[needs comments] normalize better

Author: Lukas Markeffsky

compiler/rustc_middle/src/ty/sty.rs

@@ -2759,9 +2759,17 @@ impl<'tcx> Ty<'tcx> {
                 (tcx.type_of(dyn_metadata).instantiate(tcx, &[tail.into()]), false)
             },
 
-            // type parameters only have unit metadata if they're sized, so return true
-            // to make sure we double check this during confirmation
-            ty::Param(_) |  ty::Alias(..) => (tcx.types.unit, true),
+            ty::Param(_) | ty::Alias(..) => {
+                if self == tail {
+                    // type parameters only have unit metadata if they're sized, so return true
+                    // to make sure we double check this during confirmation
+                    (tcx.types.unit, true)
+                } else {
+                    let metadata = tcx.require_lang_item(LangItem::Metadata, None);
+                    let project = Ty::new_projection(tcx, metadata, [tail]);
+                    (project, false)
+                }
+            },
 
             ty::Infer(ty::TyVar(_))
             | ty::Bound(..)

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1919,7 +1919,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                         // type parameters, opaques, and unnormalized projections have pointer
                         // metadata if they're known (e.g. by the param_env) to be sized
                         ty::Param(_) | ty::Alias(..)
-                            if selcx.infcx.predicate_must_hold_modulo_regions(
+                            if self_ty != tail || selcx.infcx.predicate_must_hold_modulo_regions(
                                 &obligation.with(
                                     selcx.tcx(),
                                     ty::TraitRef::from_lang_item(selcx.tcx(), LangItem::Sized, obligation.cause.span(),[self_ty]),

tests/ui/traits/metadata-cast.rs

@@ -93,3 +93,13 @@ fn do_casts<'a>(thin: &mut i32, slice: &mut [i32], trait_object: &mut dyn Trait)
         let _: *mut dyn Send = cast(trait_object); //[traits]~ ERROR not satisfied
     }
 }
+
+struct Wrapper<T: ?Sized> {
+    _tail: T
+}
+
+// This requires normalization from `<Wrapper<T> as Pointee>::Metadata`
+// to `<T as Pointee>::Metadata`.
+pub fn normalize_better<T: ?Sized>(ptr: *mut Wrapper<T>) -> *mut T {
+    cast(ptr)
+}