Overhaul PredicateInner and Predicate.

Specifically, change `Ty` from this:
```
pub struct Predicate<'tcx> { inner: &'tcx PredicateInner<'tcx> }
```
to this:
```
pub struct Predicate<'tcx>(&'tcx Interned<PredicateS<'tcx>>)
```
where `PredicateInner` is renamed as `PredicateS`.

 This (plus a few other minor changes) makes the parallels with `Ty` and
`TyS` much clearer, and makes the uniqueness more explicit.

compiler/rustc_middle/src/arena.rs

@@ -88,7 +88,7 @@ macro_rules! arena_types {
 
             // Interned types
             [] tys: rustc_middle::ty::TyS<'tcx>,
-            [] predicates: rustc_middle::ty::PredicateInner<'tcx>,
+            [] predicates: rustc_middle::ty::PredicateS<'tcx>,
 
             // Note that this deliberately duplicates items in the `rustc_hir::arena`,
             // since we need to allocate this type on both the `rustc_hir` arena

compiler/rustc_middle/src/ty/context.rs

@@ -20,7 +20,7 @@ use crate::ty::{
     self, AdtDef, AdtKind, Binder, BindingMode, BoundVar, CanonicalPolyFnSig,
     ClosureSizeProfileData, Const, ConstVid, DefIdTree, ExistentialPredicate, FloatTy, FloatVar,
     FloatVid, GenericParamDefKind, InferConst, InferTy, IntTy, IntVar, IntVid, List, ParamConst,
-    ParamTy, PolyFnSig, Predicate, PredicateInner, PredicateKind, ProjectionTy, Region, RegionKind,
+    ParamTy, PolyFnSig, Predicate, PredicateKind, PredicateS, ProjectionTy, Region, RegionKind,
     ReprOptions, TraitObjectVisitor, Ty, TyKind, TyS, TyVar, TyVid, TypeAndMut, UintTy,
 };
 use rustc_ast as ast;
@@ -107,7 +107,7 @@ pub struct CtxtInterners<'tcx> {
     region: InternedSet<'tcx, RegionKind>,
     poly_existential_predicates:
         InternedSet<'tcx, List<ty::Binder<'tcx, ExistentialPredicate<'tcx>>>>,
-    predicate: InternedSet<'tcx, PredicateInner<'tcx>>,
+    predicate: InternedSet<'tcx, PredicateS<'tcx>>,
     predicates: InternedSet<'tcx, List<Predicate<'tcx>>>,
     projs: InternedSet<'tcx, List<ProjectionKind>>,
     place_elems: InternedSet<'tcx, List<PlaceElem<'tcx>>>,
@@ -170,15 +170,13 @@ impl<'tcx> CtxtInterners<'tcx> {
     }
 
     #[inline(never)]
-    fn intern_predicate(
-        &self,
-        kind: Binder<'tcx, PredicateKind<'tcx>>,
-    ) -> &'tcx PredicateInner<'tcx> {
+    fn intern_predicate(&self, kind: Binder<'tcx, PredicateKind<'tcx>>) -> Predicate<'tcx> {
+        Predicate(Interned::new_unchecked(
             self.predicate
                 .intern(kind, |kind| {
                     let flags = super::flags::FlagComputation::for_predicate(kind);
 
-                let predicate_struct = PredicateInner {
+                    let predicate_struct = PredicateS {
                         kind,
                         flags: flags.flags,
                         outer_exclusive_binder: flags.outer_exclusive_binder,
@@ -186,7 +184,8 @@ impl<'tcx> CtxtInterners<'tcx> {
 
                     InternedInSet(self.arena.alloc(predicate_struct))
                 })
-            .0
+                .0,
+        ))
     }
 }
 
