Auto merge of #137944 - davidtwco:sized-hierarchy, r=oli-obk

Sized Hierarchy: Part I

This patch implements the non-const parts of rust-lang/rfcs#3729. It introduces two new traits to the standard library, `MetaSized` and `PointeeSized`. See the RFC for the rationale behind these traits and to discuss whether this change makes sense in the abstract.

These traits are unstable (as is their constness), so users cannot refer to them without opting-in to `feature(sized_hierarchy)`. These traits are not behind `cfg`s as this would make implementation unfeasible, there would simply be too many `cfg`s required to add the necessary bounds everywhere. So, like `Sized`, these traits are automatically implemented by the compiler.

RFC 3729 describes changes which are necessary to preserve backwards compatibility given the introduction of these traits, which are implemented and as follows:

- `?Sized` is rewritten as `MetaSized`
- `MetaSized` is added as a default supertrait for all traits w/out an explicit sizedness supertrait already.

There are no edition migrations implemented in this,  as these are primarily required for the constness parts of the RFC and prior to stabilisation of this (and so will come in follow-up PRs alongside the const parts). All diagnostic output should remain the same (showing `?Sized` even if the compiler sees `MetaSized`) unless the `sized_hierarchy` feature is enabled.

Due to the use of unstable extern types in the standard library and rustc, some bounds in both projects have had to be relaxed already - this is unfortunate but unavoidable so that these extern types can continue to be used where they were before. Performing these relaxations in the standard library and rustc are desirable longer-term anyway, but some bounds are not as relaxed as they ideally would be due to the inability to relax `Deref::Target` (this will be investigated separately).

It is hoped that this is implemented such that it could be merged and these traits could exist "under the hood" without that being observable to the user (other than in any performance impact this has on the compiler, etc). Some details might leak through due to the standard library relaxations, but this has not been observed in test output.

**Notes:**

- Any commits starting with "upstream:" can be ignored, as these correspond to other upstream PRs that this is based on which have yet to be merged.
- This best reviewed commit-by-commit. I've attempted to make the implementation easy to follow and keep similar changes and test output updates together.
  - Each commit has a short description describing its purpose.
  - This patch is large but it's primarily in the test suite.
- I've worked on the performance of this patch and a few optimisations are implemented so that the performance impact is neutral-to-minor.
- `PointeeSized` is a different name from the RFC just to make it more obvious that it is different from `std::ptr::Pointee` but all the names are yet to be bikeshed anyway.
- `@nikomatsakis` has confirmed [that this can proceed as an experiment from the t-lang side](https://rust-lang.zulipchat.com/#narrow/channel/435869-project-goals/topic/SVE.20and.20SME.20on.20AArch64.20.28goals.23270.29/near/506196491)
- FCP in https://github.com/rust-lang/rust/pull/137944#issuecomment-2912207485

Fixes rust-lang/rust#79409.

r? `@ghost` (I'll discuss this with relevant teams to find a reviewer)

14 files changed, 75 insertions, 17 deletions

diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
index f9f82b80041..419839067f9 100644
--- a/src/bootstrap/src/core/build_steps/test.rs
+++ b/src/bootstrap/src/core/build_steps/test.rs
@@ -2574,7 +2574,7 @@ fn prepare_cargo_test(
     // by `Cargo::new` and that actually makes things go wrong.
     if builder.kind != Kind::Miri {
         let mut dylib_paths = builder.rustc_lib_paths(compiler);
-        dylib_paths.push(PathBuf::from(&builder.sysroot_target_libdir(compiler, target)));
+        dylib_paths.push(builder.sysroot_target_libdir(compiler, target));
         helpers::add_dylib_path(dylib_paths, &mut cargo);
     }
 
diff --git a/src/bootstrap/src/core/build_steps/tool.rs b/src/bootstrap/src/core/build_steps/tool.rs
index 237efaefada..0088e851d39 100644
--- a/src/bootstrap/src/core/build_steps/tool.rs
+++ b/src/bootstrap/src/core/build_steps/tool.rs
@@ -589,7 +589,7 @@ impl ErrorIndex {
         let compiler = builder.compiler_for(builder.top_stage, host, host);
         let mut cmd = command(builder.ensure(ErrorIndex { compiler }).tool_path);
         let mut dylib_paths = builder.rustc_lib_paths(compiler);
-        dylib_paths.push(PathBuf::from(&builder.sysroot_target_libdir(compiler, compiler.host)));
+        dylib_paths.push(builder.sysroot_target_libdir(compiler, compiler.host));
         add_dylib_path(dylib_paths, &mut cmd);
         cmd
     }
diff --git a/src/librustdoc/clean/inline.rs b/src/librustdoc/clean/inline.rs
index 9b5491310b4..dcd499d5f0d 100644
--- a/src/librustdoc/clean/inline.rs
+++ b/src/librustdoc/clean/inline.rs
@@ -265,13 +265,27 @@ pub(crate) fn build_trait(cx: &mut DocContext<'_>, did: DefId) -> clean::Trait {
         .collect();
 
