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)

10 files changed, 222 insertions, 65 deletions

diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs
index 275b580d794..39f115ce0cd 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs
@@ -199,7 +199,10 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 // avoid inundating the user with unnecessary errors, but we now
                 // check upstream for type errors and don't add the obligations to
                 // begin with in those cases.
-                if self.tcx.is_lang_item(trait_pred.def_id(), LangItem::Sized) {
+                if matches!(
+                    self.tcx.as_lang_item(trait_pred.def_id()),
+                    Some(LangItem::Sized | LangItem::MetaSized)
+                ) {
                     match self.tainted_by_errors() {
                         None => {
                             let err = self.emit_inference_failure_err(
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
index 78f9287b407..d4cc1ceb280 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
@@ -13,7 +13,7 @@ use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_errors::{Applicability, Diag, E0038, E0276, MultiSpan, struct_span_code_err};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::Visitor;
-use rustc_hir::{self as hir, AmbigArg, LangItem};
+use rustc_hir::{self as hir, AmbigArg};
 use rustc_infer::traits::solve::Goal;
 use rustc_infer::traits::{
     DynCompatibilityViolation, Obligation, ObligationCause, ObligationCauseCode,
@@ -160,17 +160,24 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             })
             .collect();
 
-        // Ensure `T: Sized` and `T: WF` obligations come last. This lets us display diagnostics
-        // with more relevant type information and hide redundant E0282 errors.
-        errors.sort_by_key(|e| match e.obligation.predicate.kind().skip_binder() {
-            ty::PredicateKind::Clause(ty::ClauseKind::Trait(pred))
-                if self.tcx.is_lang_item(pred.def_id(), LangItem::Sized) =>
-            {
-                1
+        // Ensure `T: Sized`, `T: MetaSized`, `T: PointeeSized` and `T: WF` obligations come last.
+        // This lets us display diagnostics with more relevant type information and hide redundant
+        // E0282 errors.
+        errors.sort_by_key(|e| {
+            let maybe_sizedness_did = match e.obligation.predicate.kind().skip_binder() {
+                ty::PredicateKind::Clause(ty::ClauseKind::Trait(pred)) => Some(pred.def_id()),
+                ty::PredicateKind::Clause(ty::ClauseKind::HostEffect(pred)) => Some(pred.def_id()),
+                _ => None,
+            };
+
+            match e.obligation.predicate.kind().skip_binder() {
+                _ if maybe_sizedness_did == self.tcx.lang_items().sized_trait() => 1,
+                _ if maybe_sizedness_did == self.tcx.lang_items().meta_sized_trait() => 2,
+                _ if maybe_sizedness_did == self.tcx.lang_items().pointee_sized_trait() => 3,
+                ty::PredicateKind::Coerce(_) => 4,
+                ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(_)) => 5,
+                _ => 0,
             }
-            ty::PredicateKind::Coerce(_) => 2,
-            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(_)) => 3,
-            _ => 0,
         });
 
         for (index, error) in errors.iter().enumerate() {
@@ -327,19 +334,26 @@ pub(crate) fn to_pretty_impl_header(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Opti
     let trait_ref = tcx.impl_trait_ref(impl_def_id)?.instantiate_identity();
     let mut w = "impl".to_owned();
 
-    let args = ty::GenericArgs::identity_for_item(tcx, impl_def_id);
+    #[derive(Debug, Default)]
+    struct SizednessFound {
+        sized: bool,
+        meta_sized: bool,
+    }
 
-    // FIXME: Currently only handles ?Sized.
-    //        Needs to support ?Move and ?DynSized when they are implemented.
-    let mut types_without_default_bounds = FxIndexSet::default();
-    let sized_trait = tcx.lang_items().sized_trait();
+    let mut types_with_sizedness_bounds = FxIndexMap::<_, SizednessFound>::default();
+
+    let args = ty::GenericArgs::identity_for_item(tcx, impl_def_id);
 
     let arg_names = args.iter().map(|k| k.to_string()).filter(|k| k != "'_").collect::<Vec<_>>();
     if !arg_names.is_empty() {
-        types_without_default_bounds.extend(args.types());
         w.push('<');
         w.push_str(&arg_names.join(", "));
         w.push('>');
+
+        for ty in args.types() {
+            // `PointeeSized` params might have no predicates.
+            types_with_sizedness_bounds.insert(ty, SizednessFound::default());
+        }
     }
 
     write!(
@@ -351,24 +365,47 @@ pub(crate) fn to_pretty_impl_header(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Opti
     )
     .unwrap();
 
