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

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];
@@ -1007,13 +1007,15 @@ fn univariant<
  // Calculates a sort key to group fields by their alignment or possibly some
  // size-derived pseudo-alignment.
  let alignment_group_key = |layout: &F| {
+ // The two branches here return values that cannot be meaningfully compared with
+ // each other. However, we know that consistently for all executions of
+ // `alignment_group_key`, one or the other branch will be taken, so this is okay.
  if let Some(pack) = pack {
  // Return the packed alignment in bytes.
  layout.align.abi.min(pack).bytes()
  } else {
- // Returns `log2(effective-align)`. This is ok since `pack` applies to all
- // fields equally. The calculation assumes that size is an integer multiple of
- // align, except for ZSTs.
+ // Returns `log2(effective-align)`. The calculation assumes that size is an
+ // integer multiple of align, except for ZSTs.
  let align = layout.align.abi.bytes();
  let size = layout.size.bytes();
  let niche_size = layout.largest_niche.map(|n| n.available(dl)).unwrap_or(0);

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.

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)

src/tools/miri/tests/fail/reading_half_a_pointer.rs

@@ -1,7 +1,7 @@
 #![allow(dead_code)]
 
 // We use packed structs to get around alignment restrictions
-#[repr(packed)]
+#[repr(C, packed)]
 struct Data {
  pad: u8,
  ptr: &'static i32,

src/tools/miri/tests/fail/unaligned_pointers/field_requires_parent_struct_alignment2.rs

@@ -7,7 +7,7 @@ pub struct Aligned {
  _pad: [u8; 11],
  packed: Packed,
 }
-#[repr(packed)]
+#[repr(C, packed)]
 #[derive(Default, Copy, Clone)]
 pub struct Packed {
  _pad: [u8; 5],

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,