     let generics = clean_ty_generics(cx, did);
-    let (generics, supertrait_bounds) = separate_self_bounds(generics);
+    let (generics, mut supertrait_bounds) = separate_self_bounds(generics);
+
+    supertrait_bounds.retain(|b| {
+        // FIXME(sized-hierarchy): Always skip `MetaSized` bounds so that only `?Sized`
+        // is shown and none of the new sizedness traits leak into documentation.
+        !b.is_meta_sized_bound(cx)
+    });
+
     clean::Trait { def_id: did, generics, items: trait_items, bounds: supertrait_bounds }
 }
 
 fn build_trait_alias(cx: &mut DocContext<'_>, did: DefId) -> clean::TraitAlias {
     let generics = clean_ty_generics(cx, did);
-    let (generics, bounds) = separate_self_bounds(generics);
+    let (generics, mut bounds) = separate_self_bounds(generics);
+
+    bounds.retain(|b| {
+        // FIXME(sized-hierarchy): Always skip `MetaSized` bounds so that only `?Sized`
+        // is shown and none of the new sizedness traits leak into documentation.
+        !b.is_meta_sized_bound(cx)
+    });
+
     clean::TraitAlias { generics, bounds }
 }
 
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index db4bcdaeb6c..d77bdf09d01 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -39,9 +39,9 @@ use rustc_ast::tokenstream::{TokenStream, TokenTree};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet, IndexEntry};
 use rustc_errors::codes::*;
 use rustc_errors::{FatalError, struct_span_code_err};
-use rustc_hir::PredicateOrigin;
 use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::def_id::{DefId, DefIdMap, DefIdSet, LOCAL_CRATE, LocalDefId};
+use rustc_hir::{LangItem, PredicateOrigin};
 use rustc_hir_analysis::hir_ty_lowering::FeedConstTy;
 use rustc_hir_analysis::{lower_const_arg_for_rustdoc, lower_ty};
 use rustc_middle::metadata::Reexport;
@@ -886,6 +886,10 @@ fn clean_ty_generics_inner<'tcx>(
             if b.is_sized_bound(cx) {
                 has_sized = true;
                 false
+            } else if b.is_meta_sized_bound(cx) {
+                // FIXME(sized-hierarchy): Always skip `MetaSized` bounds so that only `?Sized`
+                // is shown and none of the new sizedness traits leak into documentation.
+                false
             } else {
                 true
             }
@@ -1448,6 +1452,13 @@ pub(crate) fn clean_middle_assoc_item(assoc_item: &ty::AssocItem, cx: &mut DocCo
                     }
                     _ => true,
                 });
+
+                bounds.retain(|b| {
+                    // FIXME(sized-hierarchy): Always skip `MetaSized` bounds so that only `?Sized`
+                    // is shown and none of the new sizedness traits leak into documentation.
+                    !b.is_meta_sized_bound(cx)
+                });
+
                 // Our Sized/?Sized bound didn't get handled when creating the generics
                 // because we didn't actually get our whole set of bounds until just now
                 // (some of them may have come from the trait). If we do have a sized
@@ -2276,6 +2287,12 @@ fn clean_middle_opaque_bounds<'tcx>(
                 _ => return None,
             };
 
+            // FIXME(sized-hierarchy): Always skip `MetaSized` bounds so that only `?Sized`
+            // is shown and none of the new sizedness traits leak into documentation.
+            if cx.tcx.is_lang_item(trait_ref.def_id(), LangItem::MetaSized) {
+                return None;
+            }
+
             if let Some(sized) = cx.tcx.lang_items().sized_trait()
                 && trait_ref.def_id() == sized
             {
diff --git a/src/librustdoc/clean/simplify.rs b/src/librustdoc/clean/simplify.rs
index 40efa997868..f813e6c5517 100644
--- a/src/librustdoc/clean/simplify.rs
+++ b/src/librustdoc/clean/simplify.rs
@@ -135,11 +135,17 @@ pub(crate) fn sized_bounds(cx: &mut DocContext<'_>, generics: &mut clean::Generi
     // don't actually know the set of associated types right here so that
     // should be handled when cleaning associated types.
     generics.where_predicates.retain(|pred| {
-        if let WP::BoundPredicate { ty: clean::Generic(param), bounds, .. } = pred
-            && bounds.iter().any(|b| b.is_sized_bound(cx))
-        {
+        let WP::BoundPredicate { ty: clean::Generic(param), bounds, .. } = pred else {
+            return true;
+        };
+
+        if bounds.iter().any(|b| b.is_sized_bound(cx)) {
             sized_params.insert(*param);
             false
+        } else if bounds.iter().any(|b| b.is_meta_sized_bound(cx)) {
+            // FIXME(sized-hierarchy): Always skip `MetaSized` bounds so that only `?Sized`
+            // is shown and none of the new sizedness traits leak into documentation.
+            false
         } else {
             true
         }
diff --git a/src/librustdoc/clean/types.rs b/src/librustdoc/clean/types.rs
index 58e05bd1e85..2d9670a3d10 100644
--- a/src/librustdoc/clean/types.rs
+++ b/src/librustdoc/clean/types.rs
@@ -1289,11 +1289,19 @@ impl GenericBound {
     }
 