-    // The predicates will contain default bounds like `T: Sized`. We need to
-    // remove these bounds, and add `T: ?Sized` to any untouched type parameters.
     let predicates = tcx.predicates_of(impl_def_id).predicates;
-    let mut pretty_predicates =
-        Vec::with_capacity(predicates.len() + types_without_default_bounds.len());
+    let mut pretty_predicates = Vec::with_capacity(predicates.len());
+
+    let sized_trait = tcx.lang_items().sized_trait();
+    let meta_sized_trait = tcx.lang_items().meta_sized_trait();
 
     for (p, _) in predicates {
-        if let Some(poly_trait_ref) = p.as_trait_clause() {
-            if Some(poly_trait_ref.def_id()) == sized_trait {
-                // FIXME(#120456) - is `swap_remove` correct?
-                types_without_default_bounds.swap_remove(&poly_trait_ref.self_ty().skip_binder());
+        // Accumulate the sizedness bounds for each self ty.
+        if let Some(trait_clause) = p.as_trait_clause() {
+            let self_ty = trait_clause.self_ty().skip_binder();
+            let sizedness_of = types_with_sizedness_bounds.entry(self_ty).or_default();
+            if Some(trait_clause.def_id()) == sized_trait {
+                sizedness_of.sized = true;
+                continue;
+            } else if Some(trait_clause.def_id()) == meta_sized_trait {
+                sizedness_of.meta_sized = true;
                 continue;
             }
         }
+
         pretty_predicates.push(p.to_string());
     }
 
-    pretty_predicates.extend(types_without_default_bounds.iter().map(|ty| format!("{ty}: ?Sized")));
+    for (ty, sizedness) in types_with_sizedness_bounds {
+        if !tcx.features().sized_hierarchy() {
+            if sizedness.sized {
+                // Maybe a default bound, don't write anything.
+            } else {
+                pretty_predicates.push(format!("{ty}: ?Sized"));
+            }
+        } else {
+            if sizedness.sized {
+                // Maybe a default bound, don't write anything.
+                pretty_predicates.push(format!("{ty}: Sized"));
+            } else if sizedness.meta_sized {
+                pretty_predicates.push(format!("{ty}: MetaSized"));
+            } else {
+                pretty_predicates.push(format!("{ty}: PointeeSized"));
+            }
+        }
+    }
 
     if !pretty_predicates.is_empty() {
         write!(w, "\n  where {}", pretty_predicates.join(", ")).unwrap();
@@ -519,7 +556,7 @@ fn attempt_dyn_to_enum_suggestion(
             let Some(impl_type) = tcx.type_of(*impl_id).no_bound_vars() else { return None };
 
             // Obviously unsized impl types won't be usable in an enum.
-            // Note: this doesn't use `Ty::is_trivially_sized` because that function
+            // Note: this doesn't use `Ty::has_trivial_sizedness` because that function
             // defaults to assuming that things are *not* sized, whereas we want to
             // fall back to assuming that things may be sized.
             match impl_type.kind() {
diff --git a/compiler/rustc_trait_selection/src/solve/delegate.rs b/compiler/rustc_trait_selection/src/solve/delegate.rs
index 69a0c0809b5..b247c2c2968 100644
--- a/compiler/rustc_trait_selection/src/solve/delegate.rs
+++ b/compiler/rustc_trait_selection/src/solve/delegate.rs
@@ -12,7 +12,7 @@ use rustc_infer::traits::solve::Goal;
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::traits::solve::Certainty;
 use rustc_middle::ty::{
-    self, Ty, TyCtxt, TypeFlags, TypeFoldable, TypeVisitableExt as _, TypingMode,
+    self, SizedTraitKind, Ty, TyCtxt, TypeFlags, TypeFoldable, TypeVisitableExt as _, TypingMode,
 };
 use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span};
 
