Shrink ParamEnv to 16 bytes

Author: Mark-Simulacrum

src/librustc_infer/infer/outlives/mod.rs

@@ -11,7 +11,7 @@ pub fn explicit_outlives_bounds<'tcx>(
     param_env: ty::ParamEnv<'tcx>,
 ) -> impl Iterator<Item = OutlivesBound<'tcx>> + 'tcx {
     debug!("explicit_outlives_bounds()");
-    param_env.caller_bounds.into_iter().filter_map(move |predicate| match predicate.kind() {
+    param_env.caller_bounds().into_iter().filter_map(move |predicate| match predicate.kind() {
         ty::PredicateKind::Projection(..)
         | ty::PredicateKind::Trait(..)
         | ty::PredicateKind::Subtype(..)

src/librustc_infer/infer/outlives/verify.rs

@@ -221,7 +221,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         // dubious for projections, but it will work for simple cases
         // like `T` and `T::Item`. It may not work as well for things
         // like `<T as Foo<'a>>::Item`.
-        let c_b = self.param_env.caller_bounds;
+        let c_b = self.param_env.caller_bounds();
         let param_bounds = self.collect_outlives_from_predicate_list(&compare_ty, c_b.into_iter());
 
         // Next, collect regions we scraped from the well-formedness

src/librustc_infer/traits/mod.rs

@@ -57,7 +57,7 @@ pub type TraitObligation<'tcx> = Obligation<'tcx, ty::PolyTraitPredicate<'tcx>>;
 
 // `PredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(target_arch = "x86_64")]
-static_assert_size!(PredicateObligation<'_>, 48);
+static_assert_size!(PredicateObligation<'_>, 40);
 
 pub type Obligations<'tcx, O> = Vec<Obligation<'tcx, O>>;
 pub type PredicateObligations<'tcx> = Vec<PredicateObligation<'tcx>>;

src/librustc_middle/ty/layout.rs

@@ -1588,7 +1588,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
         // Ignore layouts that are done with non-empty environments or
         // non-monomorphic layouts, as the user only wants to see the stuff
         // resulting from the final codegen session.
-        if layout.ty.has_param_types_or_consts() || !self.param_env.caller_bounds.is_empty() {
+        if layout.ty.has_param_types_or_consts() || !self.param_env.caller_bounds().is_empty() {
             return;
         }
 

src/librustc_middle/ty/mod.rs

@@ -1,3 +1,5 @@
+// ignore-tidy-filelength
+
 pub use self::fold::{TypeFoldable, TypeVisitor};
 pub use self::AssocItemContainer::*;
 pub use self::BorrowKind::*;
@@ -45,6 +47,7 @@ use std::cell::RefCell;
 use std::cmp::Ordering;
 use std::fmt;
 use std::hash::{Hash, Hasher};
+use std::marker::PhantomData;
 use std::ops::Range;
 use std::ptr;
 
@@ -1571,24 +1574,92 @@ pub type PlaceholderConst = Placeholder<BoundVar>;
 /// When type checking, we use the `ParamEnv` to track
 /// details about the set of where-clauses that are in scope at this
 /// particular point.
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable, TypeFoldable)]
+#[derive(Copy, Clone)]
 pub struct ParamEnv<'tcx> {
+    // We pack the caller_bounds List pointer and a Reveal enum into this usize.
+    // Specifically, the low bit represents Reveal, with 0 meaning `UserFacing`
+    // and 1 meaning `All`. The rest is the pointer.
+    //
+    // This relies on the List<ty::Predicate<'tcx>> type having at least 2-byte
+    // alignment. Lists start with a usize and are repr(C) so this should be
+    // fine; there is a debug_assert in the constructor as well.
+    //
+    // Note that the choice of 0 for UserFacing is intentional -- since it is the
+    // first variant in Reveal this means that joining the pointer is a simple `or`.
+    packed_data: usize,
+
     /// `Obligation`s that the caller must satisfy. This is basically
     /// the set of bounds on the in-scope type parameters, translated
     /// into `Obligation`s, and elaborated and normalized.
-    pub caller_bounds: &'tcx List<ty::Predicate<'tcx>>,
+    ///
+    /// Note: This is packed into the `packed_data` usize above, use the
+    /// `caller_bounds()` method to access it.
+    caller_bounds: PhantomData<&'tcx List<ty::Predicate<'tcx>>>,
 
     /// Typically, this is `Reveal::UserFacing`, but during codegen we
-    /// want `Reveal::All` -- note that this is always paired with an
-    /// empty environment. To get that, use `ParamEnv::reveal()`.
-    pub reveal: traits::Reveal,
+    /// want `Reveal::All`.
+    ///
+    /// Note: This is packed into the caller_bounds usize above, use the reveal()
+    /// method to access it.
+    reveal: PhantomData<traits::Reveal>,
 
