Auto merge of #14872 - lowr:fix/ref-pat-with-type-var, r=HKalbasi

fix: introduce new type var when expectation for ref pat is not ref

Fixes #14840

When we infer the type of ref patterns, its expected type may not be reference type: 1) expected type is an unresolved inference variable, or 2) expected type is erroneously other kind of type. In either case, we should produce a reference type with a new type variable rather than an error type so that we can continue inferring the inner patterns without further errors because of the (possible) type mismatch of this pattern.

crates/hir-ty/src/infer/pat.rs

@@ -313,16 +313,23 @@ impl<'a> InferenceContext<'a> {
 
     fn infer_ref_pat(
         &mut self,
-        pat: PatId,
+        inner_pat: PatId,
         mutability: Mutability,
         expected: &Ty,
         default_bm: BindingMode,
     ) -> Ty {
         let expectation = match expected.as_reference() {
             Some((inner_ty, _lifetime, _exp_mut)) => inner_ty.clone(),
-            _ => self.result.standard_types.unknown.clone(),
+            None => {
+                let inner_ty = self.table.new_type_var();
+                let ref_ty =
+                    TyKind::Ref(mutability, static_lifetime(), inner_ty.clone()).intern(Interner);
+                // Unification failure will be reported by the caller.
+                self.unify(&ref_ty, expected);
+                inner_ty
+            }
         };
-        let subty = self.infer_pat(pat, &expectation, default_bm);
+        let subty = self.infer_pat(inner_pat, &expectation, default_bm);
         TyKind::Ref(mutability, static_lifetime(), subty).intern(Interner)
     }
 

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

@@ -1,6 +1,6 @@
 use expect_test::expect;
 
-use super::{check, check_infer, check_infer_with_mismatches, check_types};
+use super::{check, check_infer, check_infer_with_mismatches, check_no_mismatches, check_types};
 
 #[test]
 fn infer_pattern() {
@@ -240,6 +240,21 @@ fn infer_pattern_match_ergonomics_ref() {
     );
 }
 
+#[test]
+fn ref_pat_with_inference_variable() {
+    check_no_mismatches(
+        r#"
+enum E { A }
+fn test() {
+    let f = |e| match e {
+        &E::A => {}
+    };
+    f(&E::A);
+}
+"#,
+    );
+}
+
 #[test]
 fn infer_pattern_match_slice() {
     check_infer(