cache type info for ParamEnv

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

@@ -165,10 +165,7 @@ pub(super) fn explicit_item_bounds_with_filter(
     ty::EarlyBinder::bind(bounds)
 }
 
-pub(super) fn item_bounds(
+pub(super) fn item_bounds(tcx: TyCtxt<'_>, def_id: DefId) -> ty::EarlyBinder<ty::Clauses<'_>> {
-    tcx: TyCtxt<'_>,
-    def_id: DefId,
-) -> ty::EarlyBinder<&'_ ty::List<ty::Clause<'_>>> {
     tcx.explicit_item_bounds(def_id).map_bound(|bounds| {
         tcx.mk_clauses_from_iter(util::elaborate(tcx, bounds.iter().map(|&(bound, _span)| bound)))
     })
@@ -177,7 +174,7 @@ pub(super) fn item_bounds(
 pub(super) fn item_super_predicates(
     tcx: TyCtxt<'_>,
     def_id: DefId,
-) -> ty::EarlyBinder<&'_ ty::List<ty::Clause<'_>>> {
+) -> ty::EarlyBinder<ty::Clauses<'_>> {
     tcx.explicit_item_super_predicates(def_id).map_bound(|bounds| {
         tcx.mk_clauses_from_iter(
             util::elaborate(tcx, bounds.iter().map(|&(bound, _span)| bound)).filter_only_self(),
@@ -188,12 +185,12 @@ pub(super) fn item_super_predicates(
 pub(super) fn item_non_self_assumptions(
     tcx: TyCtxt<'_>,
     def_id: DefId,
-) -> ty::EarlyBinder<&'_ ty::List<ty::Clause<'_>>> {
+) -> ty::EarlyBinder<ty::Clauses<'_>> {
     let all_bounds: FxIndexSet<_> = tcx.item_bounds(def_id).skip_binder().iter().collect();
     let own_bounds: FxIndexSet<_> =
         tcx.item_super_predicates(def_id).skip_binder().iter().collect();
     if all_bounds.len() == own_bounds.len() {
-        ty::EarlyBinder::bind(ty::List::empty())
+        ty::EarlyBinder::bind(ty::ListWithCachedTypeInfo::empty())
     } else {
         ty::EarlyBinder::bind(tcx.mk_clauses_from_iter(all_bounds.difference(&own_bounds).copied()))
     }

compiler/rustc_middle/src/query/erase.rs

@@ -67,6 +67,10 @@ impl<T> EraseType for &'_ ty::List<T> {
     type Result = [u8; size_of::<&'static ty::List<()>>()];
 }
 
+impl<T> EraseType for &'_ ty::ListWithCachedTypeInfo<T> {
+    type Result = [u8; size_of::<&'static ty::ListWithCachedTypeInfo<()>>()];
+}
+
 impl<I: rustc_index::Idx, T> EraseType for &'_ rustc_index::IndexSlice<I, T> {
     type Result = [u8; size_of::<&'static rustc_index::IndexSlice<u32, ()>>()];
 }

compiler/rustc_middle/src/query/keys.rs

@@ -432,7 +432,7 @@ impl<'tcx> Key for (Ty<'tcx>, Ty<'tcx>) {
     }
 }
 
-impl<'tcx> Key for &'tcx ty::List<ty::Clause<'tcx>> {
+impl<'tcx> Key for ty::Clauses<'tcx> {
     type Cache<V> = DefaultCache<Self, V>;
 
     fn default_span(&self, _: TyCtxt<'_>) -> Span {

compiler/rustc_middle/src/query/mod.rs

@@ -398,15 +398,15 @@ rustc_queries! {
     /// ```
     ///
     /// Bounds from the parent (e.g. with nested impl trait) are not included.
-    query item_bounds(key: DefId) -> ty::EarlyBinder<&'tcx ty::List<ty::Clause<'tcx>>> {
+    query item_bounds(key: DefId) -> ty::EarlyBinder<ty::Clauses<'tcx>> {
         desc { |tcx| "elaborating item bounds for `{}`", tcx.def_path_str(key) }
     }
 
-    query item_super_predicates(key: DefId) -> ty::EarlyBinder<&'tcx ty::List<ty::Clause<'tcx>>> {
+    query item_super_predicates(key: DefId) -> ty::EarlyBinder<ty::Clauses<'tcx>> {
         desc { |tcx| "elaborating item assumptions for `{}`", tcx.def_path_str(key) }
     }
 
-    query item_non_self_assumptions(key: DefId) -> ty::EarlyBinder<&'tcx ty::List<ty::Clause<'tcx>>> {
+    query item_non_self_assumptions(key: DefId) -> ty::EarlyBinder<ty::Clauses<'tcx>> {
         desc { |tcx| "elaborating item assumptions for `{}`", tcx.def_path_str(key) }
     }
 
