document test changes

src/test/ui/type-sizes.rs

@@ -103,11 +103,19 @@ enum Option2<A, B> {
     None
 }
 
+// Two layouts are considered for `CanBeNicheFilledButShouldnt`:
+//   Niche-filling:
+//     { u32 (4 bytes), NonZeroU8 + tag in niche (1 byte), padding (3 bytes) }
+//   Tagged:
+//     { tag (1 byte), NonZeroU8 (1 byte), padding (2 bytes), u32 (4 bytes) }
+// Both are the same size (due to padding),
+// but the tagged layout is better as the tag creates a niche with 254 invalid values,
+// allowing types like `Option<Option<CanBeNicheFilledButShouldnt>>` to fit into 8 bytes.
 pub enum CanBeNicheFilledButShouldnt {
     A(NonZeroU8, u32),
     B
 }
-pub enum AlwaysTagged {
+pub enum AlwaysTaggedBecauseItHasNoNiche {
     A(u8, u32),
     B
 }
@@ -159,7 +167,7 @@ pub fn main() {
     assert_eq!(size_of::<CanBeNicheFilledButShouldnt>(), 8);
     assert_eq!(size_of::<Option<CanBeNicheFilledButShouldnt>>(), 8);
     assert_eq!(size_of::<Option<Option<CanBeNicheFilledButShouldnt>>>(), 8);
-    assert_eq!(size_of::<AlwaysTagged>(), 8);
-    assert_eq!(size_of::<Option<AlwaysTagged>>(), 8);
-    assert_eq!(size_of::<Option<Option<AlwaysTagged>>>(), 8);
+    assert_eq!(size_of::<AlwaysTaggedBecauseItHasNoNiche>(), 8);
+    assert_eq!(size_of::<Option<AlwaysTaggedBecauseItHasNoNiche>>(), 8);
+    assert_eq!(size_of::<Option<Option<AlwaysTaggedBecauseItHasNoNiche>>>(), 8);
 }