Auto merge of #125360 - RalfJung:packed-field-reorder, r=fmease

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

`inhibit_struct_field_reordering_opt` being false means we exclude this type from random field shuffling. However, `packed(1)` types can still be shuffled! The logic was added in https://github.com/rust-lang/rust/pull/48528 since it's pointless to reorder fields in packed(1) types (there's no padding that could be saved) -- but that shouldn't inhibit `-Zrandomize-layout` (which did not exist at the time).

We could add an optimization elsewhere to not bother sorting the fields for `repr(packed)` types, but I don't think that's worth the effort.

This *does* change the behavior in that we may now reorder fields of `packed(1)` structs (e.g. if there are niches, we'll try to move them to the start/end, according to `NicheBias`).  We were always allowed to do that but so far we didn't. Quoting the [reference](https://doc.rust-lang.org/reference/type-layout.html):

> On their own, align and packed do not provide guarantees about the order of fields in the layout of a struct or the layout of an enum variant, although they may be combined with representations (such as C) which do provide such guarantees.

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],