@@ -2156,7 +2156,7 @@ rustc_queries! {
         desc { "resolving instance `{}`", ty::Instance::new(key.value.0, key.value.1) }
     }
 
-    query reveal_opaque_types_in_bounds(key: &'tcx ty::List<ty::Clause<'tcx>>) -> &'tcx ty::List<ty::Clause<'tcx>> {
+    query reveal_opaque_types_in_bounds(key: ty::Clauses<'tcx>) -> ty::Clauses<'tcx> {
         desc { "revealing opaque types in `{:?}`", key }
     }
 

compiler/rustc_middle/src/ty/codec.rs

@@ -414,7 +414,9 @@ impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for ty::List<ty
     }
 }
 
-impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for ty::List<ty::Clause<'tcx>> {
+impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D>
+    for ty::ListWithCachedTypeInfo<ty::Clause<'tcx>>
+{
     fn decode(decoder: &mut D) -> &'tcx Self {
         let len = decoder.read_usize();
         decoder.interner().mk_clauses_from_iter(
@@ -461,7 +463,7 @@ impl_decodable_via_ref! {
     &'tcx mir::BorrowCheckResult<'tcx>,
     &'tcx mir::coverage::CodeRegion,
     &'tcx ty::List<ty::BoundVariableKind>,
-    &'tcx ty::List<ty::Clause<'tcx>>,
+    &'tcx ty::ListWithCachedTypeInfo<ty::Clause<'tcx>>,
     &'tcx ty::List<FieldIdx>,
     &'tcx ty::List<(VariantIdx, FieldIdx)>,
 }

compiler/rustc_middle/src/ty/context.rs

@@ -25,10 +25,10 @@ use crate::traits::solve::{
     ExternalConstraints, ExternalConstraintsData, PredefinedOpaques, PredefinedOpaquesData,
 };
 use crate::ty::{
-    self, AdtDef, AdtDefData, AdtKind, Binder, Clause, Const, ConstData, GenericParamDefKind,
+    self, AdtDef, AdtDefData, AdtKind, Binder, Clause, Clauses, Const, ConstData,
-    ImplPolarity, List, ParamConst, ParamTy, PolyExistentialPredicate, PolyFnSig, Predicate,
+    GenericParamDefKind, ImplPolarity, List, ListWithCachedTypeInfo, ParamConst, ParamTy,
-    PredicateKind, PredicatePolarity, Region, RegionKind, ReprOptions, TraitObjectVisitor, Ty,
+    PolyExistentialPredicate, PolyFnSig, Predicate, PredicateKind, PredicatePolarity, Region,
-    TyKind, TyVid, TypeVisitable, Visibility,
+    RegionKind, ReprOptions, TraitObjectVisitor, Ty, TyKind, TyVid, TypeVisitable, Visibility,
 };
 use crate::ty::{GenericArg, GenericArgs, GenericArgsRef};
 use rustc_ast::{self as ast, attr};
@@ -130,6 +130,7 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
     type SubtypePredicate = ty::SubtypePredicate<'tcx>;
     type CoercePredicate = ty::CoercePredicate<'tcx>;
     type ClosureKind = ty::ClosureKind;
+    type Clauses = ty::Clauses<'tcx>;
 
     fn mk_canonical_var_infos(self, infos: &[ty::CanonicalVarInfo<Self>]) -> Self::CanonicalVars {
         self.mk_canonical_var_infos(infos)
@@ -152,7 +153,7 @@ pub struct CtxtInterners<'tcx> {
     region: InternedSet<'tcx, RegionKind<'tcx>>,
     poly_existential_predicates: InternedSet<'tcx, List<PolyExistentialPredicate<'tcx>>>,
     predicate: InternedSet<'tcx, WithCachedTypeInfo<ty::Binder<'tcx, PredicateKind<'tcx>>>>,
-    clauses: InternedSet<'tcx, List<Clause<'tcx>>>,
+    clauses: InternedSet<'tcx, ListWithCachedTypeInfo<Clause<'tcx>>>,
     projs: InternedSet<'tcx, List<ProjectionKind>>,
     place_elems: InternedSet<'tcx, List<PlaceElem<'tcx>>>,
     const_: InternedSet<'tcx, WithCachedTypeInfo<ConstData<'tcx>>>,
@@ -286,6 +287,24 @@ impl<'tcx> CtxtInterners<'tcx> {
                 .0,
         ))
     }
+
+    fn intern_clauses(&self, clauses: &[Clause<'tcx>]) -> Clauses<'tcx> {
+        if clauses.is_empty() {
+            ListWithCachedTypeInfo::empty()
+        } else {
+            self.clauses
+                .intern_ref(clauses, || {
+                    let flags = super::flags::FlagComputation::for_clauses(clauses);
+
+                    InternedInSet(ListWithCachedTypeInfo::from_arena(
+                        &*self.arena,
+                        flags.into(),
+                        clauses,
+                    ))
+                })
+                .0
+        }
+    }
 }
 