@@ -79,7 +79,14 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
                     Some(LangItem::Sized)
                         if self
                             .resolve_vars_if_possible(trait_pred.self_ty().skip_binder())
-                            .is_trivially_sized(self.0.tcx) =>
+                            .has_trivial_sizedness(self.0.tcx, SizedTraitKind::Sized) =>
+                    {
+                        return Some(Certainty::Yes);
+                    }
+                    Some(LangItem::MetaSized)
+                        if self
+                            .resolve_vars_if_possible(trait_pred.self_ty().skip_binder())
+                            .has_trivial_sizedness(self.0.tcx, SizedTraitKind::MetaSized) =>
                     {
                         return Some(Certainty::Yes);
                     }
diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index 93c7dae9c5b..81893cdcc7e 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -462,6 +462,7 @@ fn impl_intersection_has_negative_obligation(
     let param_env = infcx.resolve_vars_if_possible(param_env);
 
     util::elaborate(tcx, tcx.predicates_of(impl2_def_id).instantiate(tcx, impl2_header.impl_args))
+        .elaborate_sized()
         .any(|(clause, _)| try_prove_negated_where_clause(infcx, clause, param_env))
 }
 
diff --git a/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs b/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs
index 47d207e8d41..ee30956295a 100644
--- a/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs
+++ b/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs
@@ -7,15 +7,15 @@
 use std::ops::ControlFlow;
 
 use rustc_errors::FatalError;
-use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
+use rustc_hir::{self as hir, LangItem};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{
     self, EarlyBinder, GenericArgs, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable,
     TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor, TypingMode, Upcast,
     elaborate,
 };
-use rustc_span::Span;
+use rustc_span::{DUMMY_SP, Span};
 use smallvec::SmallVec;
 use tracing::{debug, instrument};
 
@@ -543,11 +543,11 @@ fn receiver_for_self_ty<'tcx>(
 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result in
 /// a new check that `Trait` is dyn-compatible, creating a cycle.
 /// Instead, we emulate a placeholder by introducing a new type parameter `U` such that
-/// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
+/// `Self: Unsize<U>` and `U: Trait + MetaSized`, and use `U` in place of `dyn Trait`.
 ///
 /// Written as a chalk-style query:
 /// ```ignore (not-rust)
-/// forall (U: Trait + ?Sized) {
+/// forall (U: Trait + MetaSized) {
 ///     if (Self: Unsize<U>) {
 ///         Receiver: DispatchFromDyn<Receiver[Self => U]>
 ///     }
@@ -567,9 +567,10 @@ fn receiver_is_dispatchable<'tcx>(
 ) -> bool {
     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 
-    let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
-    let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
-        debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
+    let (Some(unsize_did), Some(dispatch_from_dyn_did)) =
+        (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait())
+    else {
+        debug!("receiver_is_dispatchable: Missing `Unsize` or `DispatchFromDyn` traits");
         return false;
     };
 
@@ -583,7 +584,7 @@ fn receiver_is_dispatchable<'tcx>(
         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 
     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` (and all of
-    // its supertraits) added to caller bounds. `U: ?Sized` is already implied here.
+    // its supertraits) added to caller bounds. `U: MetaSized` is already implied here.
     let param_env = {
         // N.B. We generally want to emulate the construction of the `unnormalized_param_env`
         // in the param-env query here. The fact that we don't just start with the clauses
@@ -612,6 +613,12 @@ fn receiver_is_dispatchable<'tcx>(
         let trait_predicate = ty::TraitRef::new_from_args(tcx, trait_def_id, args);
         predicates.push(trait_predicate.upcast(tcx));
 
+        let meta_sized_predicate = {
+            let meta_sized_did = tcx.require_lang_item(LangItem::MetaSized, DUMMY_SP);
+            ty::TraitRef::new(tcx, meta_sized_did, [unsized_self_ty]).upcast(tcx)
+        };
+        predicates.push(meta_sized_predicate);
+
         normalize_param_env_or_error(
             tcx,
             ty::ParamEnv::new(tcx.mk_clauses(&predicates)),
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index 97ecf9702e6..81ce58a93fa 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -14,7 +14,7 @@ use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_hir::{self as hir, CoroutineDesugaring, CoroutineKind};
 use rustc_infer::traits::{Obligation, PolyTraitObligation, SelectionError};
 use rustc_middle::ty::fast_reject::DeepRejectCtxt;
-use rustc_middle::ty::{self, Ty, TypeVisitableExt, TypingMode, elaborate};
+use rustc_middle::ty::{self, SizedTraitKind, Ty, TypeVisitableExt, TypingMode, elaborate};
 use rustc_middle::{bug, span_bug};
 use tracing::{debug, instrument, trace};
 
