improve normalization of Pointee:Metadata

Author: Lukas Markeffsky

compiler/rustc_middle/src/ty/sty.rs

@@ -2759,9 +2759,19 @@ 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 {
+                    // normalize from `Wrapper<T>::Metadata` to `T::Metadata` to allow
+                    // casting between `*mut Wrapper<T>` and `*mut T`
+                    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-fail-traits.rs

@@ -43,13 +43,3 @@ fn do_casts<'a>(thin: &mut i32, slice: &mut [i32], trait_object: &mut dyn Trait)
     // may be allowed in the future
     let _: *mut dyn Send = cast(trait_object); //~ ERROR not satisfied
 }
-
-struct Wrapper<T: ?Sized> {
-    _tail: T
-}
-
-// This requires normalization from `<Wrapper<T> as Pointee>::Metadata`
-// to `<T as Pointee>::Metadata`.
-fn normalize_better<T: ?Sized>(ptr: *mut Wrapper<T>) -> *mut T {
-    cast(ptr) //~ ERROR not satisfied
-}

tests/ui/traits/metadata-cast-fail-traits.stderr

@@ -226,28 +226,6 @@ LL | where
 LL |     T: PointerCast<U>,
    |        ^^^^^^^^^^^^^^ required by this bound in `cast`
 
-error[E0277]: the trait bound `<Wrapper<T> as Pointee>::Metadata: MetadataCast<<T as Pointee>::Metadata>` is not satisfied
-  --> $DIR/metadata-cast-fail-traits.rs:54:10
-   |
-LL |     cast(ptr)
-   |     ---- ^^^ the trait `MetadataCast<<T as Pointee>::Metadata>` is not implemented for `<Wrapper<T> as Pointee>::Metadata`
-   |     |
-   |     required by a bound introduced by this call
-   |
-   = note: required for `Wrapper<T>` to implement `PointerCast<T>`
-note: required by a bound in `cast`
-  --> $DIR/metadata-cast-fail-traits.rs:7:8
-   |
-LL | fn cast<T: ?Sized, U: ?Sized>(_: *mut T) -> *mut U
-   |    ---- required by a bound in this function
-LL | where
-LL |     T: PointerCast<U>,
-   |        ^^^^^^^^^^^^^^ required by this bound in `cast`
-help: consider further restricting the associated type
-   |
-LL | fn normalize_better<T: ?Sized>(ptr: *mut Wrapper<T>) -> *mut T where <Wrapper<T> as Pointee>::Metadata: MetadataCast<<T as Pointee>::Metadata> {
-   |                                                                +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
-
-error: aborting due to 15 previous errors
+error: aborting due to 14 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/metadata-cast-pass.rs

@@ -77,3 +77,13 @@ where
 fn to_thin<T: ?Sized, U: ?Sized + Thin>(ptr: *mut T) -> *mut U {
     cast(ptr)
 }
+
+struct Wrapper<T: ?Sized> {
+    _tail: T
+}
+
+// This requires normalization from `<Wrapper<T> as Pointee>::Metadata`
+// to `<T as Pointee>::Metadata`.
+fn normalize_better<T: ?Sized>(ptr: *mut Wrapper<T>) -> *mut T {
+    cast(ptr)
+}