     /// If this `ParamEnv` comes from a call to `tcx.param_env(def_id)`,
     /// register that `def_id` (useful for transitioning to the chalk trait
     /// solver).
     pub def_id: Option<DefId>,
 }
 
+impl<'tcx> fmt::Debug for ParamEnv<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("ParamEnv")
+            .field("caller_bounds", &self.caller_bounds())
+            .field("reveal", &self.reveal())
+            .field("def_id", &self.def_id)
+            .finish()
+    }
+}
+
+impl<'tcx> Hash for ParamEnv<'tcx> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.caller_bounds().hash(state);
+        self.reveal().hash(state);
+        self.def_id.hash(state);
+    }
+}
+
+impl<'tcx> PartialEq for ParamEnv<'tcx> {
+    fn eq(&self, other: &Self) -> bool {
+        self.caller_bounds() == other.caller_bounds()
+            && self.reveal() == other.reveal()
+            && self.def_id == other.def_id
+    }
+}
+impl<'tcx> Eq for ParamEnv<'tcx> {}
+
+impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ParamEnv<'tcx> {
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        self.caller_bounds().hash_stable(hcx, hasher);
+        self.reveal().hash_stable(hcx, hasher);
+        self.def_id.hash_stable(hcx, hasher);
+    }
+}
+
+impl<'tcx> TypeFoldable<'tcx> for ParamEnv<'tcx> {
+    fn super_fold_with<F: ty::fold::TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
+        ParamEnv::new(
+            self.caller_bounds().fold_with(folder),
+            self.reveal().fold_with(folder),
+            self.def_id.fold_with(folder),
+        )
+    }
+
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
+        self.caller_bounds().visit_with(visitor)
+            || self.reveal().visit_with(visitor)
+            || self.def_id.visit_with(visitor)
+    }
+}
+
 impl<'tcx> ParamEnv<'tcx> {
     /// Construct a trait environment suitable for contexts where
     /// there are no where-clauses in scope. Hidden types (like `impl
@@ -1599,6 +1670,17 @@ impl<'tcx> ParamEnv<'tcx> {
         Self::new(List::empty(), Reveal::UserFacing, None)
     }
 
+    #[inline]
+    pub fn caller_bounds(self) -> &'tcx List<ty::Predicate<'tcx>> {
+        // mask out bottom bit
+        unsafe { &*((self.packed_data & (!1)) as *const _) }
+    }
+
+    #[inline]
+    pub fn reveal(self) -> traits::Reveal {
+        if self.packed_data & 1 == 0 { traits::Reveal::UserFacing } else { traits::Reveal::All }
+    }
+
     /// Construct a trait environment with no where-clauses in scope
     /// where the values of all `impl Trait` and other hidden types
     /// are revealed. This is suitable for monomorphized, post-typeck
@@ -1618,7 +1700,25 @@ impl<'tcx> ParamEnv<'tcx> {
         reveal: Reveal,
         def_id: Option<DefId>,
     ) -> Self {
-        ty::ParamEnv { caller_bounds, reveal, def_id }
+        let packed_data = caller_bounds as *const _ as usize;
+        // Check that we can pack the reveal data into the pointer.
+        debug_assert!(packed_data & 1 == 0);
+        ty::ParamEnv {
+            packed_data: packed_data
+                | match reveal {
+                    Reveal::UserFacing => 0,
+                    Reveal::All => 1,
+                },
+            caller_bounds: PhantomData,
+            reveal: PhantomData,
+            def_id,
+        }
+    }
+
+    pub fn with_user_facing(mut self) -> Self {
+        // clear bottom bit
+        self.packed_data &= !1;
+        self
     }
 
     /// Returns a new parameter environment with the same clauses, but
@@ -1627,13 +1727,14 @@ impl<'tcx> ParamEnv<'tcx> {
     /// the desired behavior during codegen and certain other special
     /// contexts; normally though we want to use `Reveal::UserFacing`,
     /// which is the default.
-    pub fn with_reveal_all(self) -> Self {
-        ty::ParamEnv { reveal: Reveal::All, ..self }
+    pub fn with_reveal_all(mut self) -> Self {
+        self.packed_data |= 1;
+        self
     }
 
     /// Returns this same environment but with no caller bounds.
     pub fn without_caller_bounds(self) -> Self {
-        ty::ParamEnv { caller_bounds: List::empty(), ..self }
+        Self::new(List::empty(), self.reveal(), self.def_id)
     }
 
     /// Creates a suitable environment in which to perform trait
@@ -1649,7 +1750,7 @@ impl<'tcx> ParamEnv<'tcx> {
     /// satisfiable. We generally want to behave as if they were true,
     /// although the surrounding function is never reachable.
     pub fn and<T: TypeFoldable<'tcx>>(self, value: T) -> ParamEnvAnd<'tcx, T> {
-        match self.reveal {
+        match self.reveal() {
             Reveal::UserFacing => ParamEnvAnd { param_env: self, value },
 
