Auto merge of #55672 - RalfJung:miri-extern-types, r=eddyb

miri: accept extern types in structs if they are the only field

Fixes https://github.com/rust-lang/rust/issues/55541

Cc @oli-obk @eddyb https://github.com/rust-lang/rust/issues/43467

src/librustc_mir/interpret/eval_context.rs

@@ -341,8 +341,21 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
                 // the last field).  Can't have foreign types here, how would we
                 // adjust alignment and size for them?
                 let field = layout.field(self, layout.fields.count() - 1)?;
-                let (unsized_size, unsized_align) = self.size_and_align_of(metadata, field)?
-                    .expect("Fields cannot be extern types");
+                let (unsized_size, unsized_align) = 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 {
+                            bug!("Fields cannot be extern types, unless they are at offset 0")
+                        }
+                    }
+                };
 
                 // FIXME (#26403, #27023): We should be adding padding
                 // to `sized_size` (to accommodate the `unsized_align`

src/librustc_mir/interpret/place.rs

@@ -351,8 +351,17 @@ where
         // Offset may need adjustment for unsized fields
         let (meta, offset) = if field_layout.is_unsized() {
             // re-use parent metadata to determine dynamic field layout
-            let (_, align) = self.size_and_align_of(base.meta, field_layout)?
-                .expect("Fields cannot be extern types");
+            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,
+                None =>
+                    bug!("Cannot compute offset for extern type field at non-0 offset"),
+            };
             (base.meta, offset.abi_align(align))
         } else {
             // base.meta could be present; we might be accessing a sized field of an unsized

src/test/ui/consts/const-eval/issue-55541.rs

@@ -0,0 +1,27 @@
+// compile-pass
+
+// Test that we can handle newtypes wrapping extern types
+
+#![feature(extern_types, const_transmute)]
+
+use std::marker::PhantomData;
+
+extern "C" {
+  pub type ExternType;
+}
+unsafe impl Sync for ExternType {}
+static MAGIC_FFI_STATIC: u8 = 42;
+
+#[repr(transparent)]
+pub struct Wrapper(ExternType);
+pub static MAGIC_FFI_REF: &'static Wrapper = unsafe {
+  std::mem::transmute(&MAGIC_FFI_STATIC)
+};
+
+#[repr(transparent)]
+pub struct Wrapper2(PhantomData<Vec<i32>>, ExternType);
+pub static MAGIC_FFI_REF2: &'static Wrapper2 = unsafe {
+  std::mem::transmute(&MAGIC_FFI_STATIC)
+};
+
+fn main() {}