wip: trait_sel: add sizedness traits to fast path

There's an existing fast path for the `type_op_prove_predicate` predicate
which can be extended to support the new sizedness traits and host effect
predicates, avoiding lots of machinery. This fast path logic can also be
reused as a fast path for within-query trait solving.

Author: davidtwco

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -23,7 +23,8 @@ use rustc_lint::builtin::SELF_CONSTRUCTOR_FROM_OUTER_ITEM;
 use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability};
 use rustc_middle::ty::{
     self, AdtKind, CanonicalUserType, GenericArgKind, GenericArgsRef, GenericParamDefKind,
-    IsIdentity, Ty, TyCtxt, TypeFoldable, TypeVisitable, TypeVisitableExt, UserArgs, UserSelfTy,
+    IsIdentity, SizedTraitKind, Ty, TyCtxt, TypeFoldable, TypeVisitable, TypeVisitableExt,
+    UserArgs, UserSelfTy,
 };
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint;
@@ -439,7 +440,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 || {},
             );
             // Sized types have static alignment, and so do slices.
-            if tail.is_trivially_sized(self.tcx) || matches!(tail.kind(), ty::Slice(..)) {
+            if tail.has_trivial_sizedness(self.tcx, SizedTraitKind::Sized)
+                || matches!(tail.kind(), ty::Slice(..))
+            {
                 // Nothing else is required here.
             } else {
                 // We can't be sure, let's required full `Sized`.

compiler/rustc_middle/src/ty/sty.rs

@@ -16,6 +16,7 @@ use rustc_hir::def_id::DefId;
 use rustc_macros::{HashStable, TyDecodable, TyEncodable, TypeFoldable, extension};
 use rustc_span::{DUMMY_SP, Span, Symbol, sym};
 use rustc_type_ir::TyKind::*;
+use rustc_type_ir::solve::SizedTraitKind;
 use rustc_type_ir::{self as ir, BoundVar, CollectAndApply, DynKind, TypeVisitableExt, elaborate};
 use tracing::instrument;
 use ty::util::{AsyncDropGlueMorphology, IntTypeExt};
@@ -1783,7 +1784,7 @@ impl<'tcx> Ty<'tcx> {
         let Some(pointee_ty) = self.builtin_deref(true) else {
             bug!("Type {self:?} is not a pointer or reference type")
         };
-        if pointee_ty.is_trivially_sized(tcx) {
+        if pointee_ty.has_trivial_sizedness(tcx, SizedTraitKind::Sized) {
             tcx.types.unit
         } else {
             match pointee_ty.ptr_metadata_ty_or_tail(tcx, |x| x) {
@@ -1886,17 +1887,17 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
-    /// Fast path helper for testing if a type is `Sized`.
+    /// Fast path helper for testing if a type is `Sized`, `MetaSized` or `PointeeSized`.
     ///
-    /// Returning true means the type is known to be sized. Returning
-    /// `false` means nothing -- could be sized, might not be.
+    /// Returning true means the type is known to implement the sizedness trait. Returning `false`
+    /// means nothing -- could be sized, might not be.
     ///
-    /// Note that we could never rely on the fact that a type such as `[_]` is
-    /// trivially `!Sized` because we could be in a type environment with a
-    /// bound such as `[_]: Copy`. A function with such a bound obviously never
-    /// can be called, but that doesn't mean it shouldn't typecheck. This is why
-    /// this method doesn't return `Option<bool>`.
-    pub fn is_trivially_sized(self, tcx: TyCtxt<'tcx>) -> bool {
+    /// Note that we could never rely on the fact that a type such as `[_]` is trivially `!Sized`
+    /// because we could be in a type environment with a bound such as `[_]: Copy`. A function with
+    /// such a bound obviously never can be called, but that doesn't mean it shouldn't typecheck.
+    /// This is why this method doesn't return `Option<bool>`.
+    #[instrument(skip(tcx), level = "debug")]
+    pub fn has_trivial_sizedness(self, tcx: TyCtxt<'tcx>, sizedness: SizedTraitKind) -> bool {
         match self.kind() {
             ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
             | ty::Uint(_)
@@ -1919,20 +1920,36 @@ impl<'tcx> Ty<'tcx> {
             | ty::Error(_)
             | ty::Dynamic(_, _, ty::DynStar) => true,
 
-            ty::Str | ty::Slice(_) | ty::Dynamic(_, _, ty::Dyn) | ty::Foreign(..) => false,
+            ty::Str | ty::Slice(_) | ty::Dynamic(_, _, ty::Dyn) => match sizedness {
+                SizedTraitKind::Sized => false,
+                SizedTraitKind::MetaSized | SizedTraitKind::PointeeSized => true,
+            },
+
+            ty::Foreign(..) => match sizedness {
+                SizedTraitKind::Sized | SizedTraitKind::MetaSized => false,
+                SizedTraitKind::PointeeSized => true,
+            },
 
