Tweak `Clause` definition and HRTBs

src/librustc/ich/impls_ty.rs

@@ -1392,6 +1392,12 @@ fn hash_stable<W: StableHasherResult>(&self,
     }
 }
 
+impl_stable_hash_for!(
+    impl<'tcx> for struct traits::ProgramClause<'tcx> {
+        goal, hypotheses
+    }
+);
+
 impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for traits::Clause<'tcx> {
     fn hash_stable<W: StableHasherResult>(&self,
                                           hcx: &mut StableHashingContext<'a>,
@@ -1400,11 +1406,7 @@ fn hash_stable<W: StableHasherResult>(&self,
 
         mem::discriminant(self).hash_stable(hcx, hasher);
         match self {
-            Implies(hypotheses, goal) => {
-                hypotheses.hash_stable(hcx, hasher);
-                goal.hash_stable(hcx, hasher);
-            }
-            DomainGoal(domain_goal) => domain_goal.hash_stable(hcx, hasher),
+            Implies(clause) => clause.hash_stable(hcx, hasher),
             ForAll(clause) => clause.hash_stable(hcx, hasher),
         }
     }

src/librustc/traits/mod.rs

@@ -272,6 +272,8 @@ pub enum DomainGoal<'tcx> {
     TypeOutlives(ty::TypeOutlivesPredicate<'tcx>),
 }
 
+pub type PolyDomainGoal<'tcx> = ty::Binder<DomainGoal<'tcx>>;
+
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 pub enum QuantifierKind {
     Universal,
@@ -294,9 +296,15 @@ fn from(domain_goal: DomainGoal<'tcx>) -> Self {
     }
 }
 
-impl<'tcx> From<DomainGoal<'tcx>> for Clause<'tcx> {
-    fn from(domain_goal: DomainGoal<'tcx>) -> Self {
-        Clause::DomainGoal(domain_goal)
+impl<'tcx> From<PolyDomainGoal<'tcx>> for Goal<'tcx> {
+    fn from(domain_goal: PolyDomainGoal<'tcx>) -> Self {
+        match domain_goal.no_late_bound_regions() {
+            Some(p) => p.into(),
+            None => Goal::Quantified(
+                QuantifierKind::Universal,
+                Box::new(domain_goal.map_bound(|p| p.into()))
+            ),
+        }
     }
 }
 
@@ -304,10 +312,23 @@ fn from(domain_goal: DomainGoal<'tcx>) -> Self {
 /// Harrop Formulas".
 #[derive(Clone, PartialEq, Eq, Hash, Debug)]
 pub enum Clause<'tcx> {
-    // FIXME: again, use interned refs instead of `Box`
-    Implies(Vec<Goal<'tcx>>, DomainGoal<'tcx>),
-    DomainGoal(DomainGoal<'tcx>),
-    ForAll(Box<ty::Binder<Clause<'tcx>>>),
+    Implies(ProgramClause<'tcx>),
+    ForAll(ty::Binder<ProgramClause<'tcx>>),
+}
+
+/// A "program clause" has the form `D :- G1, ..., Gn`. It is saying
+/// that the domain goal `D` is true if `G1...Gn` are provable. This
+/// is equivalent to the implication `G1..Gn => D`; we usually write
+/// it with the reverse implication operator `:-` to emphasize the way
+/// that programs are actually solved (via backchaining, which starts
+/// with the goal to solve and proceeds from there).
+#[derive(Clone, PartialEq, Eq, Hash, Debug)]
+pub struct ProgramClause<'tcx> {
+    /// This goal will be considered true...
+    pub goal: DomainGoal<'tcx>,
+
+    /// ...if we can prove these hypotheses (there may be no hypotheses at all):
+    pub hypotheses: Vec<Goal<'tcx>>,
 }
 
 pub type Selection<'tcx> = Vtable<'tcx, PredicateObligation<'tcx>>;

src/librustc/traits/structural_impls.rs

@@ -493,25 +493,29 @@ fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
     }
 }
 
