Fix repr for enum

Summary:
[repr(x) formats its argument as a string](https://github.com/bazelbuild/starlark/blob/master/spec.md#repr). However, for enum variants, it showed the internal value only but it should also show the surrounding enum's name.

I think under the hood, [str()](https://github.com/bazelbuild/starlark/blob/master/spec.md#str) which formats its argument as a string also calls `collect_repr` somehow so its behavior is affected (hence tests are replaced). This makes sense to me since they behave similarly but let me know if that is not the expected behavior.

Reviewed By: stepancheg, wendy728

Differential Revision: D50360889

fbshipit-source-id: 449add8f256fce69afbca1ea25cf66c51f5afa86

starlark/src/tests/bc/golden/speculative_exec_enum_inline.golden

@@ -14,6 +14,6 @@ def test(x):
 
 Max stack size: 1
 Instructions:
-  0: EqConst &x "red" &1
+  0: EqConst &x MyEnum("red") &1
   24: Return &1
   32: End

starlark/src/values/types/enumeration/enum_type.rs

@@ -410,7 +410,7 @@ def g(x):
 
 g(Season[0])
 "#,
-            r#"Value `"SPRING"` of type `enum` does not match the type annotation `Color` for argument `x`"#,
+            r#"Value `Season("SPRING")` of type `enum` does not match the type annotation `Color` for argument `x`"#,
         );
     }
 

starlark/src/values/types/enumeration/globals.rs

@@ -122,7 +122,7 @@ assert_eq([x.value for x in enum_type], ["option1","option2"])"#,
 enum_type = enum("option1","option2")
 x = enum_type("option1")
 assert_eq(str(enum_type), "enum(\"option1\", \"option2\")")
-assert_eq(str(x), "\"option1\"")
+assert_eq(str(x), "enum_type(\"option1\")")
 "#,
         );
         assert::pass(
@@ -191,7 +191,8 @@ assert_ne(rt("one"), diff("one"))
         assert::pass(
             r#"
 enum_type = enum("option1", "option2")
-assert_eq("\"option1\"", repr(enum_type("option1")))
+assert_eq("enum_type(\"option1\")", repr(enum_type("option1")))
+assert_eq("enum()(\"option1\")", repr(enum("option1", "option2")("option1")))
 "#,
         );
     }

starlark/src/values/types/enumeration/value.rs

@@ -66,9 +66,31 @@ pub struct EnumValueGen<V> {
     pub(crate) id: TypeInstanceId,
 }
 
-impl<V: Display> Display for EnumValueGen<V> {
+impl<'v, V: ValueLike<'v> + 'v> Display for EnumValueGen<V> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.value.fmt(f)
+        let ty_enum_data = match self.get_enum_type() {
+            Either::Left(x) => x.ty_enum_data(),
+            Either::Right(x) => x.ty_enum_data(),
+        };
+        match ty_enum_data {
+            Some(ty_enum_data) => {
+                {
+                    write!(f, "{}", &ty_enum_data.name)?;
+                    write!(f, "(")?;
+                    Display::fmt(&self.value, f)?;
+                    write!(f, ")")?
+                };
+                Ok(())
+            }
+            None => {
+                {
+                    write!(f, "enum()(")?;
+                    Display::fmt(&self.value, f)?;
+                    write!(f, ")")?
+                };
+                Ok(())
+            }
+        }
     }
 }