-            ty::Tuple(tys) => tys.last().is_none_or(|ty| ty.is_trivially_sized(tcx)),
+            ty::Tuple(tys) => tys.last().is_none_or(|ty| ty.has_trivial_sizedness(tcx, sizedness)),
 
-            ty::Adt(def, args) => def
-                .sized_constraint(tcx)
-                .is_none_or(|ty| ty.instantiate(tcx, args).is_trivially_sized(tcx)),
+            ty::Adt(def, args) => {
+                let constraint = match sizedness {
+                    SizedTraitKind::Sized => def.sized_constraint(tcx),
+                    SizedTraitKind::MetaSized => def.meta_sized_constraint(tcx),
+                    SizedTraitKind::PointeeSized => def.pointee_sized_constraint(tcx),
+                };
+
+                constraint.is_none_or(|ty| {
+                    ty.instantiate(tcx, args).has_trivial_sizedness(tcx, sizedness)
+                })
+            }
 
             ty::Alias(..) | ty::Param(_) | ty::Placeholder(..) | ty::Bound(..) => false,
 
             ty::Infer(ty::TyVar(_)) => false,
 
             ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
-                bug!("`is_trivially_sized` applied to unexpected type: {:?}", self)
+                bug!("`has_trivial_sizedness` applied to unexpected type: {:?}", self)
             }
         }
     }

compiler/rustc_middle/src/ty/util.rs

@@ -16,6 +16,7 @@ use rustc_index::bit_set::GrowableBitSet;
 use rustc_macros::{HashStable, TyDecodable, TyEncodable, extension};
 use rustc_session::Limit;
 use rustc_span::sym;
+use rustc_type_ir::solve::SizedTraitKind;
 use smallvec::{SmallVec, smallvec};
 use tracing::{debug, instrument};
 
@@ -1189,7 +1190,8 @@ impl<'tcx> Ty<'tcx> {
     /// strange rules like `<T as Foo<'static>>::Bar: Sized` that
     /// actually carry lifetime requirements.
     pub fn is_sized(self, tcx: TyCtxt<'tcx>, typing_env: ty::TypingEnv<'tcx>) -> bool {
-        self.is_trivially_sized(tcx) || tcx.is_sized_raw(typing_env.as_query_input(self))
+        self.has_trivial_sizedness(tcx, SizedTraitKind::Sized)
+            || tcx.is_sized_raw(typing_env.as_query_input(self))
     }
 
     /// Checks whether values of this type `T` implement the `Freeze`

compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs

@@ -196,7 +196,8 @@ where
 /// trait to be implemented (see the trait goal for that), as these are orthogonal. This means that
 /// the effect predicate can succeed while the trait predicate can fail - this is unintuitive but
 /// allows this function to be much simpler.
-// NOTE: Keep this in sync with `evaluate_host_effect_for_sizedness_goal` in the old solver.
+// NOTE: Keep this in sync with `evaluate_host_effect_for_sizedness_goal` in the old solver and
+// `ProvePredicate::try_fast_path`
 #[instrument(level = "trace", skip(cx), ret)]
 pub(in crate::solve) fn const_conditions_for_sizedness<I: Interner>(
     cx: I,

compiler/rustc_trait_selection/src/traits/effects.rs

@@ -253,7 +253,8 @@ fn evaluate_host_effect_from_builtin_impls<'tcx>(
     }
 }
 
-// NOTE: Keep this in sync with `const_conditions_for_sizedness` in the new solver.
+// NOTE: Keep this in sync with `const_conditions_for_sizedness` in the new solver and
+// `ProvePredicate::try_fast_path`
 fn evaluate_host_effect_for_sizedness_goal<'tcx>(
     selcx: &mut SelectionContext<'_, 'tcx>,
     obligation: &HostEffectObligation<'tcx>,

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -24,10 +24,10 @@ use super::{
 };
 use crate::error_reporting::InferCtxtErrorExt;
 use crate::infer::{InferCtxt, TyOrConstInferVar};
-use crate::traits::EvaluateConstErr;
 use crate::traits::normalize::normalize_with_depth_to;
 use crate::traits::project::{PolyProjectionObligation, ProjectionCacheKeyExt as _};
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
+use crate::traits::{EvaluateConstErr, sizedness_fast_path};
 
 pub(crate) type PendingPredicateObligations<'tcx> = ThinVec<PendingPredicateObligation<'tcx>>;
 