@@ -1684,7 +1683,7 @@ nop_lift! {type_; Ty<'a> => Ty<'tcx>}
 nop_lift_old! {region; Region<'a> => Region<'tcx>}
 nop_lift_old! {const_; &'a Const<'a> => &'tcx Const<'tcx>}
 nop_lift_old! {const_allocation; &'a Allocation => &'tcx Allocation}
-nop_lift_old! {predicate; &'a PredicateInner<'a> => &'tcx PredicateInner<'tcx>}
+nop_lift! {predicate; Predicate<'a> => Predicate<'tcx>}
 
 nop_list_lift! {type_list; Ty<'a> => Ty<'tcx>}
 nop_list_lift! {poly_existential_predicates; ty::Binder<'a, ExistentialPredicate<'a>> => ty::Binder<'tcx, ExistentialPredicate<'tcx>>}
@@ -2040,23 +2039,23 @@ impl<'tcx> Hash for InternedInSet<'tcx, TyS<'tcx>> {
     }
 }
 
-impl<'tcx> Borrow<Binder<'tcx, PredicateKind<'tcx>>> for InternedInSet<'tcx, PredicateInner<'tcx>> {
+impl<'tcx> Borrow<Binder<'tcx, PredicateKind<'tcx>>> for InternedInSet<'tcx, PredicateS<'tcx>> {
     fn borrow<'a>(&'a self) -> &'a Binder<'tcx, PredicateKind<'tcx>> {
         &self.0.kind
     }
 }
 
-impl<'tcx> PartialEq for InternedInSet<'tcx, PredicateInner<'tcx>> {
-    fn eq(&self, other: &InternedInSet<'tcx, PredicateInner<'tcx>>) -> bool {
+impl<'tcx> PartialEq for InternedInSet<'tcx, PredicateS<'tcx>> {
+    fn eq(&self, other: &InternedInSet<'tcx, PredicateS<'tcx>>) -> bool {
         // The `Borrow` trait requires that `x.borrow() == y.borrow()` equals
         // `x == y`.
         self.0.kind == other.0.kind
     }
 }
 
-impl<'tcx> Eq for InternedInSet<'tcx, PredicateInner<'tcx>> {}
+impl<'tcx> Eq for InternedInSet<'tcx, PredicateS<'tcx>> {}
 
-impl<'tcx> Hash for InternedInSet<'tcx, PredicateInner<'tcx>> {
+impl<'tcx> Hash for InternedInSet<'tcx, PredicateS<'tcx>> {
     fn hash<H: Hasher>(&self, s: &mut H) {
         // The `Borrow` trait requires that `x.borrow().hash(s) == x.hash(s)`.
         self.0.kind.hash(s)
@@ -2237,8 +2236,7 @@ impl<'tcx> TyCtxt<'tcx> {
 
     #[inline]
     pub fn mk_predicate(self, binder: Binder<'tcx, PredicateKind<'tcx>>) -> Predicate<'tcx> {
-        let inner = self.interners.intern_predicate(binder);
-        Predicate { inner }
+        self.interners.intern_predicate(binder)
     }
 
     #[inline]

compiler/rustc_middle/src/ty/fold.rs

@@ -1181,7 +1181,7 @@ impl<'tcx> TypeVisitor<'tcx> for HasEscapingVarsVisitor {
 
     #[inline]
     fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
-        if predicate.inner.outer_exclusive_binder > self.outer_index {
+        if predicate.outer_exclusive_binder() > self.outer_index {
             ControlFlow::Break(FoundEscapingVars)
         } else {
             ControlFlow::CONTINUE
@@ -1263,9 +1263,11 @@ impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor {
     fn visit_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
         debug!(
             "HasTypeFlagsVisitor: predicate={:?} predicate.flags={:?} self.flags={:?}",
-            predicate, predicate.inner.flags, self.flags
+            predicate,
+            predicate.flags(),
+            self.flags
         );
-        if predicate.inner.flags.intersects(self.flags) {
+        if predicate.flags().intersects(self.flags) {
             ControlFlow::Break(FoundFlags)
         } else {
             ControlFlow::CONTINUE

compiler/rustc_middle/src/ty/mod.rs

@@ -43,9 +43,9 @@ use rustc_span::symbol::{kw, Ident, Symbol};
 use rustc_span::{sym, Span};
 use rustc_target::abi::Align;
 