@@ -87,7 +87,21 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     candidates.vec.push(BuiltinCandidate { has_nested: false });
                 }
                 Some(LangItem::Sized) => {
-                    self.assemble_builtin_sized_candidate(obligation, &mut candidates);
+                    self.assemble_builtin_sized_candidate(
+                        obligation,
+                        &mut candidates,
+                        SizedTraitKind::Sized,
+                    );
+                }
+                Some(LangItem::MetaSized) => {
+                    self.assemble_builtin_sized_candidate(
+                        obligation,
+                        &mut candidates,
+                        SizedTraitKind::MetaSized,
+                    );
+                }
+                Some(LangItem::PointeeSized) => {
+                    bug!("`PointeeSized` is removed during lowering");
                 }
                 Some(LangItem::Unsize) => {
                     self.assemble_candidates_for_unsizing(obligation, &mut candidates);
@@ -201,6 +215,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     }
 
                     selcx.infcx.probe(|_| {
+                        let bound = util::lazily_elaborate_sizedness_candidate(
+                            selcx.infcx,
+                            obligation,
+                            bound,
+                        );
+
                         // We checked the polarity already
                         match selcx.match_normalize_trait_ref(
                             obligation,
@@ -245,14 +265,21 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .caller_bounds()
             .iter()
             .filter_map(|p| p.as_trait_clause())
-            // Micro-optimization: filter out predicates relating to different traits.
-            .filter(|p| p.def_id() == stack.obligation.predicate.def_id())
+            // Micro-optimization: filter out predicates with different polarities.
             .filter(|p| p.polarity() == stack.obligation.predicate.polarity());
 
         let drcx = DeepRejectCtxt::relate_rigid_rigid(self.tcx());
         let obligation_args = stack.obligation.predicate.skip_binder().trait_ref.args;
         // Keep only those bounds which may apply, and propagate overflow if it occurs.
         for bound in bounds {
+            let bound =
+                util::lazily_elaborate_sizedness_candidate(self.infcx, stack.obligation, bound);
+
+            // Micro-optimization: filter out predicates relating to different traits.
+            if bound.def_id() != stack.obligation.predicate.def_id() {
+                continue;
+            }
+
             let bound_trait_ref = bound.map_bound(|t| t.trait_ref);
             if !drcx.args_may_unify(obligation_args, bound_trait_ref.skip_binder().args) {
                 continue;
@@ -1086,15 +1113,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
-    /// Assembles the trait which are built-in to the language itself:
-    /// `Copy`, `Clone` and `Sized`.
+    /// Assembles the `Sized` and `MetaSized` traits which are built-in to the language itself.
     #[instrument(level = "debug", skip(self, candidates))]
     fn assemble_builtin_sized_candidate(
         &mut self,
         obligation: &PolyTraitObligation<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
+        sizedness: SizedTraitKind,
     ) {
-        match self.sized_conditions(obligation) {
+        match self.sizedness_conditions(obligation, sizedness) {
             BuiltinImplConditions::Where(nested) => {
                 candidates
                     .vec
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 786afd7cf48..80f71c78993 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -14,7 +14,9 @@ use rustc_hir::lang_items::LangItem;
 use rustc_infer::infer::{DefineOpaqueTypes, HigherRankedType, InferOk};
 use rustc_infer::traits::ObligationCauseCode;
 use rustc_middle::traits::{BuiltinImplSource, SignatureMismatchData};
-use rustc_middle::ty::{self, GenericArgsRef, Region, Ty, TyCtxt, Upcast, elaborate};
+use rustc_middle::ty::{
+    self, GenericArgsRef, Region, SizedTraitKind, Ty, TyCtxt, Upcast, elaborate,
+};
 use rustc_middle::{bug, span_bug};
 use rustc_span::def_id::DefId;
 use thin_vec::thin_vec;
@@ -164,10 +166,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             )
             .break_value()
             .expect("expected to index into clause that exists");
-        let candidate = candidate_predicate
+        let candidate_predicate = candidate_predicate
             .as_trait_clause()
-            .expect("projection candidate is not a trait predicate")
-            .map_bound(|t| t.trait_ref);
+            .expect("projection candidate is not a trait predicate");
+        let candidate_predicate =
+            util::lazily_elaborate_sizedness_candidate(self.infcx, obligation, candidate_predicate);
+
+        let candidate = candidate_predicate.map_bound(|t| t.trait_ref);
 
         let candidate = self.infcx.instantiate_binder_with_fresh_vars(
             obligation.cause.span,
@@ -224,6 +229,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> PredicateObligations<'tcx> {
         debug!(?obligation, ?param, "confirm_param_candidate");
 
