Allow unsizing pattern types with pointer base

Author: oli-obk

src/unsize.rs

@@ -131,6 +131,11 @@ pub(crate) fn coerce_unsized_into<'tcx>(
         dst.write_cvalue(fx, CValue::by_val_pair(base, info, dst.layout()));
     };
     match (&src_ty.kind(), &dst_ty.kind()) {
+        (ty::Pat(a, _), ty::Pat(b, _)) => {
+            let src = src.cast_pat_ty_to_base(fx.layout_of(*a));
+            let dst = dst.place_transmute_type(fx, *b);
+            return coerce_unsized_into(fx, src, dst);
+        }
         (&ty::Ref(..), &ty::Ref(..))
         | (&ty::Ref(..), &ty::RawPtr(..))
         | (&ty::RawPtr(..), &ty::RawPtr(..)) => coerce_ptr(),

src/value_and_place.rs

@@ -342,6 +342,14 @@ impl<'tcx> CValue<'tcx> {
         assert_eq!(self.layout().backend_repr, layout.backend_repr);
         CValue(self.0, layout)
     }
+
+    pub(crate) fn cast_pat_ty_to_base(self, layout: TyAndLayout<'tcx>) -> Self {
+        let ty::Pat(base, _) = *self.layout().ty.kind() else {
+            panic!("not a pattern type: {:#?}", self.layout())
+        };
+        assert_eq!(layout.ty, base);
+        CValue(self.0, layout)
+    }
 }
 
 /// A place where you can write a value to or read a value from