+impl<'tcx> fmt::Display for traits::ProgramClause<'tcx> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        let traits::ProgramClause { goal, hypotheses } = self;
+        write!(fmt, "{}", goal)?;
+        if !hypotheses.is_empty() {
+            write!(fmt, " :- ")?;
+            for (index, condition) in hypotheses.iter().enumerate() {
+                if index > 0 {
+                    write!(fmt, ", ")?;
+                }
+                write!(fmt, "{}", condition)?;
+            }
+        }
+        write!(fmt, ".")
+    }
+}
+
 impl<'tcx> fmt::Display for traits::Clause<'tcx> {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         use traits::Clause::*;
 
         match self {
-            Implies(hypotheses, goal) => {
-                write!(fmt, "{}", goal)?;
-                if !hypotheses.is_empty() {
-                    write!(fmt, " :- ")?;
-                    for (index, condition) in hypotheses.iter().enumerate() {
-                        if index > 0 {
-                            write!(fmt, ", ")?;
-                        }
-                        write!(fmt, "{}", condition)?;
-                    }
-                }
-                write!(fmt, ".")
-            }
-            DomainGoal(domain_goal) => write!(fmt, "{}.", domain_goal),
+            Implies(clause) => write!(fmt, "{}", clause),
             ForAll(clause) => {
                 // FIXME: appropriate binder names
                 write!(fmt, "forall<> {{ {} }}", clause.skip_binder())
@@ -553,10 +557,16 @@ impl<'tcx> TypeFoldable<'tcx> for traits::Goal<'tcx> {
     }
 }
 
+BraceStructTypeFoldableImpl! {
+    impl<'tcx> TypeFoldable<'tcx> for traits::ProgramClause<'tcx> {
+        goal,
+        hypotheses
+    }
+}
+
 EnumTypeFoldableImpl! {
     impl<'tcx> TypeFoldable<'tcx> for traits::Clause<'tcx> {
-        (traits::Clause::Implies)(hypotheses, goal),
-        (traits::Clause::DomainGoal)(domain_goal),
+        (traits::Clause::Implies)(clause),
         (traits::Clause::ForAll)(clause),
     }
 }

src/librustc_traits/lowering.rs

@@ -13,7 +13,7 @@
 use rustc::hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc::ty::{self, TyCtxt};
 use rustc::ty::subst::Substs;
-use rustc::traits::{QuantifierKind, Goal, DomainGoal, Clause, WhereClauseAtom};
+use rustc::traits::{WhereClauseAtom, PolyDomainGoal, DomainGoal, ProgramClause, Clause};
 use syntax::ast;
 use rustc_data_structures::sync::Lrc;
 
@@ -61,28 +61,19 @@ fn lower(&self) -> DomainGoal<'tcx> {
 /// `ty::Binder` is used for wrapping a rustc construction possibly containing generic
 /// lifetimes, e.g. `for<'a> T: Fn(&'a i32)`. Instead of representing higher-ranked things
 /// in that leaf-form (i.e. `Holds(Implemented(Binder<TraitPredicate>))` in the previous
-/// example), we model them with quantified goals, e.g. as for the previous example:
+/// example), we model them with quantified domain goals, e.g. as for the previous example:
 /// `forall<'a> { T: Fn(&'a i32) }` which corresponds to something like
 /// `Binder<Holds(Implemented(TraitPredicate))>`.
-///
-/// Also, if `self` does not contain generic lifetimes, we can safely drop the binder and we
-/// can directly lower to a leaf goal instead of a quantified goal.
-impl<'tcx, T> Lower<Goal<'tcx>> for ty::Binder<T>
-    where T: Lower<DomainGoal<'tcx>> + ty::fold::TypeFoldable<'tcx> + Copy
+impl<'tcx, T> Lower<PolyDomainGoal<'tcx>> for ty::Binder<T>
+    where T: Lower<DomainGoal<'tcx>> + ty::fold::TypeFoldable<'tcx>
 {
-    fn lower(&self) -> Goal<'tcx> {
-        match self.no_late_bound_regions() {
-            Some(p) => p.lower().into(),
-            None => Goal::Quantified(
-                QuantifierKind::Universal,
-                Box::new(self.map_bound(|p| p.lower().into()))
-            ),
-        }
+    fn lower(&self) -> PolyDomainGoal<'tcx> {
+        self.map_bound_ref(|p| p.lower())
     }
 }
 
