reject projecting to fields whose offset we cannot compute

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -99,6 +99,8 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
         let offset = self.layout.fields.offset(ix);
         let effective_field_align = self.align.restrict_for_offset(offset);
 
+        // `simple` is called when we don't need to adjust the offset to
+        // the dynamic alignment of the field.
         let mut simple = || {
             let llval = match self.layout.abi {
                 _ if offset.bytes() == 0 => {
@@ -141,19 +143,13 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
         };
 
         // Simple cases, which don't need DST adjustment:
-        //   * no metadata available - just log the case
-        //   * known alignment - sized types, `[T]`, `str` or a foreign type
+        //   * known alignment - sized types, `[T]`, `str`
+        //   * offset 0 -- rounding up to alignment cannot change the offset
         //   * packed struct - there is no alignment padding
         match field.ty.kind() {
-            _ if self.llextra.is_none() => {
-                debug!(
-                    "unsized field `{}`, of `{:?}` has no metadata for adjustment",
-                    ix, self.llval
-                );
-                return simple();
-            }
             _ if field.is_sized() => return simple(),
-            ty::Slice(..) | ty::Str | ty::Foreign(..) => return simple(),
+            ty::Slice(..) | ty::Str => return simple(),
+            _ if offset.bytes() == 0 => return simple(),
             ty::Adt(def, _) => {
                 if def.repr().packed() {
                     // FIXME(eddyb) generalize the adjustment when we

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -164,12 +164,15 @@ where
             // happens at run-time so that's okay.
             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));
+                None if offset == Size::ZERO => {
+                    // If the offset is 0, then rounding it up to alignment wouldn't change anything,
+                    // so we can do this even for types where we cannot determine the alignment.
                     (base_meta, offset)
                 }
+                None => {
+                    // We don't know the alignment of this field, so we cannot adjust.
+                    throw_unsup_format!("`extern type` does not have a known offset")
+                }
             }
         } else {
             // base_meta could be present; we might be accessing a sized field of an unsized

tests/ui/consts/const-eval/issue-91827-extern-types-field-offset.rs

@@ -0,0 +1,32 @@
+// Test that we can handle unsized types with an extern type tail part.
+// Regression test for issue #91827.
+
+#![feature(extern_types)]
+
+use std::ptr::addr_of;
+
+extern "C" {
+    type Opaque;
+}
+
+struct Newtype(Opaque);
+
+struct S {
+    i: i32,
+    a: Opaque,
+}
+
+const NEWTYPE: () = unsafe {
+    // Projecting to the newtype works, because it is always at offset 0.
+    let x: &Newtype = unsafe { &*(1usize as *const Newtype) };
+    let field = &x.0;
+};
+
+const OFFSET: () = unsafe {
+    // This needs to compute the field offset, but we don't know the type's alignment, so this fail.
+    let x: &S = unsafe { &*(1usize as *const S) };
+    let field = &x.a; //~ERROR: evaluation of constant value failed
+    //~| does not have a known offset
+};
+
+fn main() {}

tests/ui/consts/const-eval/issue-91827-extern-types-field-offset.stderr

@@ -0,0 +1,9 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/issue-91827-extern-types-field-offset.rs:28:17
+   |
+LL |     let field = &x.a;
+   |                 ^^^^ `extern type` does not have a known offset
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.

tests/ui/extern/extern-types-field-offset.rs

@@ -0,0 +1,26 @@
+// run-fail
+// check-run-results
+// normalize-stderr-test "panicking\.rs:\d+:\d+:" -> "panicking.rs:"
+#![feature(extern_types)]
+
+extern "C" {
+    type Opaque;
+}
+
+struct Newtype(Opaque);
+
+struct S {
+    i: i32,
+    a: Opaque,
+}
+
+fn main() {
+    // Projecting to the newtype works, because it is always at offset 0.
+    let x: &Newtype = unsafe { &*(1usize as *const Newtype) };
+    let field = &x.0;
+
+    // This needs to compute the field offset, but we don't know the type's alignment,
+    // so this panics.
+    let x: &S = unsafe { &*(1usize as *const S) };
+    let field = &x.a;
+}

tests/ui/extern/extern-types-field-offset.run.stderr

@@ -0,0 +1,4 @@
+thread 'main' panicked at library/core/src/panicking.rs:
+attempted to compute the size or alignment of extern type `Opaque`
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+thread caused non-unwinding panic. aborting.