don't inhibit random field reordering on repr(packed(1))

Author: RalfJung

Diff for: compiler/rustc_abi/src/layout.rs

@@ -970,7 +970,7 @@ fn univariant<
     let mut align = if pack.is_some() { dl.i8_align } else { dl.aggregate_align };
     let mut max_repr_align = repr.align;
     let mut inverse_memory_index: IndexVec<u32, FieldIdx> = fields.indices().collect();
-    let optimize = !repr.inhibit_struct_field_reordering_opt();
+    let optimize = !repr.inhibit_struct_field_reordering();
     if optimize && fields.len() > 1 {
         let end = if let StructKind::MaybeUnsized = kind { fields.len() - 1 } else { fields.len() };
         let optimizing = &mut inverse_memory_index.raw[..end];

Diff for: compiler/rustc_abi/src/lib.rs

@@ -137,23 +137,16 @@ impl ReprOptions {
         self.c() || self.int.is_some()
     }
 
-    /// Returns `true` if this `#[repr()]` should inhibit struct field reordering
-    /// optimizations, such as with `repr(C)`, `repr(packed(1))`, or `repr(<int>)`.
-    pub fn inhibit_struct_field_reordering_opt(&self) -> bool {
-        if let Some(pack) = self.pack {
-            if pack.bytes() == 1 {
-                return true;
-            }
-        }
-
+    /// Returns `true` if this `#[repr()]` guarantees a fixed field order,
+    /// e.g. `repr(C)` or `repr(<int>)`.
+    pub fn inhibit_struct_field_reordering(&self) -> bool {
         self.flags.intersects(ReprFlags::IS_UNOPTIMISABLE) || self.int.is_some()
     }
 
     /// Returns `true` if this type is valid for reordering and `-Z randomize-layout`
     /// was enabled for its declaration crate.
     pub fn can_randomize_type_layout(&self) -> bool {
-        !self.inhibit_struct_field_reordering_opt()
-            && self.flags.contains(ReprFlags::RANDOMIZE_LAYOUT)
+        !self.inhibit_struct_field_reordering() && self.flags.contains(ReprFlags::RANDOMIZE_LAYOUT)
     }
 
     /// Returns `true` if this `#[repr()]` should inhibit union ABI optimisations.

Diff for: src/tools/clippy/clippy_lints/src/transmute/transmute_undefined_repr.rs

@@ -278,7 +278,7 @@ fn reduce_ty<'tcx>(cx: &LateContext<'tcx>, mut ty: Ty<'tcx>) -> ReducedTy<'tcx>
                     ty = sized_ty;
                     continue;
                 }
-                if def.repr().inhibit_struct_field_reordering_opt() {
+                if def.repr().inhibit_struct_field_reordering() {
                     ReducedTy::OrderedFields(Some(sized_ty))
                 } else {
                     ReducedTy::UnorderedFields(ty)

Diff for: tests/mir-opt/const_allocation3.rs

@@ -7,7 +7,7 @@ fn main() {
     FOO;
 }
 
-#[repr(packed)]
+#[repr(C, packed)]
 struct Packed {
     a: [u8; 28],
     b: &'static i32,