-impl<'tcx> Lower<Goal<'tcx>> for ty::Predicate<'tcx> {
-    fn lower(&self) -> Goal<'tcx> {
+impl<'tcx> Lower<PolyDomainGoal<'tcx>> for ty::Predicate<'tcx> {
+    fn lower(&self) -> PolyDomainGoal<'tcx> {
         use rustc::ty::Predicate::*;
 
         match self {
@@ -90,7 +81,7 @@ fn lower(&self) -> Goal<'tcx> {
             RegionOutlives(predicate) => predicate.lower(),
             TypeOutlives(predicate) => predicate.lower(),
             Projection(predicate) => predicate.lower(),
-            WellFormed(ty) => DomainGoal::WellFormedTy(*ty).into(),
+            WellFormed(ty) => ty::Binder::dummy(DomainGoal::WellFormedTy(*ty)),
             ObjectSafe(..) |
             ClosureKind(..) |
             Subtype(..) |
@@ -134,13 +125,16 @@ fn program_clauses_for_trait<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefI
         }
     };
     // `FromEnv(Self: Trait<P1..Pn>)`
-    let from_env = Goal::DomainGoal(DomainGoal::FromEnv(trait_pred.lower()));
+    let from_env = DomainGoal::FromEnv(trait_pred.lower()).into();
     // `Implemented(Self: Trait<P1..Pn>)`
     let impl_trait = DomainGoal::Holds(WhereClauseAtom::Implemented(trait_pred));
 
     // `Implemented(Self: Trait<P1..Pn>) :- FromEnv(Self: Trait<P1..Pn>)`
-    let clause = Clause::Implies(vec![from_env], impl_trait);
-    Lrc::new(vec![clause])
+    let clause = ProgramClause {
+        goal: impl_trait,
+        hypotheses: vec![from_env],
+    };
+    Lrc::new(vec![Clause::ForAll(ty::Binder::dummy(clause))])
 }
 
 fn program_clauses_for_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId)
@@ -167,8 +161,11 @@ fn program_clauses_for_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId
     let where_clauses = tcx.predicates_of(def_id).predicates.lower();
 
      // `Implemented(A0: Trait<A1..An>) :- WC`
-    let clause = Clause::Implies(where_clauses, trait_pred);
-    Lrc::new(vec![clause])
+    let clause = ProgramClause {
+        goal: trait_pred,
+        hypotheses: where_clauses.into_iter().map(|wc| wc.into()).collect()
+    };
+    Lrc::new(vec![Clause::ForAll(ty::Binder::dummy(clause))])
 }
 
 pub fn dump_program_clauses<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
@@ -184,14 +181,19 @@ struct ClauseDumper<'a, 'tcx: 'a> {
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 }
 
-impl <'a, 'tcx> ClauseDumper<'a, 'tcx > {
+impl<'a, 'tcx> ClauseDumper<'a, 'tcx > {
     fn process_attrs(&mut self, node_id: ast::NodeId, attrs: &[ast::Attribute]) {
         let def_id = self.tcx.hir.local_def_id(node_id);
         for attr in attrs {
             if attr.check_name("rustc_dump_program_clauses") {
                 let clauses = self.tcx.program_clauses_for(def_id);
                 for clause in &*clauses {
-                    self.tcx.sess.struct_span_err(attr.span, &format!("{}", clause)).emit();
+                    let program_clause = match clause {
+                        Clause::Implies(program_clause) => program_clause,
+                        Clause::ForAll(program_clause) => program_clause.skip_binder(),
+                    };
+                    // Skip the top-level binder for a less verbose output
+                    self.tcx.sess.struct_span_err(attr.span, &format!("{}", program_clause)).emit();
                 }
             }
         }