Rollup merge of rust-lang#91910 - tmiasko:miri-extern-type, r=RalfJung

miri: lift restriction on extern types being the only field in a struct

Fixes rust-lang#91827.

r? ```@RalfJung```

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -616,19 +616,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
  match self.size_and_align_of(metadata, &field)? {
  Some(size_and_align) => size_and_align,
  None => {
- // A field with extern type. If this field is at offset 0, we behave
- // like the underlying extern type.
- // FIXME: Once we have made decisions for how to handle size and alignment
- // of `extern type`, this should be adapted. It is just a temporary hack
- // to get some code to work that probably ought to work.
- if sized_size == Size::ZERO {
- return Ok(None);
- } else {
- span_bug!(
- self.cur_span(),
- "Fields cannot be extern types, unless they are at offset 0"
- )
- }
+ // A field with an extern type. We don't know the actual dynamic size
+ // or the alignment.
+ return Ok(None);
  }
  };
 

compiler/rustc_const_eval/src/interpret/place.rs

@@ -362,21 +362,15 @@ where
  // Re-use parent metadata to determine dynamic field layout.
  // With custom DSTS, this *will* execute user-defined code, but the same
  // happens at run-time so that's okay.
- let align = match self.size_and_align_of(&base.meta, &field_layout)? {
- Some((_, align)) => align,
- None if offset == Size::ZERO => {
- // An extern type at offset 0, we fall back to its static alignment.
- // FIXME: Once we have made decisions for how to handle size and alignment
- // of `extern type`, this should be adapted. It is just a temporary hack
- // to get some code to work that probably ought to work.
- field_layout.align.abi
+ match self.size_and_align_of(&base.meta, &field_layout)? {
+ Some((_, align)) => (base.meta, offset.align_to(align)),
+ None => {
+ // For unsized types with an extern type tail we perform no adjustments.
+ // NOTE: keep this in sync with `PlaceRef::project_field` in the codegen backend.
+ assert!(matches!(base.meta, MemPlaceMeta::None));
+ (base.meta, offset)
  }
- None => span_bug!(
- self.cur_span(),
- "cannot compute offset for extern type field at non-0 offset"
- ),
- };
- (base.meta, offset.align_to(align))
+ }
  } else {
  // base.meta could be present; we might be accessing a sized field of an unsized
  // struct.

src/test/ui/consts/const-eval/issue-91827-extern-types.rs

@@ -0,0 +1,58 @@
+// run-pass
+//
+// Test that we can handle unsized types with an extern type tail part.
+// Regression test for issue #91827.
+
+#![feature(const_ptr_offset_from)]
+#![feature(const_slice_from_raw_parts)]
+#![feature(extern_types)]
+
+use std::ptr::addr_of;
+
+extern "C" {
+ type Opaque;
+}
+
+unsafe impl Sync for Opaque {}
+
+#[repr(C)]
+pub struct List<T> {
+ len: usize,
+ data: [T; 0],
+ tail: Opaque,
+}
+
+#[repr(C)]
+pub struct ListImpl<T, const N: usize> {
+ len: usize,
+ data: [T; N],
+}
+
+impl<T> List<T> {
+ const fn as_slice(&self) -> &[T] {
+ unsafe { std::slice::from_raw_parts(self.data.as_ptr(), self.len) }
+ }
+}
+
+impl<T, const N: usize> ListImpl<T, N> {
+ const fn as_list(&self) -> &List<T> {
+ unsafe { std::mem::transmute(self) }
+ }
+}
+
+pub static A: ListImpl<u128, 3> = ListImpl {
+ len: 3,
+ data: [5, 6, 7],
+};
+pub static A_REF: &'static List<u128> = A.as_list();
+pub static A_TAIL_OFFSET: isize = tail_offset(A.as_list());
+
+const fn tail_offset<T>(list: &List<T>) -> isize {
+ unsafe { (addr_of!(list.tail) as *const u8).offset_from(list as *const List<T> as *const u8) }
+}
+
+fn main() {
+ assert_eq!(A_REF.as_slice(), &[5, 6, 7]);
+ // Check that interpreter and code generation agree about the position of the tail field.
+ assert_eq!(A_TAIL_OFFSET, tail_offset(A_REF));
+}