 // For these preinterned values, an alternative would be to have
@@ -1775,6 +1794,29 @@ impl<'tcx, T: Hash> Hash for InternedInSet<'tcx, List<T>> {
     }
 }
 
+impl<'tcx, T> Borrow<[T]> for InternedInSet<'tcx, ListWithCachedTypeInfo<T>> {
+    fn borrow(&self) -> &[T] {
+        &self.0[..]
+    }
+}
+
+impl<'tcx, T: PartialEq> PartialEq for InternedInSet<'tcx, ListWithCachedTypeInfo<T>> {
+    fn eq(&self, other: &InternedInSet<'tcx, ListWithCachedTypeInfo<T>>) -> bool {
+        // The `Borrow` trait requires that `x.borrow() == y.borrow()` equals
+        // `x == y`.
+        self.0[..] == other.0[..]
+    }
+}
+
+impl<'tcx, T: Eq> Eq for InternedInSet<'tcx, ListWithCachedTypeInfo<T>> {}
+
+impl<'tcx, T: Hash> Hash for InternedInSet<'tcx, ListWithCachedTypeInfo<T>> {
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        // The `Borrow` trait requires that `x.borrow().hash(s) == x.hash(s)`.
+        self.0[..].hash(s)
+    }
+}
+
 macro_rules! direct_interners {
     ($($name:ident: $vis:vis $method:ident($ty:ty): $ret_ctor:ident -> $ret_ty:ty,)+) => {
         $(impl<'tcx> Borrow<$ty> for InternedInSet<'tcx, $ty> {
@@ -1850,7 +1892,6 @@ slice_interners!(
     type_lists: pub mk_type_list(Ty<'tcx>),
     canonical_var_infos: pub mk_canonical_var_infos(CanonicalVarInfo<'tcx>),
     poly_existential_predicates: intern_poly_existential_predicates(PolyExistentialPredicate<'tcx>),
-    clauses: intern_clauses(Clause<'tcx>),
     projs: pub mk_projs(ProjectionKind),
     place_elems: pub mk_place_elems(PlaceElem<'tcx>),
     bound_variable_kinds: pub mk_bound_variable_kinds(ty::BoundVariableKind),
@@ -2155,11 +2196,11 @@ impl<'tcx> TyCtxt<'tcx> {
         self.intern_poly_existential_predicates(eps)
     }
 
-    pub fn mk_clauses(self, clauses: &[Clause<'tcx>]) -> &'tcx List<Clause<'tcx>> {
+    pub fn mk_clauses(self, clauses: &[Clause<'tcx>]) -> Clauses<'tcx> {
         // FIXME consider asking the input slice to be sorted to avoid
         // re-interning permutations, in which case that would be asserted
         // here.
-        self.intern_clauses(clauses)
+        self.interners.intern_clauses(clauses)
     }
 
     pub fn mk_local_def_ids(self, clauses: &[LocalDefId]) -> &'tcx List<LocalDefId> {
@@ -2223,7 +2264,7 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn mk_clauses_from_iter<I, T>(self, iter: I) -> T::Output
     where
         I: Iterator<Item = T>,
-        T: CollectAndApply<Clause<'tcx>, &'tcx List<Clause<'tcx>>>,
+        T: CollectAndApply<Clause<'tcx>, Clauses<'tcx>>,
     {
         T::collect_and_apply(iter, |xs| self.mk_clauses(xs))
     }

compiler/rustc_middle/src/ty/flags.rs

@@ -35,6 +35,15 @@ impl FlagComputation {
         result
     }
 
+    pub fn for_clauses(clauses: &[ty::Clause<'_>]) -> FlagComputation {
+        let mut result = FlagComputation::new();
+        for c in clauses {
+            result.add_flags(c.as_predicate().flags());
+            result.add_exclusive_binder(c.as_predicate().outer_exclusive_binder());
+        }
+        result
+    }
+
     fn add_flags(&mut self, flags: TypeFlags) {
         self.flags = self.flags | flags;
     }

compiler/rustc_middle/src/ty/impls_ty.rs

@@ -55,6 +55,16 @@ where
     }
 }
 
+impl<'a, 'tcx, T> HashStable<StableHashingContext<'a>> for &'tcx ty::ListWithCachedTypeInfo<T>
+where
+    T: HashStable<StableHashingContext<'a>>,
+{
+    #[inline]
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        self.as_list().hash_stable(hcx, hasher);
+    }
+}
+
 impl<'a> ToStableHashKey<StableHashingContext<'a>> for SimplifiedType {
     type KeyType = Fingerprint;
 

compiler/rustc_middle/src/ty/list.rs

@@ -1,7 +1,8 @@
+use super::flags::FlagComputation;
+use super::{DebruijnIndex, DebugWithInfcx, InferCtxtLike, TyCtxt, TypeFlags, WithInfcx};
 use crate::arena::Arena;
 use rustc_data_structures::aligned::{align_of, Aligned};
 use rustc_serialize::{Encodable, Encoder};
-use rustc_type_ir::{InferCtxtLike, WithInfcx};
 use std::alloc::Layout;
 use std::cmp::Ordering;
 use std::fmt;
@@ -12,6 +13,9 @@ use std::ops::Deref;
 use std::ptr;
 use std::slice;
 
