Allow extern types in #[repr(transparent)] structs

Fixes rust-lang#55541

src/librustc/ty/sty.rs

@@ -1578,6 +1578,13 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
         }
     }
 
+    pub fn is_transparent(&self) -> bool {
+        match self.sty {
+            Adt(def, _) => def.repr.transparent(),
+            _ => false,
+        }
+    }
+
     pub fn sequence_element_type(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>) -> Ty<'tcx> {
         match self.sty {
             Array(ty, _) | Slice(ty) => ty,

src/librustc_mir/interpret/eval_context.rs

@@ -368,10 +368,15 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
                 );
 
                 // Recurse to get the size of the dynamically sized field (must be
-                // the last field).  Can't have foreign types here, how would we
-                // adjust alignment and size for them?
+                // the last field).  Can't have foreign types here unless they're
+                // in a #[repr(transparent)] struct, otherwise 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)?
+                let unsized_size_and_align = self.size_and_align_of(metadata, field)?;
+                if unsized_size_and_align.is_none() && layout.ty.is_transparent() {
+                    return Ok(None);
+                }
+                let (unsized_size, unsized_align) = unsized_size_and_align
                     .expect("Fields cannot be extern types");
 
                 // FIXME (#26403, #27023): We should be adding padding

src/librustc_mir/interpret/place.rs

@@ -359,9 +359,13 @@ 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");
-            (base.meta, offset.abi_align(align))
+            let size_and_align = self.size_and_align_of(base.meta, field_layout)?;
+            if size_and_align.is_none() && base.layout.ty.is_transparent() {
+                (base.meta, offset)
+            } else {
+                let (_, align) = size_and_align.expect("Fields cannot be extern types");
+                (base.meta, offset.abi_align(align))
+            }
         } else {
             // base.meta could be present; we might be accessing a sized field of an unsized
             // struct.

src/test/run-pass/extern/extern-types-in-transparent.rs

@@ -0,0 +1,39 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// run-pass
+#![allow(dead_code)]
+
+// Test that extern types can be used as fields within a transparent struct. See issue #55541.
+
+#![feature(const_transmute, extern_types)]
+
+extern {
+    type A;
+}
+unsafe impl Sync for A {}
+
+#[repr(transparent)]
+struct Foo(A);
+
+#[repr(transparent)]
+struct Bar(std::marker::PhantomData<u64>, A);
+
+static FOO: &'static Foo = {
+    static VALUE: usize = b'F' as usize;
+    unsafe { std::mem::transmute(&VALUE) }
+};
+
+static BAR: &'static Bar = {
+    static VALUE: usize = b'B' as usize;
+    unsafe { std::mem::transmute(&VALUE) }
+};
+
+fn main() {}