Fix taking address of truly unsized type field of unsized adt

Author: bjorn3

Diff for: example/issue-91827-extern-types.rs

@@ -0,0 +1,60 @@
+// Copied from rustc ui test suite
+
+// 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));
+}

Diff for: scripts/tests.sh

@@ -35,6 +35,10 @@ function base_sysroot_tests() {
     $MY_RUSTC example/arbitrary_self_types_pointers_and_wrappers.rs --crate-name arbitrary_self_types_pointers_and_wrappers --crate-type bin --target "$TARGET_TRIPLE"
     $RUN_WRAPPER ./target/out/arbitrary_self_types_pointers_and_wrappers
 
+    echo "[AOT] issue_91827_extern_types"
+    $MY_RUSTC example/issue-91827-extern-types.rs --crate-name issue_91827_extern_types --crate-type bin --target "$TARGET_TRIPLE"
+    $RUN_WRAPPER ./target/out/issue_91827_extern_types
+
     echo "[AOT] alloc_system"
     $MY_RUSTC example/alloc_system.rs --crate-type lib --target "$TARGET_TRIPLE"
 

Diff for: src/value_and_place.rs

@@ -672,7 +672,12 @@ impl<'tcx> CPlace<'tcx> {
 
         let (field_ptr, field_layout) = codegen_field(fx, base, extra, layout, field);
         if field_layout.is_unsized() {
-            CPlace::for_ptr_with_extra(field_ptr, extra.unwrap(), field_layout)
+            if let ty::Foreign(_) = field_layout.ty.kind() {
+                assert!(extra.is_none());
+                CPlace::for_ptr(field_ptr, field_layout)
+            } else {
+                CPlace::for_ptr_with_extra(field_ptr, extra.unwrap(), field_layout)
+            }
         } else {
             CPlace::for_ptr(field_ptr, field_layout)
         }