Auto traits

chalk-parse/src/ast.rs

@@ -35,6 +35,7 @@ pub struct TraitDefn {
     pub parameter_kinds: Vec<ParameterKind>,
     pub where_clauses: Vec<WhereClause>,
     pub assoc_ty_defns: Vec<AssocTyDefn>,
+    pub auto: bool,
 }
 
 pub struct AssocTyDefn {
@@ -93,7 +94,7 @@ impl Kinded for Parameter {
 
 pub struct Impl {
     pub parameter_kinds: Vec<ParameterKind>,
-    pub trait_ref: TraitRef,
+    pub trait_ref: PolarizedTraitRef,
     pub where_clauses: Vec<WhereClause>,
     pub assoc_ty_values: Vec<AssocTyValue>,
 }
@@ -139,6 +140,21 @@ pub struct TraitRef {
     pub args: Vec<Parameter>,
 }
 
+pub enum PolarizedTraitRef {
+    Positive(TraitRef),
+    Negative(TraitRef),
+}
+
+impl PolarizedTraitRef {
+    pub fn from_bool(polarity: bool, trait_ref: TraitRef) -> PolarizedTraitRef {
+        if polarity {
+            PolarizedTraitRef::Positive(trait_ref)
+        } else {
+            PolarizedTraitRef::Negative(trait_ref)
+        }
+    }
+}
+
 #[derive(Copy, Clone, Debug)]
 pub struct Identifier {
     pub str: InternedString,

chalk-parse/src/parser.lalrpop

@@ -47,12 +47,17 @@ StructDefn: StructDefn = {
     }
 };
 
+AutoKeyword: () = "#" "[" "auto" "]";
+
 TraitDefn: TraitDefn = {
-    "trait" <n:Id><p:Angle<ParameterKind>> <w:WhereClauses> "{" <a:AssocTyDefn*> "}" => TraitDefn {
+    <auto:AutoKeyword?> "trait" <n:Id><p:Angle<ParameterKind>> <w:WhereClauses> "{"
+        <a:AssocTyDefn*> "}" =>
+    TraitDefn {
         name: n,
         parameter_kinds: p,
         where_clauses: w,
         assoc_ty_defns: a,
+        auto: auto.is_some(),
     }
 };
 
@@ -64,14 +69,17 @@ AssocTyDefn: AssocTyDefn = {
 };
 
 Impl: Impl = {
-    "impl" <p:Angle<ParameterKind>> <t:Id> <a:Angle<Parameter>> "for" <s:Ty> <w:WhereClauses> "{"
-        <assoc:AssocTyValue*>"}" =>
+    "impl" <p:Angle<ParameterKind>> <mark:"!"?> <t:Id> <a:Angle<Parameter>> "for" <s:Ty> <w:WhereClauses> "{"
+        <assoc:AssocTyValue*> "}" =>
     {
         let mut args = vec![Parameter::Ty(s)];
         args.extend(a);
         Impl {
             parameter_kinds: p,
-            trait_ref: TraitRef { trait_name: t, args: args },
+            trait_ref: PolarizedTraitRef::from_bool(mark.is_none(), TraitRef {
+                trait_name: t,
+                args: args,
+            }),
             where_clauses: w,
             assoc_ty_values: assoc,
         }

src/coherence/solve.rs

@@ -17,12 +17,12 @@ impl Program {
 
         // Create a vector of references to impl datums, sorted by trait ref
         let impl_data = self.impl_data.iter().sorted_by(|&(_, lhs), &(_, rhs)| {
-            lhs.binders.value.trait_ref.trait_id.cmp(&rhs.binders.value.trait_ref.trait_id)
+            lhs.binders.value.trait_ref.trait_ref().trait_id.cmp(&rhs.binders.value.trait_ref.trait_ref().trait_id)
         });
 
         // Group impls by trait.
         let impl_groupings = impl_data.into_iter().group_by(|&(_, impl_datum)| {
-            impl_datum.binders.value.trait_ref.trait_id
+            impl_datum.binders.value.trait_ref.trait_ref().trait_id
         });
 
