old solver: improve normalization of Pointee::Metadata

Author: Lukas Markeffsky

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -1985,10 +1985,9 @@ 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 }
@@ -2000,10 +1999,9 @@ 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

@@ -377,12 +377,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 // to fields, which can yield non-normalized types. So we need to provide a
                 // normalization function.
                 let normalize = |ty| self.tcx.normalize_erasing_regions(self.param_env, ty);
-                let (meta, only_if_sized) = ty.ptr_metadata_ty(*self.tcx, normalize);
-                assert!(
-                    !only_if_sized,
-                    "there should be no more 'maybe has that metadata' types during interpretation"
-                );
-                meta
+                ty.ptr_metadata_ty(*self.tcx, normalize)
             };
             return Ok(meta_ty(caller) == meta_ty(callee));
         }

compiler/rustc_middle/src/ty/sty.rs

@@ -1957,12 +1957,12 @@ 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 determined.
+    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
@@ -1984,31 +1984,47 @@ impl<'tcx> Ty<'tcx> {
             | ty::Error(_)
             // Extern types have metadata = ().
             | ty::Foreign(..)
-            // `dyn*` has no metadata
+            // `dyn*` has 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

@@ -1916,10 +1916,11 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                         // Integers and floats are always Sized, and so have unit type metadata.
                         | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
 
-                        // type parameters, opaques, and unnormalized projections have pointer
-                        // metadata if they're known (e.g. by the param_env) to be sized
+                        // We normalize from `Wrapper<Tail>::Metadata` to `Tail::Metadata` if able.
+                        // Otherwise, type parameters, opaques, and unnormalized projections have
+                        // unit 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 +2290,7 @@ 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,
@@ -2298,16 +2299,26 @@ fn confirm_builtin_candidate<'cx, 'tcx>(
                 ty,
                 &mut obligations,
             )
+        };
+        let metadata_ty = self_ty.ptr_metadata_ty_or_tail(tcx, normalize).unwrap_or_else(|tail| {
+            if tail == self_ty {
+                // This is the fallback case for type parameters, unnormalizable projections
+                // and opaque types.
+                // If the `self_ty` is `Sized`, then the metadata is `()`.
+                let sized_predicate = ty::TraitRef::from_lang_item(
+                    tcx,
+                    LangItem::Sized,
+                    obligation.cause.span(),
+                    [self_ty],
+                );
+                obligations.push(obligation.with(tcx, sized_predicate));
+                tcx.types.unit
+            } else {
+                // We know that `self_ty` has the same metadata as `tail`. This allows us
+                // to prove predicates like `Wrapper<Tail>::Metadata == Tail::Metadata`.
+                Ty::new_projection(tcx, metadata_def_id, [tail])
+            }
         });
-        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)
     } else {
         bug!("unexpected builtin trait with associated type: {:?}", obligation.predicate);

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

@@ -0,0 +1,54 @@
+// 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);
+
+// normalize `Wrapper<T>::Metadata` -> `T::Metadata`
+fn wrapper_to_tail<T: ?Sized>(ptr: *const T) -> *const Wrapper<T> {
+    cast_same_meta(ptr)
+}
+
+// normalize `Wrapper<T>::Metadata` -> `T::Metadata` -> `()`
+fn wrapper_to_unit<T>(ptr: *const ()) -> *const Wrapper<T> {
+    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_tail<T: ?Sized + Project>(ptr: *const T::Assoc) -> *const WrapperProject<T> {
+    cast_same_meta(ptr)
+}
+
+// normalize `WrapperProject<T>::Metadata` -> `T::Assoc::Metadata` -> `()`
+fn wrapper_project_unit<T: ?Sized + Project>(ptr: *const ()) -> *const WrapperProject<T>
+where
+    T::Assoc: Sized,
+{
+    cast_same_meta(ptr)
+}
+
+// normalize `<[T] as Pointee>::Metadata` -> `usize`, even if `[T]: Sized`
+fn sized_slice<T>(ptr: *const [T]) -> *const str
+where
+    [T]: Sized,
+{
+    cast_same_meta(ptr)
+}
+
+fn main() {}