+#[cfg(parallel_compiler)]
+use rustc_data_structures::sync::DynSync;
+
 /// `List<T>` is a bit like `&[T]`, but with some critical differences.
 /// - IMPORTANT: Every `List<T>` is *required* to have unique contents. The
 ///   type's correctness relies on this, *but it does not enforce it*.
@@ -30,13 +34,32 @@ use std::slice;
 /// - `T` must not be zero-sized.
 #[repr(C)]
 pub struct List<T> {
+    skel: ListSkeleton<T>,
+    opaque: OpaqueListContents,
+}
+
+/// This type defines the statically known field offsets and alignment of a [`List`].
+///
+/// The alignment of a `List` cannot be determined, because it has an extern type tail.
+/// We use `ListSkeleton` instead of `List` whenever computing the alignment is required,
+/// for example:
+/// - Implementing the [`Aligned`] trait, which is required for [`CopyTaggedPtr`].
+/// - Projecting from [`ListWithCachedTypeInfo`] to `List`, which requires computing the padding
+///   between the cached type info and the list, which requires computing the list's alignment.
+///
+/// Note that `ListSkeleton` is `Sized`, but **it's size is not correct**, as it is missing the
+/// dynamically sized list tail. Do not create a `ListSkeleton` on the stack.
+///
+/// FIXME: This can be removed once we properly support `!Sized + Aligned + Thin` types.
+///
+/// [`CopyTaggedPtr`]: rustc_data_structures::tagged_ptr::CopyTaggedPtr
+#[repr(C)]
+struct ListSkeleton<T> {
     len: usize,
 
     /// Although this claims to be a zero-length array, in practice `len`
     /// elements are actually present.
     data: [T; 0],
-
-    opaque: OpaqueListContents,
 }
 
 extern "C" {
@@ -49,25 +72,15 @@ impl<T> List<T> {
     /// Returns a reference to the (unique, static) empty list.
     #[inline(always)]
     pub fn empty<'a>() -> &'a List<T> {
-        #[repr(align(64))]
+        ListWithCachedTypeInfo::empty()
-        struct MaxAlign;
-
-        assert!(mem::align_of::<T>() <= mem::align_of::<MaxAlign>());
-
-        #[repr(C)]
-        struct InOrder<T, U>(T, U);
-
-        // The empty slice is static and contains a single `0` usize (for the
-        // length) that is 64-byte aligned, thus featuring the necessary
-        // trailing padding for elements with up to 64-byte alignment.
-        static EMPTY_SLICE: InOrder<usize, MaxAlign> = InOrder(0, MaxAlign);
-        unsafe { &*(std::ptr::addr_of!(EMPTY_SLICE) as *const List<T>) }
     }
 
+    #[inline(always)]
     pub fn len(&self) -> usize {
-        self.len
+        self.skel.len
     }
 
+    #[inline(always)]
     pub fn as_slice(&self) -> &[T] {
         self
     }
@@ -94,10 +107,10 @@ impl<T: Copy> List<T> {
         let mem = arena.dropless.alloc_raw(layout) as *mut List<T>;
         unsafe {
             // Write the length
-            ptr::addr_of_mut!((*mem).len).write(slice.len());
+            ptr::addr_of_mut!((*mem).skel.len).write(slice.len());
 
             // Write the elements
-            ptr::addr_of_mut!((*mem).data)
+            ptr::addr_of_mut!((*mem).skel.data)
                 .cast::<T>()
                 .copy_from_nonoverlapping(slice.as_ptr(), slice.len());
 
@@ -120,7 +133,7 @@ impl<T: fmt::Debug> fmt::Debug for List<T> {
         (**self).fmt(f)
     }
 }
-impl<'tcx, T: super::DebugWithInfcx<TyCtxt<'tcx>>> super::DebugWithInfcx<TyCtxt<'tcx>> for List<T> {
+impl<'tcx, T: DebugWithInfcx<TyCtxt<'tcx>>> DebugWithInfcx<TyCtxt<'tcx>> for List<T> {
     fn fmt<Infcx: InferCtxtLike<Interner = TyCtxt<'tcx>>>(
         this: WithInfcx<'_, Infcx, &Self>,
         f: &mut core::fmt::Formatter<'_>,
@@ -178,7 +191,7 @@ impl<T> Hash for List<T> {
     fn hash<H: Hasher>(&self, s: &mut H) {
         // Pointer hashing is sufficient (due to the unique contents
         // assumption).
-        (self as *const List<T>).hash(s)
+        ptr::from_ref(self).hash(s)
     }
 }
 