             Reveal::All => {

src/librustc_middle/ty/structural_impls.rs

@@ -521,11 +521,8 @@ impl<'tcx, T: Lift<'tcx>> Lift<'tcx> for ty::Binder<T> {
 impl<'a, 'tcx> Lift<'tcx> for ty::ParamEnv<'a> {
     type Lifted = ty::ParamEnv<'tcx>;
     fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
-        tcx.lift(&self.caller_bounds).map(|caller_bounds| ty::ParamEnv {
-            reveal: self.reveal,
-            caller_bounds,
-            def_id: self.def_id,
-        })
+        tcx.lift(&self.caller_bounds())
+            .map(|caller_bounds| ty::ParamEnv::new(caller_bounds, self.reveal(), self.def_id))
     }
 }
 

src/librustc_mir/const_eval/eval_queries.rs

@@ -226,9 +226,9 @@ pub fn const_eval_validated_provider<'tcx>(
     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 ) -> ::rustc_middle::mir::interpret::ConstEvalResult<'tcx> {
     // see comment in const_eval_raw_provider for what we're doing here
-    if key.param_env.reveal == Reveal::All {
+    if key.param_env.reveal() == Reveal::All {
         let mut key = key;
-        key.param_env.reveal = Reveal::UserFacing;
+        key.param_env = key.param_env.with_user_facing();
         match tcx.const_eval_validated(key) {
             // try again with reveal all as requested
             Err(ErrorHandled::TooGeneric) => {}
@@ -267,9 +267,9 @@ pub fn const_eval_raw_provider<'tcx>(
     // information being available.
 
     // In case we fail in the `UserFacing` variant, we just do the real computation.
-    if key.param_env.reveal == Reveal::All {
+    if key.param_env.reveal() == Reveal::All {
         let mut key = key;
-        key.param_env.reveal = Reveal::UserFacing;
+        key.param_env = key.param_env.with_user_facing();
         match tcx.const_eval_raw(key) {
             // try again with reveal all as requested
             Err(ErrorHandled::TooGeneric) => {}
@@ -326,7 +326,7 @@ pub fn const_eval_raw_provider<'tcx>(
                 // this is `Reveal::UserFacing`, then it's expected that we could get a
                 // `TooGeneric` error. When we fall back to `Reveal::All`, then it will either
                 // succeed or we'll report this error then.
-                if key.param_env.reveal == Reveal::All {
+                if key.param_env.reveal() == Reveal::All {
                     tcx.sess.delay_span_bug(
                         err.span,
                         &format!("static eval failure did not emit an error: {:#?}", v),

src/librustc_mir/util/elaborate_drops.rs

@@ -279,7 +279,7 @@ where
                 let subpath = self.elaborator.field_subpath(variant_path, field);
                 let tcx = self.tcx();
 
-                assert_eq!(self.elaborator.param_env().reveal, Reveal::All);
+                assert_eq!(self.elaborator.param_env().reveal(), Reveal::All);
                 let field_ty =
                     tcx.normalize_erasing_regions(self.elaborator.param_env(), f.ty(tcx, substs));
                 (tcx.mk_place_field(base_place, field, field_ty), subpath)

src/librustc_trait_selection/traits/auto_trait.rs

@@ -281,8 +281,8 @@ impl AutoTraitFinder<'tcx> {
             },
         }));
 
-        let computed_preds = param_env.caller_bounds.iter();
-        let mut user_computed_preds: FxHashSet<_> = user_env.caller_bounds.iter().collect();
+        let computed_preds = param_env.caller_bounds().iter();
+        let mut user_computed_preds: FxHashSet<_> = user_env.caller_bounds().iter().collect();
 
         let mut new_env = param_env;
         let dummy_cause = ObligationCause::dummy();
@@ -368,12 +368,12 @@ impl AutoTraitFinder<'tcx> {
             )
             .map(|o| o.predicate);
             new_env =
-                ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal, None);
+                ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal(), None);
         }
 
         let final_user_env = ty::ParamEnv::new(
             tcx.mk_predicates(user_computed_preds.into_iter()),
-            user_env.reveal,
+            user_env.reveal(),
             None,
         );
         debug!(

src/librustc_trait_selection/traits/chalk_fulfill.rs

@@ -137,7 +137,7 @@ fn in_environment(
 
     let environment = match obligation.param_env.def_id {
         Some(def_id) => environment(infcx.tcx, def_id),
-        None if obligation.param_env.caller_bounds.is_empty() => ty::List::empty(),
+        None if obligation.param_env.caller_bounds().is_empty() => ty::List::empty(),
         // FIXME(chalk): this is hit in ui/where-clauses/where-clause-constraints-are-local-for-trait-impl
         // and ui/generics/generic-static-methods
         //_ => bug!("non-empty `ParamEnv` with no def-id"),