     pub(crate) fn is_sized_bound(&self, cx: &DocContext<'_>) -> bool {
+        self.is_bounded_by_lang_item(cx, LangItem::Sized)
+    }
+
+    pub(crate) fn is_meta_sized_bound(&self, cx: &DocContext<'_>) -> bool {
+        self.is_bounded_by_lang_item(cx, LangItem::MetaSized)
+    }
+
+    fn is_bounded_by_lang_item(&self, cx: &DocContext<'_>, lang_item: LangItem) -> bool {
         if let GenericBound::TraitBound(
             PolyTrait { ref trait_, .. },
             rustc_hir::TraitBoundModifiers::NONE,
         ) = *self
-            && Some(trait_.def_id()) == cx.tcx.lang_items().sized_trait()
+            && cx.tcx.is_lang_item(trait_.def_id(), lang_item)
         {
             return true;
         }
diff --git a/src/tools/clippy/clippy_lints/src/methods/unnecessary_to_owned.rs b/src/tools/clippy/clippy_lints/src/methods/unnecessary_to_owned.rs
index fdccf1fb33d..769526d131b 100644
--- a/src/tools/clippy/clippy_lints/src/methods/unnecessary_to_owned.rs
+++ b/src/tools/clippy/clippy_lints/src/methods/unnecessary_to_owned.rs
@@ -388,9 +388,11 @@ fn check_other_call_arg<'tcx>(
         && let (input, n_refs) = peel_middle_ty_refs(*input)
         && let (trait_predicates, _) = get_input_traits_and_projections(cx, callee_def_id, input)
         && let Some(sized_def_id) = cx.tcx.lang_items().sized_trait()
+        && let Some(meta_sized_def_id) = cx.tcx.lang_items().meta_sized_trait()
         && let [trait_predicate] = trait_predicates
             .iter()
             .filter(|trait_predicate| trait_predicate.def_id() != sized_def_id)
+            .filter(|trait_predicate| trait_predicate.def_id() != meta_sized_def_id)
             .collect::<Vec<_>>()[..]
         && let Some(deref_trait_id) = cx.tcx.get_diagnostic_item(sym::Deref)
         && let Some(as_ref_trait_id) = cx.tcx.get_diagnostic_item(sym::AsRef)
diff --git a/src/tools/clippy/clippy_lints/src/needless_borrows_for_generic_args.rs b/src/tools/clippy/clippy_lints/src/needless_borrows_for_generic_args.rs
index 2efb55b9880..8d2f8029112 100644
--- a/src/tools/clippy/clippy_lints/src/needless_borrows_for_generic_args.rs
+++ b/src/tools/clippy/clippy_lints/src/needless_borrows_for_generic_args.rs
@@ -174,6 +174,7 @@ fn needless_borrow_count<'tcx>(
 ) -> usize {
     let destruct_trait_def_id = cx.tcx.lang_items().destruct_trait();
     let sized_trait_def_id = cx.tcx.lang_items().sized_trait();
+    let meta_sized_trait_def_id = cx.tcx.lang_items().meta_sized_trait();
     let drop_trait_def_id = cx.tcx.lang_items().drop_trait();
 
     let fn_sig = cx.tcx.fn_sig(fn_id).instantiate_identity().skip_binder();
@@ -209,6 +210,7 @@ fn needless_borrow_count<'tcx>(
         .all(|trait_def_id| {
             Some(trait_def_id) == destruct_trait_def_id
                 || Some(trait_def_id) == sized_trait_def_id
+                || Some(trait_def_id) == meta_sized_trait_def_id
                 || cx.tcx.is_diagnostic_item(sym::Any, trait_def_id)
         })
     {
diff --git a/src/tools/clippy/clippy_lints/src/needless_pass_by_value.rs b/src/tools/clippy/clippy_lints/src/needless_pass_by_value.rs
index 95623467b81..9aede1dec93 100644
--- a/src/tools/clippy/clippy_lints/src/needless_pass_by_value.rs
+++ b/src/tools/clippy/clippy_lints/src/needless_pass_by_value.rs
@@ -116,13 +116,16 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
         ];
 
