Get rid of niche selection's dependence on fields's order

compiler/rustc_abi/src/layout.rs

@@ -527,15 +527,10 @@ struct TmpLayout<FieldIdx: Idx, VariantIdx: Idx> {
             let count =
                 (niche_variants.end().index() as u128 - niche_variants.start().index() as u128) + 1;
 
-            // Find the field with the largest niche
+            // Use the largest niche in the largest variant.
-            let (field_index, niche, (niche_start, niche_scalar)) = variants[largest_variant_index]
+            let niche = variant_layouts[largest_variant_index].largest_niche?;
-                .iter()
+            let (niche_start, niche_scalar) = niche.reserve(dl, count)?;
-                .enumerate()
+            let niche_offset = niche.offset;
-                .filter_map(|(j, field)| Some((j, field.largest_niche?)))
-                .max_by_key(|(_, niche)| niche.available(dl))
-                .and_then(|(j, niche)| Some((j, niche, niche.reserve(dl, count)?)))?;
-            let niche_offset =
-                niche.offset + variant_layouts[largest_variant_index].fields.offset(field_index);
             let niche_size = niche.value.size(dl);
             let size = variant_layouts[largest_variant_index].size.align_to(align.abi);
 

tests/ui/structs-enums/type-sizes.rs

@@ -209,6 +209,23 @@ struct ReorderEndNiche {
     b: MiddleNiche4,
 }
 
+// We want that the niche selection doesn't depend on order of the fields. See issue #125630.
+pub enum NicheFieldOrder1 {
+    A {
+        x: NonZero<u64>,
+        y: [NonZero<u64>; 2],
+    },
+    B([u64; 2]),
+}
+
+pub enum NicheFieldOrder2 {
+    A {
+        y: [NonZero<u64>; 2],
+        x: NonZero<u64>,
+    },
+    B([u64; 2]),
+}
+
 
 // standins for std types which we want to be laid out in a reasonable way
 struct RawVecDummy {
@@ -260,6 +277,9 @@ pub fn main() {
                size_of::<EnumWithMaybeUninhabitedVariant<()>>());
     assert_eq!(size_of::<NicheFilledEnumWithAbsentVariant>(), size_of::<&'static ()>());
 
+    assert_eq!(size_of::<NicheFieldOrder1>(), 24);
+    assert_eq!(size_of::<NicheFieldOrder2>(), 24);
+
     assert_eq!(size_of::<Option<Option<(bool, &())>>>(), size_of::<(bool, &())>());
     assert_eq!(size_of::<Option<Option<(&(), bool)>>>(), size_of::<(bool, &())>());
     assert_eq!(size_of::<Option<Option2<bool, &()>>>(), size_of::<(bool, &())>());