library/compiler: add PointeeSized bounds

As core uses an extern type (`ptr::VTable`), the default `?Sized` to
`MetaSized` migration isn't sufficient, and some code that previously
accepted `VTable` needs relaxed to continue to accept extern types.

Similarly, the compiler uses many extern types in `rustc_codegen_llvm`
and in the `rustc_middle::ty::List` implementation (`OpaqueListContents`)
some bounds must be relaxed to continue to accept these types.

Unfortunately, due to the current inability to relax `Deref::Target`,
some of the bounds in the standard library are forced to be stricter than
they ideally would be.

Author: davidtwco

compiler/rustc_data_structures/src/aligned.rs

@@ -1,3 +1,4 @@
+use std::marker::PointeeSized;
 use std::ptr::Alignment;
 
 /// Returns the ABI-required minimum alignment of a type in bytes.
@@ -19,7 +20,7 @@ pub const fn align_of<T: ?Sized + Aligned>() -> Alignment {
 /// example `[T]` has alignment of `T`.
 ///
 /// [`mem::align_of<Self>()`]: std::mem::align_of
-pub unsafe trait Aligned {
+pub unsafe trait Aligned: PointeeSized {
     /// Alignment of `Self`.
     const ALIGN: Alignment;
 }

compiler/rustc_data_structures/src/lib.rs

@@ -33,6 +33,7 @@
 #![feature(ptr_alignment_type)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
+#![feature(sized_hierarchy)]
 #![feature(test)]
 #![feature(thread_id_value)]
 #![feature(type_alias_impl_trait)]

compiler/rustc_data_structures/src/marker.rs

@@ -1,3 +1,5 @@
+use std::marker::PointeeSized;
+
 #[rustc_on_unimplemented(message = "`{Self}` doesn't implement `DynSend`. \
             Add it to `rustc_data_structures::marker` or use `IntoDynSyncSend` if it's already `Send`")]
 // This is an auto trait for types which can be sent across threads if `sync::is_dyn_thread_safe()`
@@ -13,7 +15,7 @@ pub unsafe auto trait DynSend {}
 pub unsafe auto trait DynSync {}
 
 // Same with `Sync` and `Send`.
-unsafe impl<T: DynSync + ?Sized> DynSend for &T {}
+unsafe impl<T: DynSync + ?Sized + PointeeSized> DynSend for &T {}
 
 macro_rules! impls_dyn_send_neg {
     ($([$t1: ty $(where $($generics1: tt)*)?])*) => {
@@ -25,9 +27,9 @@ macro_rules! impls_dyn_send_neg {
 impls_dyn_send_neg!(
     [std::env::Args]
     [std::env::ArgsOs]
-    [*const T where T: ?Sized]
-    [*mut T where T: ?Sized]
-    [std::ptr::NonNull<T> where T: ?Sized]
+    [*const T where T: ?Sized + PointeeSized]
+    [*mut T where T: ?Sized + PointeeSized]
+    [std::ptr::NonNull<T> where T: ?Sized + PointeeSized]
     [std::rc::Rc<T> where T: ?Sized]
     [std::rc::Weak<T> where T: ?Sized]
     [std::sync::MutexGuard<'_, T> where T: ?Sized]
@@ -90,12 +92,12 @@ macro_rules! impls_dyn_sync_neg {
 impls_dyn_sync_neg!(
     [std::env::Args]
     [std::env::ArgsOs]
-    [*const T where T: ?Sized]
-    [*mut T where T: ?Sized]
+    [*const T where T: ?Sized + PointeeSized]
+    [*mut T where T: ?Sized + PointeeSized]
     [std::cell::Cell<T> where T: ?Sized]
     [std::cell::RefCell<T> where T: ?Sized]
     [std::cell::UnsafeCell<T> where T: ?Sized]
-    [std::ptr::NonNull<T> where T: ?Sized]
+    [std::ptr::NonNull<T> where T: ?Sized + PointeeSized]
     [std::rc::Rc<T> where T: ?Sized]
     [std::rc::Weak<T> where T: ?Sized]
     [std::cell::OnceCell<T> where T]
@@ -157,10 +159,10 @@ impl_dyn_sync!(
     [thin_vec::ThinVec<T> where T: DynSync]
 );
 
-pub fn assert_dyn_sync<T: ?Sized + DynSync>() {}
-pub fn assert_dyn_send<T: ?Sized + DynSend>() {}
-pub fn assert_dyn_send_val<T: ?Sized + DynSend>(_t: &T) {}
-pub fn assert_dyn_send_sync_val<T: ?Sized + DynSync + DynSend>(_t: &T) {}
+pub fn assert_dyn_sync<T: ?Sized + PointeeSized + DynSync>() {}
+pub fn assert_dyn_send<T: ?Sized + PointeeSized + DynSend>() {}
+pub fn assert_dyn_send_val<T: ?Sized + PointeeSized + DynSend>(_t: &T) {}
+pub fn assert_dyn_send_sync_val<T: ?Sized + PointeeSized + DynSync + DynSend>(_t: &T) {}
 
 #[derive(Copy, Clone)]
 pub struct FromDyn<T>(T);
@@ -200,10 +202,10 @@ impl<T> std::ops::Deref for FromDyn<T> {
 // an instance of `DynSend` and `DynSync`, since the compiler cannot infer
 // it automatically in some cases. (e.g. Box<dyn Send / Sync>)
 #[derive(Copy, Clone)]
-pub struct IntoDynSyncSend<T: ?Sized>(pub T);
+pub struct IntoDynSyncSend<T: ?Sized + PointeeSized>(pub T);
 