-use std::hash::{Hash, Hasher};
+use std::hash::Hash;
 use std::ops::ControlFlow;
-use std::{fmt, ptr, str};
+use std::{fmt, str};
 
 pub use crate::ty::diagnostics::*;
 pub use rustc_type_ir::InferTy::*;
@@ -466,51 +466,50 @@ impl ty::EarlyBoundRegion {
     }
 }
 
+/// Represents a predicate.
+///
+/// See comments on `TyS`, which apply here too (albeit for
+/// `PredicateS`/`Predicate` rather than `TyS`/`Ty`).
 #[derive(Debug)]
-crate struct PredicateInner<'tcx> {
+crate struct PredicateS<'tcx> {
     kind: Binder<'tcx, PredicateKind<'tcx>>,
     flags: TypeFlags,
     /// See the comment for the corresponding field of [TyS].
     outer_exclusive_binder: ty::DebruijnIndex,
 }
 
+// This type is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-static_assert_size!(PredicateInner<'_>, 56);
+static_assert_size!(PredicateS<'_>, 56);
 
-#[derive(Clone, Copy, Lift)]
-pub struct Predicate<'tcx> {
-    inner: &'tcx PredicateInner<'tcx>,
-}
-
-impl<'tcx> PartialEq for Predicate<'tcx> {
-    fn eq(&self, other: &Self) -> bool {
-        // `self.kind` is always interned.
-        ptr::eq(self.inner, other.inner)
-    }
-}
-
-impl Hash for Predicate<'_> {
-    fn hash<H: Hasher>(&self, s: &mut H) {
-        (self.inner as *const PredicateInner<'_>).hash(s)
-    }
-}
-
-impl<'tcx> Eq for Predicate<'tcx> {}
+/// Use this rather than `PredicateS`, whenever possible.
+#[derive(Clone, Copy, PartialEq, Eq, Hash)]
+#[cfg_attr(not(bootstrap), rustc_pass_by_value)]
+pub struct Predicate<'tcx>(Interned<'tcx, PredicateS<'tcx>>);
 
 impl<'tcx> Predicate<'tcx> {
     /// Gets the inner `Binder<'tcx, PredicateKind<'tcx>>`.
     #[inline]
     pub fn kind(self) -> Binder<'tcx, PredicateKind<'tcx>> {
-        self.inner.kind
+        self.0.kind
+    }
+
+    #[inline(always)]
+    pub fn flags(self) -> TypeFlags {
+        self.0.flags
+    }
+
+    #[inline(always)]
+    pub fn outer_exclusive_binder(self) -> DebruijnIndex {
+        self.0.outer_exclusive_binder
     }
 
     /// Flips the polarity of a Predicate.
     ///
     /// Given `T: Trait` predicate it returns `T: !Trait` and given `T: !Trait` returns `T: Trait`.
-    pub fn flip_polarity(&self, tcx: TyCtxt<'tcx>) -> Option<Predicate<'tcx>> {
+    pub fn flip_polarity(self, tcx: TyCtxt<'tcx>) -> Option<Predicate<'tcx>> {
         let kind = self
-            .inner
-            .kind
+            .kind()
             .map_bound(|kind| match kind {
                 PredicateKind::Trait(TraitPredicate { trait_ref, constness, polarity }) => {
                     Some(PredicateKind::Trait(TraitPredicate {
@@ -530,14 +529,14 @@ impl<'tcx> Predicate<'tcx> {
 
 impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for Predicate<'tcx> {
     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
-        let PredicateInner {
+        let PredicateS {
             ref kind,
 
             // The other fields just provide fast access to information that is
             // also contained in `kind`, so no need to hash them.
             flags: _,
             outer_exclusive_binder: _,
-        } = self.inner;
+        } = self.0.0;
 
         kind.hash_stable(hcx, hasher);
     }

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -1112,12 +1112,12 @@ impl<'tcx> TypeFoldable<'tcx> for ty::Predicate<'tcx> {
         self,
         folder: &mut F,
     ) -> Result<Self, F::Error> {
-        let new = self.inner.kind.try_fold_with(folder)?;
+        let new = self.kind().try_fold_with(folder)?;
         Ok(folder.tcx().reuse_or_mk_predicate(self, new))
     }
 