@@ -193,7 +206,12 @@ impl<T> Deref for List<T> {
 impl<T> AsRef<[T]> for List<T> {
     #[inline(always)]
     fn as_ref(&self) -> &[T] {
-        unsafe { slice::from_raw_parts(self.data.as_ptr(), self.len) }
+        let data_ptr = ptr::addr_of!(self.skel.data).cast::<T>();
+        // SAFETY: `data_ptr` has the same provenance as `self` and can therefore
+        // access the `self.skel.len` elements stored at `self.skel.data`.
+        // Note that we specifically don't reborrow `&self.skel.data`, because that
+        // would give us a pointer with provenance over 0 bytes.
+        unsafe { slice::from_raw_parts(data_ptr, self.skel.len) }
     }
 }
 
@@ -210,23 +228,221 @@ unsafe impl<T: Sync> Sync for List<T> {}
 
 // We need this since `List` uses extern type `OpaqueListContents`.
 #[cfg(parallel_compiler)]
-use rustc_data_structures::sync::DynSync;
-
-use super::TyCtxt;
-#[cfg(parallel_compiler)]
 unsafe impl<T: DynSync> DynSync for List<T> {}
 
 // Safety:
-// Layouts of `Equivalent<T>` and `List<T>` are the same, modulo opaque tail,
+// Layouts of `ListSkeleton<T>` and `List<T>` are the same, modulo opaque tail,
-// thus aligns of `Equivalent<T>` and `List<T>` must be the same.
+// thus aligns of `ListSkeleton<T>` and `List<T>` must be the same.
 unsafe impl<T> Aligned for List<T> {
-    const ALIGN: ptr::Alignment = {
+    const ALIGN: ptr::Alignment = align_of::<ListSkeleton<T>>();
+}
+
+/// A [`List`] that additionally stores type information inline to speed up
+/// [`TypeVisitableExt`](super::TypeVisitableExt) operations.
+#[repr(C)]
+pub struct ListWithCachedTypeInfo<T> {
+    skel: ListWithCachedTypeInfoSkeleton<T>,
+    opaque: OpaqueListContents,
+}
+
+/// The additional info that is stored in [`ListWithCachedTypeInfo`].
+#[repr(C)]
+pub struct TypeInfo {
+    flags: TypeFlags,
+    outer_exclusive_binder: DebruijnIndex,
+}
+
+impl From<FlagComputation> for TypeInfo {
+    fn from(computation: FlagComputation) -> TypeInfo {
+        TypeInfo {
+            flags: computation.flags,
+            outer_exclusive_binder: computation.outer_exclusive_binder,
+        }
+    }
+}
+
+/// This type is similar to [`ListSkeleton`], but for [`ListWithCachedTypeInfo`].
+/// It is used for computing the alignment of a [`ListWithCachedTypeInfo`].
+#[repr(C)]
+struct ListWithCachedTypeInfoSkeleton<T> {
+    info: TypeInfo,
+    // N.B.: There may be padding between these two fields. We cannot use `List` directly
+    // here, because it has an unknown alignment which makes computing the amount of padding
+    // and therefore projecting from `&ListWithCachedTypeInfo` to `&List` impossible.
+    list: ListSkeleton<T>,
+}
+
+impl<T> ListWithCachedTypeInfo<T> {
+    #[inline(always)]
+    pub fn empty<'a>() -> &'a ListWithCachedTypeInfo<T> {
+        #[repr(align(64))]
+        struct MaxAlign;
+
         #[repr(C)]
-        struct Equivalent<T> {
+        struct Empty {
-            _len: usize,
+            info: TypeInfo,
-            _data: [T; 0],
+            zero: [u8; 2 * mem::align_of::<MaxAlign>() - mem::size_of::<TypeInfo>()],
+            align: MaxAlign,
         }
 