-unsafe impl<T: ?Sized + Send> DynSend for IntoDynSyncSend<T> {}
-unsafe impl<T: ?Sized + Sync> DynSync for IntoDynSyncSend<T> {}
+unsafe impl<T: ?Sized + PointeeSized + Send> DynSend for IntoDynSyncSend<T> {}
+unsafe impl<T: ?Sized + PointeeSized + Sync> DynSync for IntoDynSyncSend<T> {}
 
 impl<T> std::ops::Deref for IntoDynSyncSend<T> {
     type Target = T;

compiler/rustc_middle/src/lib.rs

@@ -55,6 +55,7 @@
 #![feature(ptr_alignment_type)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
+#![feature(sized_hierarchy)]
 #![feature(trusted_len)]
 #![feature(try_blocks)]
 #![feature(try_trait_v2)]

compiler/rustc_middle/src/ty/codec.rs

@@ -8,7 +8,7 @@
 
 use std::hash::Hash;
 use std::intrinsics;
-use std::marker::DiscriminantKind;
+use std::marker::{DiscriminantKind, PointeeSized};
 
 use rustc_abi::{FieldIdx, VariantIdx};
 use rustc_data_structures::fx::FxHashMap;
@@ -65,7 +65,7 @@ impl<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>> EncodableWithShorthand<E> for ty::Pre
 ///
 /// `Decodable` can still be implemented in cases where `Decodable` is required
 /// by a trait bound.
-pub trait RefDecodable<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> {
+pub trait RefDecodable<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>>: PointeeSized {
     fn decode(d: &mut D) -> &'tcx Self;
 }
 

compiler/rustc_middle/src/ty/context.rs

@@ -8,7 +8,7 @@ use std::assert_matches::{assert_matches, debug_assert_matches};
 use std::borrow::Borrow;
 use std::cmp::Ordering;
 use std::hash::{Hash, Hasher};
-use std::marker::PhantomData;
+use std::marker::{PhantomData, PointeeSized};
 use std::ops::{Bound, Deref};
 use std::sync::{Arc, OnceLock};
 use std::{fmt, iter, mem};
@@ -2421,17 +2421,17 @@ impl<'tcx> TyCtxt<'tcx> {
 // this type just holds a pointer to it, but it still effectively owns it. It
 // impls `Borrow` so that it can be looked up using the original
 // (non-arena-memory-owning) types.
-struct InternedInSet<'tcx, T: ?Sized>(&'tcx T);
+struct InternedInSet<'tcx, T: ?Sized + PointeeSized>(&'tcx T);
 
-impl<'tcx, T: 'tcx + ?Sized> Clone for InternedInSet<'tcx, T> {
+impl<'tcx, T: 'tcx + ?Sized + PointeeSized> Clone for InternedInSet<'tcx, T> {
     fn clone(&self) -> Self {
         InternedInSet(self.0)
     }
 }
 
-impl<'tcx, T: 'tcx + ?Sized> Copy for InternedInSet<'tcx, T> {}
+impl<'tcx, T: 'tcx + ?Sized + PointeeSized> Copy for InternedInSet<'tcx, T> {}
 
-impl<'tcx, T: 'tcx + ?Sized> IntoPointer for InternedInSet<'tcx, T> {
+impl<'tcx, T: 'tcx + ?Sized + PointeeSized> IntoPointer for InternedInSet<'tcx, T> {
     fn into_pointer(&self) -> *const () {
         self.0 as *const _ as *const ()
     }

compiler/rustc_serialize/src/lib.rs

@@ -14,6 +14,7 @@
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(rustdoc_internals)]
+#![feature(sized_hierarchy)]
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 

compiler/rustc_serialize/src/serialize.rs

@@ -4,7 +4,7 @@ use std::borrow::Cow;
 use std::cell::{Cell, RefCell};
 use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet, VecDeque};
 use std::hash::{BuildHasher, Hash};
-use std::marker::PhantomData;
+use std::marker::{PhantomData, PointeeSized};
 use std::num::NonZero;
 use std::path;
 use std::rc::Rc;
@@ -142,7 +142,7 @@ pub trait Decoder {
 ///   `rustc_metadata::rmeta::Lazy`.
 /// * `TyEncodable` should be used for types that are only serialized in crate
 ///   metadata or the incremental cache. This is most types in `rustc_middle`.
-pub trait Encodable<S: Encoder> {
+pub trait Encodable<S: Encoder>: PointeeSized {
     fn encode(&self, s: &mut S);
 }
 
@@ -198,7 +198,7 @@ direct_serialize_impls! {
     char emit_char read_char
 }
 
-impl<S: Encoder, T: ?Sized> Encodable<S> for &T
+impl<S: Encoder, T: ?Sized + PointeeSized> Encodable<S> for &T
 where
     T: Encodable<S>,
 {

compiler/rustc_smir/src/lib.rs

@@ -15,6 +15,7 @@
 )]
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
+#![feature(sized_hierarchy)]
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end
 

compiler/rustc_smir/src/rustc_smir/mod.rs

@@ -7,6 +7,7 @@
 //!
 //! For now, we are developing everything inside `rustc`, thus, we keep this module private.
 
+use std::marker::PointeeSized;
 use std::ops::RangeInclusive;
 
 use rustc_hir::def::DefKind;
@@ -157,7 +158,7 @@ pub(crate) fn new_item_kind(kind: DefKind) -> ItemKind {
 }
 
 /// Trait used to convert between an internal MIR type to a Stable MIR type.
-pub trait Stable<'cx> {
+pub trait Stable<'cx>: PointeeSized {
     /// The stable representation of the type implementing Stable.
     type T;
     /// Converts an object to the equivalent Stable MIR representation.