preparations for unifying optional fields

src/types.rs

@@ -47,6 +47,15 @@ pub enum RecordFieldLabel {
     Optional(Lowercase),
 }
 
+impl Into<Lowercase> for RecordFieldLabel {
+    fn into(self) -> Lowercase {
+        match self {
+            RecordFieldLabel::Required(label) => label,
+            RecordFieldLabel::Optional(label) => label,
+        }
+    }
+}
+
 impl fmt::Debug for RecordFieldLabel {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self {

src/unify.rs

@@ -173,6 +173,7 @@ fn unify_record(
     rec1: RecordStructure,
     rec2: RecordStructure,
 ) {
+    // This is a bit more complicated because of optional fields
     let fields1 = rec1.fields;
     let fields2 = rec2.fields;
     let shared_fields = fields1

tests/test_infer.rs

@@ -980,23 +980,9 @@ mod test_infer {
         );
     }
 
-    #[test]
-    fn optional_field() {
-        infer_eq(
-            indoc!(
-                r#"
-                foo : { x? : Int } -> Int 
-                foo = \_ -> 42
-
-                foo {}
-            "#
-            ),
-            "Int",
-        );
-    }
-
     #[test]
     fn record_type_annotation() {
+        // check that a closed record remains closed
         infer_eq(
             indoc!(
                 r#"
@@ -1009,4 +995,19 @@ mod test_infer {
             "{ x : custom } -> custom",
         );
     }
+
+    #[test]
+    fn optional_field() {
+        infer_eq(
+            indoc!(
+                r#"
+                foo : { x? : Int } -> Int 
+                foo = \_ -> 42
+
+                foo
+            "#
+            ),
+            "{ x? : Int } -> Int",
+        );
+    }
 }