+        let param = util::lazily_elaborate_sizedness_candidate(
+            self.infcx,
+            obligation,
+            param.upcast(self.infcx.tcx),
+        )
+        .map_bound(|p| p.trait_ref);
+
         // During evaluation, we already checked that this
         // where-clause trait-ref could be unified with the obligation
         // trait-ref. Repeat that unification now without any
@@ -251,7 +263,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let obligations = if has_nested {
             let trait_def = obligation.predicate.def_id();
             let conditions = match tcx.as_lang_item(trait_def) {
-                Some(LangItem::Sized) => self.sized_conditions(obligation),
+                Some(LangItem::Sized) => {
+                    self.sizedness_conditions(obligation, SizedTraitKind::Sized)
+                }
+                Some(LangItem::MetaSized) => {
+                    self.sizedness_conditions(obligation, SizedTraitKind::MetaSized)
+                }
+                Some(LangItem::PointeeSized) => {
+                    bug!("`PointeeSized` is removing during lowering");
+                }
                 Some(LangItem::Copy | LangItem::Clone) => self.copy_clone_conditions(obligation),
                 Some(LangItem::FusedIterator) => self.fused_iterator_conditions(obligation),
                 other => bug!("unexpected builtin trait {trait_def:?} ({other:?})"),
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 1b9b68fa980..9c0ccb26e53 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -27,8 +27,8 @@ use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::TypeErrorToStringExt;
 use rustc_middle::ty::print::{PrintTraitRefExt as _, with_no_trimmed_paths};
 use rustc_middle::ty::{
-    self, DeepRejectCtxt, GenericArgsRef, PolyProjectionPredicate, Ty, TyCtxt, TypeFoldable,
-    TypeVisitableExt, TypingMode, Upcast, elaborate,
+    self, DeepRejectCtxt, GenericArgsRef, PolyProjectionPredicate, SizedTraitKind, Ty, TyCtxt,
+    TypeFoldable, TypeVisitableExt, TypingMode, Upcast, elaborate,
 };
 use rustc_span::{Symbol, sym};
 use tracing::{debug, instrument, trace};
@@ -2094,9 +2094,10 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
 }
 
 impl<'tcx> SelectionContext<'_, 'tcx> {
-    fn sized_conditions(
+    fn sizedness_conditions(
         &mut self,
         obligation: &PolyTraitObligation<'tcx>,
+        sizedness: SizedTraitKind,
     ) -> BuiltinImplConditions<'tcx> {
         use self::BuiltinImplConditions::{Ambiguous, None, Where};
 
@@ -2126,7 +2127,12 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 Where(ty::Binder::dummy(Vec::new()))
             }
 
-            ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None,
+            ty::Str | ty::Slice(_) | ty::Dynamic(..) => match sizedness {
+                SizedTraitKind::Sized => None,
+                SizedTraitKind::MetaSized => Where(ty::Binder::dummy(Vec::new())),
+            },
+
+            ty::Foreign(..) => None,
 
             ty::Tuple(tys) => Where(
                 obligation.predicate.rebind(tys.last().map_or_else(Vec::new, |&last| vec![last])),
@@ -2135,11 +2141,9 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             ty::Pat(ty, _) => Where(obligation.predicate.rebind(vec![*ty])),
 
             ty::Adt(def, args) => {
-                if let Some(sized_crit) = def.sized_constraint(self.tcx()) {
+                if let Some(crit) = def.sizedness_constraint(self.tcx(), sizedness) {
                     // (*) binder moved here
-                    Where(
-                        obligation.predicate.rebind(vec![sized_crit.instantiate(self.tcx(), args)]),
-                    )
+                    Where(obligation.predicate.rebind(vec![crit.instantiate(self.tcx(), args)]))
                 } else {
                     Where(ty::Binder::dummy(Vec::new()))
                 }
diff --git a/compiler/rustc_trait_selection/src/traits/util.rs b/compiler/rustc_trait_selection/src/traits/util.rs
index 0723aebd5d2..a05bae53566 100644
--- a/compiler/rustc_trait_selection/src/traits/util.rs
+++ b/compiler/rustc_trait_selection/src/traits/util.rs
@@ -4,10 +4,13 @@ use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_hir::LangItem;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::InferCtxt;
+use rustc_infer::traits::PolyTraitObligation;
 pub use rustc_infer::traits::util::*;
 use rustc_middle::bug;
+use rustc_middle::ty::fast_reject::DeepRejectCtxt;
 use rustc_middle::ty::{
-    self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitableExt,
+    self, PolyTraitPredicate, SizedTraitKind, TraitPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
+    TypeFolder, TypeSuperFoldable, TypeVisitableExt,
 };
 pub use rustc_next_trait_solver::placeholder::BoundVarReplacer;
 use rustc_span::Span;
@@ -362,15 +365,19 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for PlaceholderReplacer<'_, 'tcx> {
 }
 