@@ -329,6 +329,10 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
 
         let infcx = self.selcx.infcx;
 
+        if sizedness_fast_path(infcx.tcx, obligation.predicate, obligation.param_env) {
+            return ProcessResult::Changed(thin_vec::thin_vec![]);
+        }
+
         if obligation.predicate.has_aliases() {
             let mut obligations = PredicateObligations::new();
             let predicate = normalize_with_depth_to(

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -65,8 +65,8 @@ pub use self::specialize::{
 pub use self::structural_normalize::StructurallyNormalizeExt;
 pub use self::util::{
     BoundVarReplacer, PlaceholderReplacer, elaborate, expand_trait_aliases, impl_item_is_final,
-    supertrait_def_ids, supertraits, transitive_bounds_that_define_assoc_item, upcast_choices,
-    with_replaced_escaping_bound_vars,
+    sizedness_fast_path, supertrait_def_ids, supertraits, transitive_bounds_that_define_assoc_item,
+    upcast_choices, with_replaced_escaping_bound_vars,
 };
 use crate::error_reporting::InferCtxtErrorExt;
 use crate::infer::outlives::env::OutlivesEnvironment;

compiler/rustc_trait_selection/src/traits/query/type_op/prove_predicate.rs

@@ -1,4 +1,3 @@
-use rustc_hir::LangItem;
 use rustc_infer::traits::Obligation;
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::traits::query::NoSolution;
@@ -7,7 +6,7 @@ use rustc_middle::ty::{self, ParamEnvAnd, TyCtxt};
 use rustc_span::Span;
 
 use crate::infer::canonical::{CanonicalQueryInput, CanonicalQueryResponse};
-use crate::traits::ObligationCtxt;
+use crate::traits::{ObligationCtxt, sizedness_fast_path};
 
 impl<'tcx> super::QueryTypeOp<'tcx> for ProvePredicate<'tcx> {
     type QueryResponse = ();
@@ -16,15 +15,7 @@ impl<'tcx> super::QueryTypeOp<'tcx> for ProvePredicate<'tcx> {
         tcx: TyCtxt<'tcx>,
         key: &ParamEnvAnd<'tcx, Self>,
     ) -> Option<Self::QueryResponse> {
-        // 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.
-        if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_ref)) =
-            key.value.predicate.kind().skip_binder()
-            && tcx.is_lang_item(trait_ref.def_id(), LangItem::Sized)
-            && trait_ref.self_ty().is_trivially_sized(tcx)
-        {
+        if sizedness_fast_path(tcx, key.value.predicate, key.param_env) {
             return Some(());
         }
 

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -51,7 +51,9 @@ use crate::infer::{InferCtxt, InferOk, TypeFreshener};
 use crate::solve::InferCtxtSelectExt as _;
 use crate::traits::normalize::{normalize_with_depth, normalize_with_depth_to};
 use crate::traits::project::{ProjectAndUnifyResult, ProjectionCacheKeyExt};
-use crate::traits::{EvaluateConstErr, ProjectionCacheKey, Unimplemented, effects};
+use crate::traits::{
+    EvaluateConstErr, ProjectionCacheKey, Unimplemented, effects, sizedness_fast_path,
+};
 
 mod _match;
 mod candidate_assembly;
@@ -605,6 +607,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             None => self.check_recursion_limit(&obligation, &obligation)?,
         }
 
+        if sizedness_fast_path(self.tcx(), obligation.predicate, obligation.param_env) {
+            return Ok(EvaluatedToOk);
+        }
+
         ensure_sufficient_stack(|| {
             let bound_predicate = obligation.predicate.kind();
             match bound_predicate.skip_binder() {

compiler/rustc_trait_selection/src/traits/util.rs

@@ -1,12 +1,13 @@
 use std::collections::{BTreeMap, VecDeque};
 