         let sized_trait = need!(cx.tcx.lang_items().sized_trait());
+        let meta_sized_trait = need!(cx.tcx.lang_items().meta_sized_trait());
 
         let preds = traits::elaborate(cx.tcx, cx.param_env.caller_bounds().iter())
             .filter(|p| !p.is_global())
             .filter_map(|pred| {
                 // Note that we do not want to deal with qualified predicates here.
                 match pred.kind().no_bound_vars() {
-                    Some(ty::ClauseKind::Trait(pred)) if pred.def_id() != sized_trait => Some(pred),
+                    Some(ty::ClauseKind::Trait(pred))
+                        if pred.def_id() != sized_trait && pred.def_id() != meta_sized_trait
+                            => Some(pred),
                     _ => None,
                 }
             })
diff --git a/src/tools/clippy/tests/ui/def_id_nocore.rs b/src/tools/clippy/tests/ui/def_id_nocore.rs
index 40f40f7ea09..5c13d862276 100644
--- a/src/tools/clippy/tests/ui/def_id_nocore.rs
+++ b/src/tools/clippy/tests/ui/def_id_nocore.rs
@@ -7,8 +7,14 @@
 #[link(name = "c")]
 unsafe extern "C" {}
 
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-pub trait Sized {}
+pub trait Sized: MetaSized {}
 #[lang = "copy"]
 pub trait Copy {}
 #[lang = "freeze"]
diff --git a/src/tools/clippy/tests/ui/def_id_nocore.stderr b/src/tools/clippy/tests/ui/def_id_nocore.stderr
index 2718217313f..175dd075408 100644
--- a/src/tools/clippy/tests/ui/def_id_nocore.stderr
+++ b/src/tools/clippy/tests/ui/def_id_nocore.stderr
@@ -1,5 +1,5 @@
 error: methods called `as_*` usually take `self` by reference or `self` by mutable reference
-  --> tests/ui/def_id_nocore.rs:27:19
+  --> tests/ui/def_id_nocore.rs:33:19
    |
 LL |     pub fn as_ref(self) -> &'static str {
    |                   ^^^^
diff --git a/src/tools/miri/src/shims/native_lib.rs b/src/tools/miri/src/shims/native_lib.rs
index acf258f4eed..030c2e36721 100644
--- a/src/tools/miri/src/shims/native_lib.rs
+++ b/src/tools/miri/src/shims/native_lib.rs
@@ -72,7 +72,7 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
             }
             // Functions with no declared return type (i.e., the default return)
             // have the output_type `Tuple([])`.
-            ty::Tuple(t_list) if t_list.is_empty() => {
+            ty::Tuple(t_list) if (*t_list).deref().is_empty() => {
                 unsafe { ffi::call::<()>(ptr, libffi_args.as_slice()) };
                 return interp_ok(ImmTy::uninit(dest.layout));
             }
diff --git a/src/tools/miri/tests/fail/stacked_borrows/drop_in_place_retag.stderr b/src/tools/miri/tests/fail/stacked_borrows/drop_in_place_retag.stderr
index 32446a8e1ce..ec578db2d0a 100644
--- a/src/tools/miri/tests/fail/stacked_borrows/drop_in_place_retag.stderr
+++ b/src/tools/miri/tests/fail/stacked_borrows/drop_in_place_retag.stderr
@@ -1,8 +1,8 @@
 error: Undefined Behavior: trying to retag from <TAG> for Unique permission at ALLOC[0x0], but that tag only grants SharedReadOnly permission for this location
   --> RUSTLIB/core/src/ptr/mod.rs:LL:CC
    |
-LL | pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this error occurs as part of retag at ALLOC[0x0..0x1]
+LL | pub unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this error occurs as part of retag at ALLOC[0x0..0x1]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
diff --git a/src/tools/miri/tests/fail/unaligned_pointers/drop_in_place.stderr b/src/tools/miri/tests/fail/unaligned_pointers/drop_in_place.stderr
index 1a8509a0b13..72aeb0fdf8e 100644
--- a/src/tools/miri/tests/fail/unaligned_pointers/drop_in_place.stderr
+++ b/src/tools/miri/tests/fail/unaligned_pointers/drop_in_place.stderr
@@ -1,8 +1,8 @@
 error: Undefined Behavior: constructing invalid value: encountered an unaligned reference (required ALIGN byte alignment but found ALIGN)
   --> RUSTLIB/core/src/ptr/mod.rs:LL:CC
    |
-LL | pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
+LL | pub unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information