Fix unsize coerce for structs containing PhantomData like Unique

examples/mini_core.rs

@@ -12,6 +12,7 @@ pub trait Unsize<T: ?Sized> {}
 pub trait CoerceUnsized<T> {}
 
 impl<'a, 'b: 'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b T {}
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *const T {}
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for *mut T {}
 
 #[lang = "copy"]
@@ -169,6 +170,9 @@ impl<T: ?Sized> PartialEq for *const T {
     }
 }
 
+#[lang = "phantom_data"]
+pub struct PhantomData<T: ?Sized>;
+
 #[lang = "fn_once"]
 #[rustc_paren_sugar]
 pub trait FnOnce<Args> {

examples/mini_core_hello_world.rs

@@ -102,6 +102,13 @@ macro_rules! assert_eq {
     }
 }
 
+struct Unique<T: ?Sized> {
+    pointer: *const T,
+    _marker: PhantomData<T>,
+}
+
+impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> {}
+
 fn main() {
     unsafe {
         let hello: &[u8] = b"Hello\0" as &[u8; 6];
@@ -133,6 +140,11 @@ fn main() {
 
         assert!(!intrinsics::needs_drop::<u8>());
         assert!(intrinsics::needs_drop::<NoisyDrop>());
+
+        Unique {
+            pointer: 0 as *const &str,
+            _marker: PhantomData,
+        } as Unique<dyn SomeTrait>;
     }
 
     let _ = NoisyDrop {

src/common.rs

@@ -259,6 +259,17 @@ impl<'tcx> CValue<'tcx> {
                 for idx in 0..field_count {
                     let field_dest = dest.place_field(fx, mir::Field::new(idx));
                     let field_src = self.value_field(fx, mir::Field::new(idx));
+                    if field_src.layout().ty.is_phantom_data() {
+                        // Ignore PhantomData so for example `Unique<()>` can coerce to `Unique<Debug>`
+                        //
+                        // ```rust
+                        // struct Unique<T: ?Sized> {
+                        //     pointer: NonZero<*const T>,
+                        //     _marker: PhantomData<T>,
+                        // }
+                        // ```
+                        continue;
+                    }
                     if field_src.layout().ty != field_dest.layout().ty {
                         assert!(!found_unsize_field);
                         found_unsize_field = true;