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
+use rustc_hir::LangItem;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::InferCtxt;
 pub use rustc_infer::traits::util::*;
 use rustc_middle::bug;
 use rustc_middle::ty::{
-    self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitableExt,
+    self, SizedTraitKind, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitableExt,
 };
 use rustc_span::Span;
 use smallvec::{SmallVec, smallvec};
@@ -504,3 +505,68 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for PlaceholderReplacer<'_, 'tcx> {
         }
     }
 }
+
+pub fn sizedness_fast_path<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    predicate: ty::Predicate<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+) -> bool {
+    // Proving `Sized`/`MetaSized`/`PointeeSized`, 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()
+    {
+        let sizedness = if tcx.is_lang_item(trait_ref.def_id(), LangItem::Sized) {
+            Some(SizedTraitKind::Sized)
+        } else if tcx.is_lang_item(trait_ref.def_id(), LangItem::MetaSized) {
+            Some(SizedTraitKind::MetaSized)
+        } else if tcx.is_lang_item(trait_ref.def_id(), LangItem::PointeeSized) {
+            Some(SizedTraitKind::PointeeSized)
+        } else {
+            None
+        };
+
+        if let Some(sizedness) = sizedness
+            && trait_ref.self_ty().has_trivial_sizedness(tcx, sizedness)
+        {
+            debug!("fast path -- trivial sizedness");
+            return true;
+        }
+
+        if matches!(sizedness, Some(SizedTraitKind::MetaSized)) {
+            let has_sized_in_param_env = param_env
+                .caller_bounds()
+                .iter()
+                .rev() // sizedness predicates are normally at the end for diagnostics reasons
+                .filter_map(|p| p.as_trait_clause())
+                .any(|c| {
+                    trait_ref.self_ty() == c.skip_binder().self_ty()
+                        && tcx.is_lang_item(c.def_id(), LangItem::Sized)
+                });
+            if has_sized_in_param_env {
+                debug!("fast path -- metasized paramenv");
+                return true;
+            }
+        }
+    }
+
+    // Likewise, determining if a sizedness trait is implemented const-ly is a trivial
+    // determination that can happen in the fast path.
+    //
+    // NOTE: Keep this in sync with `evaluate_host_effect_for_sizedness_goal` in the old solver,
+    // `const_conditions_for_sizedness` in the new solver.
+    if let ty::PredicateKind::Clause(ty::ClauseKind::HostEffect(host_pred)) =
+        predicate.kind().skip_binder()
+    {
+        let is_sizedness = tcx.is_lang_item(host_pred.def_id(), LangItem::Sized)
+            || tcx.is_lang_item(host_pred.def_id(), LangItem::MetaSized);
+
+        if is_sizedness && !host_pred.self_ty().has_non_const_sizedness() {
+            debug!("fast path -- host effect");
+            return true;
+        }
+    }
+
+    false
+}

compiler/rustc_traits/src/codegen.rs

@@ -6,11 +6,11 @@
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::bug;
 use rustc_middle::traits::CodegenObligationError;
-use rustc_middle::ty::{self, PseudoCanonicalInput, TyCtxt, TypeVisitableExt};
+use rustc_middle::ty::{self, PseudoCanonicalInput, TyCtxt, TypeVisitableExt, Upcast};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
 use rustc_trait_selection::traits::{
     ImplSource, Obligation, ObligationCause, ObligationCtxt, ScrubbedTraitError, SelectionContext,
-    Unimplemented,
+    Unimplemented, sizedness_fast_path,
 };
 use tracing::debug;
 
@@ -34,6 +34,13 @@ pub(crate) fn codegen_select_candidate<'tcx>(
     let (infcx, param_env) = tcx.infer_ctxt().ignoring_regions().build_with_typing_env(typing_env);
     let mut selcx = SelectionContext::new(&infcx);
 
+    if sizedness_fast_path(tcx, trait_ref.upcast(tcx), param_env) {
+        return Ok(&*tcx.arena.alloc(ImplSource::Builtin(
+            ty::solve::BuiltinImplSource::Trivial,
+            Default::default(),
+        )));
+    }
+
     let obligation_cause = ObligationCause::dummy();
     let obligation = Obligation::new(tcx, obligation_cause, param_env, trait_ref);
 

compiler/rustc_traits/src/evaluate_obligation.rs

@@ -5,6 +5,7 @@ use rustc_span::DUMMY_SP;
 use rustc_trait_selection::traits::query::CanonicalPredicateGoal;
 use rustc_trait_selection::traits::{
     EvaluationResult, Obligation, ObligationCause, OverflowError, SelectionContext, TraitQueryMode,
+    sizedness_fast_path,
 };
 use tracing::debug;
 
@@ -23,6 +24,10 @@ fn evaluate_obligation<'tcx>(
     debug!("evaluate_obligation: goal={:#?}", goal);
     let ParamEnvAnd { param_env, value: predicate } = goal;
 
+    if sizedness_fast_path(tcx, predicate, param_env) {
+        return Ok(EvaluationResult::EvaluatedToOk);
+    }
+
     let mut selcx = SelectionContext::with_query_mode(infcx, TraitQueryMode::Canonical);
     let obligation = Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate);
 

tests/ui/associated-consts/assoc-const-eq-bound-var-in-ty-not-wf.rs

@@ -7,6 +7,7 @@ trait Trait<T> {
 }
 
 fn take(
+//~^ ERROR implementation of `Project` is not general enough
     _: impl Trait<
         <<for<'a> fn(&'a str) -> &'a str as Project>::Out as Discard>::Out,
         K = { () }