improve normalization of Pointee::Metadata

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -1977,10 +1977,9 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
 
         match in_elem.kind() {
             ty::RawPtr(p) => {
-                let (metadata, check_sized) = p.ty.ptr_metadata_ty(bx.tcx, |ty| {
+                let metadata = p.ty.ptr_metadata_ty(bx.tcx, |ty| {
                     bx.tcx.normalize_erasing_regions(ty::ParamEnv::reveal_all(), ty)
                 });
-                assert!(!check_sized); // we are in codegen, so we shouldn't see these types
                 require!(
                     metadata.is_unit(),
                     InvalidMonomorphization::CastFatPointer { span, name, ty: in_elem }
@@ -1992,10 +1991,9 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
         }
         match out_elem.kind() {
             ty::RawPtr(p) => {
-                let (metadata, check_sized) = p.ty.ptr_metadata_ty(bx.tcx, |ty| {
+                let metadata = p.ty.ptr_metadata_ty(bx.tcx, |ty| {
                     bx.tcx.normalize_erasing_regions(ty::ParamEnv::reveal_all(), ty)
                 });
-                assert!(!check_sized); // we are in codegen, so we shouldn't see these types
                 require!(
                     metadata.is_unit(),
                     InvalidMonomorphization::CastFatPointer { span, name, ty: out_elem }

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -372,14 +372,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         };
         if let (Some(caller), Some(callee)) = (pointee_ty(caller.ty)?, pointee_ty(callee.ty)?) {
             // This is okay if they have the same metadata type.
-            let meta_ty = |ty: Ty<'tcx>| {
-                let (meta, only_if_sized) = ty.ptr_metadata_ty(*self.tcx, |ty| ty);
-                assert!(
-                    !only_if_sized,
-                    "there should be no more 'maybe has that metadata' types during interpretation"
-                );
-                meta
-            };
+            let meta_ty = |ty: Ty<'tcx>| ty.ptr_metadata_ty(*self.tcx, |ty| ty);
             return Ok(meta_ty(caller) == meta_ty(callee));
         }
 

compiler/rustc_middle/src/ty/sty.rs

@@ -2710,12 +2710,12 @@ impl<'tcx> Ty<'tcx> {
     }
 
     /// Returns the type of metadata for (potentially fat) pointers to this type,
-    /// and a boolean signifying if this is conditional on this type being `Sized`.
-    pub fn ptr_metadata_ty(
+    /// or the struct tail if the metadata type cannot be determinded.
+    pub fn ptr_metadata_ty_or_tail(
         self,
         tcx: TyCtxt<'tcx>,
         normalize: impl FnMut(Ty<'tcx>) -> Ty<'tcx>,
-    ) -> (Ty<'tcx>, bool) {
+    ) -> Result<Ty<'tcx>, Ty<'tcx>> {
         let tail = tcx.struct_tail_with_normalize(self, normalize, || {});
         match tail.kind() {
             // Sized types
@@ -2739,29 +2739,45 @@ impl<'tcx> Ty<'tcx> {
             | ty::Foreign(..)
             // `dyn*` has no metadata
             | ty::Dynamic(_, _, ty::DynStar)
-            // If returned by `struct_tail_without_normalization` this is a unit struct
+            // If returned by `struct_tail_with_normalize` this is a unit struct
             // without any fields, or not a struct, and therefore is Sized.
             | ty::Adt(..)
-            // If returned by `struct_tail_without_normalization` this is the empty tuple,
+            // If returned by `struct_tail_with_normalize` this is the empty tuple,
             // a.k.a. unit type, which is Sized
-            | ty::Tuple(..) => (tcx.types.unit, false),
+            | ty::Tuple(..) => Ok(tcx.types.unit),
+
+            ty::Str | ty::Slice(_) => Ok(tcx.types.usize),
 
-            ty::Str | ty::Slice(_) => (tcx.types.usize, false),
             ty::Dynamic(_, _, ty::Dyn) => {
                 let dyn_metadata = tcx.require_lang_item(LangItem::DynMetadata, None);
-                (tcx.type_of(dyn_metadata).instantiate(tcx, &[tail.into()]), false)
-            },
+                Ok(tcx.type_of(dyn_metadata).instantiate(tcx, &[tail.into()]))
+            }
 
-            // 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),
+            // We don't know the metadata of `self`, but it must be equal to the
+            // metadata of `tail`.
+            ty::Param(_) | ty::Alias(..) => Err(tail),
 
             ty::Infer(ty::TyVar(_))
             | ty::Bound(..)
             | ty::Placeholder(..)
-            | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
-                bug!("`ptr_metadata_ty` applied to unexpected type: {:?} (tail = {:?})", self, tail)
-            }
+            | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => bug!(
+                "`ptr_metadata_ty_or_tail` applied to unexpected type: {self:?} (tail = {tail:?})"
+            ),
+        }
+    }
+
+    /// Returns the type of metadata for (potentially fat) pointers to this type.
+    /// Causes an ICE if the metadata type cannot be determined.
+    pub fn ptr_metadata_ty(
+        self,
+        tcx: TyCtxt<'tcx>,
+        normalize: impl FnMut(Ty<'tcx>) -> Ty<'tcx>,
+    ) -> Ty<'tcx> {
+        match self.ptr_metadata_ty_or_tail(tcx, normalize) {
+            Ok(metadata) => metadata,
+            Err(tail) => bug!(
+                "`ptr_metadata_ty` failed to get metadata for type: {self:?} (tail = {tail:?})"
+            ),
         }
     }
 