-        align_of::<Equivalent<T>>()
+        static EMPTY: Empty = Empty {
+            info: TypeInfo { flags: TypeFlags::empty(), outer_exclusive_binder: super::INNERMOST },
+            zero: [0; 2 * mem::align_of::<MaxAlign>() - mem::size_of::<TypeInfo>()],
+            align: MaxAlign,
         };
+
+        assert!(mem::align_of::<T>() <= mem::align_of::<MaxAlign>());
+
+        // The layout of the empty `ListWithCachedTypeInfo<T>` must be one of the following,
+        // depending on the alignment of `T`:
+        //
+        // On 64-bit platforms:
+        // F = flags (32 bit), B = outer_exclusive_binder (32 bit), LL = len (64 bit)
+        // align(T) <= 8: FBLL
+        // align(T) = 16: FB..LL..
+        // align(T) = 32: FB......LL......
+        // align(T) = 64: FB..............LL..............
+        //
+        // On 32-bit platforms:
+        // F = flags (32 bit), B = outer_exclusive_binder (32 bit), L = len (32 bit)
+        // align(T) <= 4: FBL
+        // align(T) =  8: FBL.
+        // align(T) = 16: FB..L...
+        // align(T) = 32: FB......L.......
+        // align(T) = 64: FB..............L...............
+        //
+        // We zero out every possible location of `len` so that `EMPTY` is a valid
+        // `ListWithCachedTypeInfo<T>` for all `T` with alignment up to 64 bytes.
+        unsafe { &*(std::ptr::addr_of!(EMPTY) as *const ListWithCachedTypeInfo<T>) }
+    }
+
+    #[inline]
+    pub(super) fn from_arena<'tcx>(
+        arena: &'tcx Arena<'tcx>,
+        info: TypeInfo,
+        slice: &[T],
+    ) -> &'tcx ListWithCachedTypeInfo<T>
+    where
+        T: Copy,
+    {
+        assert!(!mem::needs_drop::<T>());
+        assert!(mem::size_of::<T>() != 0);
+        assert!(!slice.is_empty());
+
+        let (list_layout, _offset) =
+            Layout::new::<usize>().extend(Layout::for_value::<[T]>(slice)).unwrap();
+
+        let (layout, _offset) = Layout::new::<TypeInfo>().extend(list_layout).unwrap();
+
+        let mem = arena.dropless.alloc_raw(layout) as *mut ListWithCachedTypeInfo<T>;
+        unsafe {
+            // Write the cached type info
+            ptr::addr_of_mut!((*mem).skel.info).write(info);
+
+            // Write the length
+            ptr::addr_of_mut!((*mem).skel.list.len).write(slice.len());
+
+            // Write the elements
+            ptr::addr_of_mut!((*mem).skel.list.data)
+                .cast::<T>()
+                .copy_from_nonoverlapping(slice.as_ptr(), slice.len());
+
+            &*mem
+        }
+    }
+
+    #[inline(always)]
+    pub fn as_list(&self) -> &List<T> {
+        self
+    }
+
+    #[inline(always)]
+    pub fn flags(&self) -> TypeFlags {
+        self.skel.info.flags
+    }
+
+    #[inline(always)]
+    pub fn outer_exclusive_binder(&self) -> DebruijnIndex {
+        self.skel.info.outer_exclusive_binder
+    }
+}
+
+impl<T> Deref for ListWithCachedTypeInfo<T> {
+    type Target = List<T>;
+    #[inline(always)]
+    fn deref(&self) -> &List<T> {
+        let list_ptr = ptr::addr_of!(self.skel.list) as *const List<T>;
+        unsafe { &*list_ptr }
+    }
+}
+
+impl<T> AsRef<[T]> for ListWithCachedTypeInfo<T> {
+    #[inline(always)]
+    fn as_ref(&self) -> &[T] {
+        (&**self).as_ref()
+    }
+}
+
+impl<T: fmt::Debug> fmt::Debug for ListWithCachedTypeInfo<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+impl<'tcx, T: DebugWithInfcx<TyCtxt<'tcx>>> DebugWithInfcx<TyCtxt<'tcx>>
+    for ListWithCachedTypeInfo<T>
+{
+    fn fmt<Infcx: InferCtxtLike<Interner = TyCtxt<'tcx>>>(
+        this: WithInfcx<'_, Infcx, &Self>,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> core::fmt::Result {
+        DebugWithInfcx::fmt(this.map(|this| &**this), f)
+    }
+}
+
+impl<S: Encoder, T: Encodable<S>> Encodable<S> for ListWithCachedTypeInfo<T> {
+    #[inline]
+    fn encode(&self, s: &mut S) {
+        (**self).encode(s);
+    }
+}
+
+impl<T: PartialEq> PartialEq for ListWithCachedTypeInfo<T> {
+    #[inline]
+    fn eq(&self, other: &ListWithCachedTypeInfo<T>) -> bool {
+        // Pointer equality implies list equality (due to the unique contents
+        // assumption).
+        ptr::eq(self, other)
+    }
+}
+
+impl<T: Eq> Eq for ListWithCachedTypeInfo<T> {}
+
+impl<T> Hash for ListWithCachedTypeInfo<T> {
+    #[inline]
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        // Pointer hashing is sufficient (due to the unique contents
+        // assumption).
+        ptr::from_ref(self).hash(s)
+    }
+}
+
+impl<'a, T: Copy> IntoIterator for &'a ListWithCachedTypeInfo<T> {
+    type Item = T;
+    type IntoIter = iter::Copied<<&'a [T] as IntoIterator>::IntoIter>;
+    #[inline(always)]
+    fn into_iter(self) -> Self::IntoIter {
+        (**self).into_iter()
+    }
+}
+
+unsafe impl<T: Sync> Sync for ListWithCachedTypeInfo<T> {}
+
+#[cfg(parallel_compiler)]
+unsafe impl<T: DynSync> DynSync for ListWithCachedTypeInfo<T> {}
+
+// Safety:
+// Layouts of `ListWithCachedTypeInfoSkeleton<T>` and `ListWithCachedTypeInfo<T>`
+// are the same, modulo opaque tail, thus their aligns must be the same.
+unsafe impl<T> Aligned for ListWithCachedTypeInfo<T> {
+    const ALIGN: ptr::Alignment = align_of::<ListWithCachedTypeInfoSkeleton<T>>();
 }

