unresolved, mod_path)
             }
             TypeNs::AdtSelfType(_) => {

crates/hir-ty/src/lower.rs

@@ -768,7 +768,7 @@ impl<'a> TyLoweringContext<'a> {
         }
     }
 
-    fn substs_from_path_segment(
+    pub(super) fn substs_from_path_segment(
         &self,
         segment: PathSegment<'_>,
         def: Option<GenericDefId>,

crates/hir-ty/src/tests/patterns.rs

@@ -1129,3 +1129,27 @@ fn foo() {
 "#,
     );
 }
+
+#[test]
+fn generic_alias() {
+    check_types(
+        r#"
+type Wrap<T> = T;
+
+enum X {
+    A { cool: u32, stuff: u32 },
+    B,
+}
+
+fn main() {
+    let wrapped = Wrap::<X>::A {
+        cool: 100,
+        stuff: 100,
+    };
+
+    if let Wrap::<X>::A { cool, ..} = &wrapped {}
+                        //^^^^ &u32
+}
+"#,
+    );
+}

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

@@ -354,6 +354,36 @@ fn outer(Foo { bar$0 }: Foo) {}
     )
 }
 
+
+#[test]
+fn completes_in_record_field_pat_with_generic_type_alias() {
+    check_empty(
+        r#"
+type Wrap<T> = T;
+
+enum X {
+    A { cool: u32, stuff: u32 },
+    B,
+}
+
+fn main() {
+    let wrapped = Wrap::<X>::A {
+        cool: 100,
+        stuff: 100,
+    };
+
+    if let Wrap::<X>::A { $0 } = &wrapped {};
+}
+"#,
+        expect![[r#"
+            fd cool  u32
+            fd stuff u32
+            kw mut
+            kw ref
+        "#]],
+    )
+}
+
 #[test]
 fn completes_in_fn_param() {
     check_empty(