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]),
@@ -2289,7 +2289,7 @@ fn confirm_builtin_candidate<'cx, 'tcx>(
         assert_eq!(metadata_def_id, item_def_id);
 
         let mut obligations = Vec::new();
-        let (metadata_ty, check_is_sized) = self_ty.ptr_metadata_ty(tcx, |ty| {
+        let normalize = |ty| {
             normalize_with_depth_to(
                 selcx,
                 obligation.param_env,
@@ -2298,17 +2298,33 @@ fn confirm_builtin_candidate<'cx, 'tcx>(
                 ty,
                 &mut obligations,
             )
-        });
-        if check_is_sized {
-            let sized_predicate = ty::TraitRef::from_lang_item(
-                tcx,
-                LangItem::Sized,
-                obligation.cause.span(),
-                [self_ty],
-            );
-            obligations.push(obligation.with(tcx, sized_predicate));
-        }
-        (metadata_ty.into(), obligations)
+        };
+        let metadata = match self_ty.ptr_metadata_ty_or_tail(tcx, normalize) {
+            Ok(metadata) => metadata,
+            Err(tail) => {
+                let sized_predicate = ty::TraitRef::from_lang_item(
+                    tcx,
+                    LangItem::Sized,
+                    obligation.cause.span(),
+                    [self_ty],
+                );
+                let sized_obligation = obligation.with(tcx, sized_predicate);
+                if self_ty == tail
+                    || selcx.infcx.predicate_must_hold_considering_regions(&sized_obligation)
+                {
+                    // If the `self_ty` is `Sized`, then the metadata is `()`.
+                    // We check this before projecting to the metadata of `tail`,
+                    // because we may know `self_ty: Sized`, but not `tail: Sized`.
+                    obligations.push(sized_obligation);
+                    tcx.types.unit
+                } else {
+                    // We know that `self_ty` has the same metadata as `tail`. This allows
+                    // us to prove predicates like `Wrapper<T>::Metadata == T::Metadata`.
+                    Ty::new_projection(tcx, metadata_def_id, [tail])
+                }
+            }
+        };
+        (metadata.into(), obligations)
     } else {
         bug!("unexpected builtin trait with associated type: {:?}", obligation.predicate);
     };

tests/ui/traits/pointee-normalize-equate.rs

@@ -0,0 +1,55 @@
+// check-pass
+
+#![feature(ptr_metadata)]
+
+use std::ptr::{self, Pointee};
+
+fn cast_same_meta<T: ?Sized, U: ?Sized>(ptr: *const T) -> *const U
+where
+    T: Pointee<Metadata = <U as Pointee>::Metadata>,
+{
+    let (thin, meta) = ptr.to_raw_parts();
+    ptr::from_raw_parts(thin, meta)
+}
+
+struct Wrapper<T: ?Sized>(T);
+
+// if `Wrapper<T>: ?Sized` then normalize `Wrapper<T>::Metadata` -> `T::Metadata`
+fn wrapper_to_tail<T: ?Sized>(ptr: *const T) -> *const Wrapper<T> {
+    cast_same_meta(ptr)
+}
+
+// if `Wrapper<T>: Sized` then normalize `Wrapper<T>::Metadata` -> `()`
+fn wrapper_to_unit<T: ?Sized>(ptr: *const ()) -> *const Wrapper<T>
+where
+    Wrapper<T>: Sized,
+{
+    cast_same_meta(ptr)
+}
+
+trait Project {
+    type Assoc: ?Sized;
+}
+
+struct WrapperProject<T: ?Sized + Project>(T::Assoc);
+
+// normalize `WrapperProject<T>::Metadata` -> `T::Assoc::Metadata`
+fn wrapper_project<T: ?Sized + Project>(ptr: *const T::Assoc) -> *const WrapperProject<T> {
+    cast_same_meta(ptr)
+}
+
+// In this example, `WrapperProject<&'a T>` is `Sized` modulo regions,
+// but `?Sized` considering regions.
+// Normalize `WrapperProject<&'a T>::Metadata` -> `<&'a T as Project>::Assoc::Metadata`
+// and don't require `WrapperProject<&'a T>: Sized`.
+fn sized_modulo_regions<'a, T: ?Sized + 'static>(
+    ptr: *const <&'a T as Project>::Assoc,
+) -> *const WrapperProject<&'a T>
+where
+    for<'b> &'b T: Project,
+    WrapperProject<&'static T>: Sized,
+{
+    cast_same_meta(ptr)
+}
+
+fn main() {}