 pub fn sizedness_fast_path<'tcx>(tcx: TyCtxt<'tcx>, predicate: ty::Predicate<'tcx>) -> bool {
-    // Proving `Sized` very often on "obviously sized" types like `&T`, accounts for about 60%
-    // percentage of the predicates we have to prove. No need to canonicalize and all that for
-    // such cases.
+    // Proving `Sized`/`MetaSized`, very often on "obviously sized" types like
+    // `&T`, accounts for about 60% percentage of the predicates we have to prove. No need to
+    // canonicalize and all that for such cases.
     if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_ref)) =
         predicate.kind().skip_binder()
     {
-        if tcx.is_lang_item(trait_ref.def_id(), LangItem::Sized)
-            && trait_ref.self_ty().is_trivially_sized(tcx)
-        {
+        let sizedness = match tcx.as_lang_item(trait_ref.def_id()) {
+            Some(LangItem::Sized) => SizedTraitKind::Sized,
+            Some(LangItem::MetaSized) => SizedTraitKind::MetaSized,
+            _ => return false,
+        };
+
+        if trait_ref.self_ty().has_trivial_sizedness(tcx, sizedness) {
             debug!("fast path -- trivial sizedness");
             return true;
         }
@@ -378,3 +385,39 @@ pub fn sizedness_fast_path<'tcx>(tcx: TyCtxt<'tcx>, predicate: ty::Predicate<'tc
 
     false
 }
+
+/// To improve performance, sizedness traits are not elaborated and so special-casing is required
+/// in the trait solver to find a `Sized` candidate for a `MetaSized` obligation. Returns the
+/// predicate to used in the candidate for such a `obligation`, given a `candidate`.
+pub(crate) fn lazily_elaborate_sizedness_candidate<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    obligation: &PolyTraitObligation<'tcx>,
+    candidate: PolyTraitPredicate<'tcx>,
+) -> PolyTraitPredicate<'tcx> {
+    if !infcx.tcx.is_lang_item(obligation.predicate.def_id(), LangItem::MetaSized)
+        || !infcx.tcx.is_lang_item(candidate.def_id(), LangItem::Sized)
+    {
+        return candidate;
+    }
+
+    if obligation.predicate.polarity() != candidate.polarity() {
+        return candidate;
+    }
+
+    let drcx = DeepRejectCtxt::relate_rigid_rigid(infcx.tcx);
+    if !drcx.args_may_unify(
+        obligation.predicate.skip_binder().trait_ref.args,
+        candidate.skip_binder().trait_ref.args,
+    ) {
+        return candidate;
+    }
+
+    candidate.map_bound(|c| TraitPredicate {
+        trait_ref: TraitRef::new_from_args(
+            infcx.tcx,
+            obligation.predicate.def_id(),
+            c.trait_ref.args,
+        ),
+        polarity: c.polarity,
+    })
+}
diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
index 416865e861e..d4e6a23f0eb 100644
--- a/compiler/rustc_trait_selection/src/traits/wf.rs
+++ b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -567,6 +567,14 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
         def_id: DefId,
         args: GenericArgsRef<'tcx>,
     ) -> PredicateObligations<'tcx> {
+        // PERF: `Sized`'s predicates include `MetaSized`, but both are compiler implemented marker
+        // traits, so `MetaSized` will always be WF if `Sized` is WF and vice-versa. Determining
+        // the nominal obligations of `Sized` would in-effect just elaborate `MetaSized` and make
+        // the compiler do a bunch of work needlessly.
+        if self.tcx().is_lang_item(def_id, LangItem::Sized) {
+            return Default::default();
+        }
+
         let predicates = self.tcx().predicates_of(def_id);
         let mut origins = vec![def_id; predicates.predicates.len()];
         let mut head = predicates;