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
-        self.inner.kind.visit_with(visitor)
+        self.kind().visit_with(visitor)
     }
 
     fn visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
@@ -1125,11 +1125,11 @@ impl<'tcx> TypeFoldable<'tcx> for ty::Predicate<'tcx> {
     }
 
     fn has_vars_bound_at_or_above(&self, binder: ty::DebruijnIndex) -> bool {
-        self.inner.outer_exclusive_binder > binder
+        self.outer_exclusive_binder() > binder
     }
 
     fn has_type_flags(&self, flags: ty::TypeFlags) -> bool {
-        self.inner.flags.intersects(flags)
+        self.flags().intersects(flags)
     }
 }
 

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -198,7 +198,7 @@ fn extend_cause_with_original_assoc_item_obligation<'tcx>(
     trait_ref: &ty::TraitRef<'tcx>,
     item: Option<&hir::Item<'tcx>>,
     cause: &mut traits::ObligationCause<'tcx>,
-    pred: &ty::Predicate<'tcx>,
+    pred: ty::Predicate<'tcx>,
 ) {
     debug!(
         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
@@ -319,7 +319,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
                 trait_ref,
                 item,
                 &mut cause,
-                &obligation.predicate,
+                obligation.predicate,
             );
             traits::Obligation::with_depth(cause, depth, param_env, obligation.predicate)
         };

compiler/rustc_traits/src/normalize_erasing_regions.rs

@@ -39,7 +39,7 @@ fn try_normalize_after_erasing_regions<'tcx, T: TypeFoldable<'tcx> + PartialEq +
                 // always only region relations, and we are about to
                 // erase those anyway:
                 debug_assert_eq!(
-                    normalized_obligations.iter().find(|p| not_outlives_predicate(&p.predicate)),
+                    normalized_obligations.iter().find(|p| not_outlives_predicate(p.predicate)),
                     None,
                 );
 
@@ -57,7 +57,7 @@ fn try_normalize_after_erasing_regions<'tcx, T: TypeFoldable<'tcx> + PartialEq +
     })
 }
 
-fn not_outlives_predicate<'tcx>(p: &ty::Predicate<'tcx>) -> bool {
+fn not_outlives_predicate<'tcx>(p: ty::Predicate<'tcx>) -> bool {
     match p.kind().skip_binder() {
         ty::PredicateKind::RegionOutlives(..) | ty::PredicateKind::TypeOutlives(..) => false,
         ty::PredicateKind::Trait(..)

compiler/rustc_typeck/src/check/method/suggest.rs

@@ -703,7 +703,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     let mut bound_spans = vec![];
 
                     let mut collect_type_param_suggestions =
-                        |self_ty: Ty<'tcx>, parent_pred: &ty::Predicate<'tcx>, obligation: &str| {
+                        |self_ty: Ty<'tcx>, parent_pred: ty::Predicate<'tcx>, obligation: &str| {
                             // We don't care about regions here, so it's fine to skip the binder here.
                             if let (ty::Param(_), ty::PredicateKind::Trait(p)) =
                                 (self_ty.kind(), parent_pred.kind().skip_binder())
@@ -892,7 +892,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         .filter(|(pred, _, _parent_pred)| !skip_list.contains(&pred))
                         .filter_map(|(pred, parent_pred, _cause)| {
                             format_pred(*pred).map(|(p, self_ty)| {
-                                collect_type_param_suggestions(self_ty, pred, &p);
+                                collect_type_param_suggestions(self_ty, *pred, &p);
                                 match parent_pred {
                                     None => format!("`{}`", &p),
                                     Some(parent_pred) => match format_pred(*parent_pred) {
@@ -900,7 +900,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                         Some((parent_p, _)) => {
                                             collect_type_param_suggestions(
                                                 self_ty,
-                                                parent_pred,
+                                                *parent_pred,
                                                 &p,
                                             );
                                             format!("`{}`\nwhich is required by `{}`", p, parent_p)