compiler/rustc_middle/src/ty/mod.rs

@@ -89,7 +89,7 @@ pub use self::context::{
     TyCtxt, TyCtxtFeed,
 };
 pub use self::instance::{Instance, InstanceDef, ReifyReason, ShortInstance, UnusedGenericParams};
-pub use self::list::List;
+pub use self::list::{List, ListWithCachedTypeInfo};
 pub use self::parameterized::ParameterizedOverTcx;
 pub use self::predicate::{
     Clause, ClauseKind, CoercePredicate, ExistentialPredicate, ExistentialProjection,
@@ -1034,6 +1034,18 @@ impl PlaceholderLike for PlaceholderConst {
     }
 }
 
+pub type Clauses<'tcx> = &'tcx ListWithCachedTypeInfo<Clause<'tcx>>;
+
+impl<'tcx> rustc_type_ir::visit::Flags for Clauses<'tcx> {
+    fn flags(&self) -> TypeFlags {
+        (**self).flags()
+    }
+
+    fn outer_exclusive_binder(&self) -> DebruijnIndex {
+        (**self).outer_exclusive_binder()
+    }
+}
+
 /// When interacting with the type system we must provide information about the
 /// environment. `ParamEnv` is the type that represents this information. See the
 /// [dev guide chapter][param_env_guide] for more information.
@@ -1053,7 +1065,7 @@ pub struct ParamEnv<'tcx> {
     /// want `Reveal::All`.
     ///
     /// Note: This is packed, use the reveal() method to access it.
-    packed: CopyTaggedPtr<&'tcx List<Clause<'tcx>>, ParamTag, true>,
+    packed: CopyTaggedPtr<Clauses<'tcx>, ParamTag, true>,
 }
 
 #[derive(Copy, Clone)]
@@ -1112,11 +1124,11 @@ impl<'tcx> ParamEnv<'tcx> {
     /// [param_env_guide]: https://rustc-dev-guide.rust-lang.org/param_env/param_env_summary.html
     #[inline]
     pub fn empty() -> Self {
-        Self::new(List::empty(), Reveal::UserFacing)
+        Self::new(ListWithCachedTypeInfo::empty(), Reveal::UserFacing)
     }
 
     #[inline]
-    pub fn caller_bounds(self) -> &'tcx List<Clause<'tcx>> {
+    pub fn caller_bounds(self) -> Clauses<'tcx> {
         self.packed.pointer()
     }
 
@@ -1134,12 +1146,12 @@ impl<'tcx> ParamEnv<'tcx> {
     /// or invoke `param_env.with_reveal_all()`.
     #[inline]
     pub fn reveal_all() -> Self {
-        Self::new(List::empty(), Reveal::All)
+        Self::new(ListWithCachedTypeInfo::empty(), Reveal::All)
     }
 
     /// Construct a trait environment with the given set of predicates.
     #[inline]
-    pub fn new(caller_bounds: &'tcx List<Clause<'tcx>>, reveal: Reveal) -> Self {
+    pub fn new(caller_bounds: Clauses<'tcx>, reveal: Reveal) -> Self {
         ty::ParamEnv { packed: CopyTaggedPtr::new(caller_bounds, ParamTag { reveal }) }
     }
 
@@ -1168,7 +1180,7 @@ impl<'tcx> ParamEnv<'tcx> {
     /// Returns this same environment but with no caller bounds.
     #[inline]
     pub fn without_caller_bounds(self) -> Self {
-        Self::new(List::empty(), self.reveal())
+        Self::new(ListWithCachedTypeInfo::empty(), self.reveal())
     }
 
     /// Creates a pair of param-env and value for use in queries.

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -712,7 +712,19 @@ impl<'tcx> TypeSuperVisitable<TyCtxt<'tcx>> for ty::Predicate<'tcx> {
     }
 }
 
-impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<ty::Clause<'tcx>> {
+impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for ty::Clauses<'tcx> {
+    fn visit_with<V: TypeVisitor<TyCtxt<'tcx>>>(&self, visitor: &mut V) -> V::Result {
+        visitor.visit_clauses(self)
+    }
+}
+
+impl<'tcx> TypeSuperVisitable<TyCtxt<'tcx>> for ty::Clauses<'tcx> {
+    fn super_visit_with<V: TypeVisitor<TyCtxt<'tcx>>>(&self, visitor: &mut V) -> V::Result {
+        self.as_slice().visit_with(visitor)
+    }
+}
+
+impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ty::Clauses<'tcx> {
     fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>(
         self,
         folder: &mut F,

compiler/rustc_middle/src/ty/util.rs

@@ -1608,16 +1608,18 @@ pub fn is_trivially_const_drop(ty: Ty<'_>) -> bool {
 /// let v = self.iter().map(|p| p.fold_with(folder)).collect::<SmallVec<[_; 8]>>();
 /// folder.tcx().intern_*(&v)
 /// ```
-pub fn fold_list<'tcx, F, T>(
+pub fn fold_list<'tcx, F, L, T>(
-    list: &'tcx ty::List<T>,
+    list: L,
     folder: &mut F,
-    intern: impl FnOnce(TyCtxt<'tcx>, &[T]) -> &'tcx ty::List<T>,
+    intern: impl FnOnce(TyCtxt<'tcx>, &[T]) -> L,
-) -> Result<&'tcx ty::List<T>, F::Error>
+) -> Result<L, F::Error>
 where
     F: FallibleTypeFolder<TyCtxt<'tcx>>,
+    L: AsRef<[T]>,
     T: TypeFoldable<TyCtxt<'tcx>> + PartialEq + Copy,
 {
-    let mut iter = list.iter();
+    let slice = list.as_ref();
+    let mut iter = slice.iter().copied();
     // Look for the first element that changed
     match iter.by_ref().enumerate().find_map(|(i, t)| match t.try_fold_with(folder) {
         Ok(new_t) if new_t == t => None,
@@ -1625,8 +1627,8 @@ where
     }) {
         Some((i, Ok(new_t))) => {
             // An element changed, prepare to intern the resulting list
-            let mut new_list = SmallVec::<[_; 8]>::with_capacity(list.len());
+            let mut new_list = SmallVec::<[_; 8]>::with_capacity(slice.len());
-            new_list.extend_from_slice(&list[..i]);
+            new_list.extend_from_slice(&slice[..i]);
             new_list.push(new_t);
             for t in iter {
                 new_list.push(t.try_fold_with(folder)?)
@@ -1647,8 +1649,8 @@ pub struct AlwaysRequiresDrop;
 /// with their underlying types.
 pub fn reveal_opaque_types_in_bounds<'tcx>(
     tcx: TyCtxt<'tcx>,
-    val: &'tcx ty::List<ty::Clause<'tcx>>,
+    val: ty::Clauses<'tcx>,
-) -> &'tcx ty::List<ty::Clause<'tcx>> {
+) -> ty::Clauses<'tcx> {
     let mut visitor = OpaqueTypeExpander {
         seen_opaque_tys: FxHashSet::default(),
         expanded_cache: FxHashMap::default(),

compiler/rustc_type_ir/src/interner.rs

@@ -100,6 +100,7 @@ pub trait Interner: Sized + Copy {
     type SubtypePredicate: Copy + Debug + Hash + Eq;
     type CoercePredicate: Copy + Debug + Hash + Eq;
     type ClosureKind: Copy + Debug + Hash + Eq;
+    type Clauses: Copy + Debug + Hash + Eq + TypeSuperVisitable<Self> + Flags;
 
     fn mk_canonical_var_infos(self, infos: &[CanonicalVarInfo<Self>]) -> Self::CanonicalVars;
 }

compiler/rustc_type_ir/src/visit.rs

@@ -110,6 +110,10 @@ pub trait TypeVisitor<I: Interner>: Sized {
     fn visit_predicate(&mut self, p: I::Predicate) -> Self::Result {
         p.super_visit_with(self)
     }
+
+    fn visit_clauses(&mut self, p: I::Clauses) -> Self::Result {
+        p.super_visit_with(self)
+    }
 }
 
 ///////////////////////////////////////////////////////////////////////////
@@ -423,6 +427,16 @@ impl<I: Interner> TypeVisitor<I> for HasTypeFlagsVisitor {
             ControlFlow::Continue(())
         }
     }
+
+    #[inline]
+    fn visit_clauses(&mut self, clauses: I::Clauses) -> Self::Result {
+        // Note: no `super_visit_with` call.
+        if clauses.flags().intersects(self.flags) {
+            ControlFlow::Break(FoundFlags)
+        } else {
+            ControlFlow::Continue(())
+        }
+    }
 }
 
 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
@@ -515,6 +529,15 @@ impl<I: Interner> TypeVisitor<I> for HasEscapingVarsVisitor {
             ControlFlow::Continue(())
         }
     }
+
+    #[inline]
+    fn visit_clauses(&mut self, clauses: I::Clauses) -> Self::Result {
+        if clauses.outer_exclusive_binder() > self.outer_index {
+            ControlFlow::Break(FoundEscapingVars)
+        } else {
+            ControlFlow::Continue(())
+        }
+    }
 }
 
 struct HasErrorVisitor;