 
@@ -31,6 +31,11 @@ impl Program {
             let impls: Vec<(&ItemId, &ImplDatum)> = impls.collect();
 
             for ((&l_id, lhs), (&r_id, rhs)) in impls.into_iter().tuple_combinations() {
+                // Two negative impls never overlap.
+                if !lhs.binders.value.trait_ref.is_positive() && !rhs.binders.value.trait_ref.is_positive() {
+                    continue;
+                }
+
                 // Check if the impls overlap, then if they do, check if one specializes
                 // the other. Note that specialization can only run one way - if both
                 // specialization checks return *either* true or false, that's an error.
@@ -133,6 +138,11 @@ impl Solver {
     //  }
     // }
     fn specializes(&mut self, less_special: &ImplDatum, more_special: &ImplDatum) -> bool {
+        // Negative impls cannot specialize.
+        if !less_special.binders.value.trait_ref.is_positive() || !more_special.binders.value.trait_ref.is_positive() {
+            return false;
+        }
+
         let more_len = more_special.binders.len();
 
         // Create parameter equality goals.
@@ -160,5 +170,5 @@ impl Solver {
 }
 
 fn params(impl_datum: &ImplDatum) -> &[Parameter] {
-    &impl_datum.binders.value.trait_ref.parameters
+    &impl_datum.binders.value.trait_ref.trait_ref().parameters
 }

src/fold/mod.rs

@@ -195,6 +195,7 @@ macro_rules! enum_fold {
     }
 }
 
+enum_fold!(PolarizedTraitRef[] { Positive(a), Negative(a) });
 enum_fold!(ParameterKind[T,L] { Ty(a), Lifetime(a) } where T: Fold, L: Fold);
 enum_fold!(DomainGoal[] { Implemented(a), Normalize(a), WellFormed(a) });
 enum_fold!(WellFormed[] { Ty(a), TraitRef(a) });

src/ir/mod.rs

@@ -24,8 +24,11 @@ pub struct Program {
     /// For each trait:
     pub trait_data: HashMap<ItemId, TraitDatum>,
 
-    /// For each trait:
+    /// For each associated ty:
     pub associated_ty_data: HashMap<ItemId, AssociatedTyDatum>,
+
+    /// For each default impl (automatically generated for auto traits):
+    pub default_impl_data: Vec<DefaultImplDatum>,
 }
 
 impl Program {
@@ -84,8 +87,11 @@ impl Environment {
     pub fn add_clauses<I>(&self, clauses: I) -> Arc<Environment>
         where I: IntoIterator<Item = DomainGoal>
     {
+        use std::collections::HashSet;
+
         let mut env = self.clone();
-        env.clauses.extend(clauses);
+        let env_clauses: HashSet<_> = env.clauses.into_iter().chain(clauses).collect();
+        env.clauses = env_clauses.into_iter().collect();
         Arc::new(env)
     }
 
@@ -182,12 +188,23 @@ pub struct ImplDatum {
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct ImplDatumBound {
-    pub trait_ref: TraitRef,
+    pub trait_ref: PolarizedTraitRef,
     pub where_clauses: Vec<DomainGoal>,
     pub associated_ty_values: Vec<AssociatedTyValue>,
     pub specialization_priority: usize,
 }
 
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct DefaultImplDatum {
+    pub binders: Binders<DefaultImplDatumBound>,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct DefaultImplDatumBound {
+    pub trait_ref: TraitRef,
+    pub accessible_tys: Vec<Ty>,
+}
+
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct StructDatum {
     pub binders: Binders<StructDatumBound>,
@@ -209,6 +226,7 @@ pub struct TraitDatum {
 pub struct TraitDatumBound {
     pub trait_ref: TraitRef,
     pub where_clauses: Vec<DomainGoal>,
+    pub auto: bool,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
@@ -343,6 +361,28 @@ pub struct TraitRef {
     pub parameters: Vec<Parameter>,
 }
 
+#[derive(Clone, PartialEq, Eq, Hash, PartialOrd, Ord, Debug)]
+pub enum PolarizedTraitRef {
+    Positive(TraitRef),
+    Negative(TraitRef),
+}
+
+impl PolarizedTraitRef {
+    pub fn is_positive(&self) -> bool {
+        match *self {
+            PolarizedTraitRef::Positive(_) => true,
+            PolarizedTraitRef::Negative(_) => false,
+        }
+    }
+
+    pub fn trait_ref(&self) -> &TraitRef {
+        match *self {
+            PolarizedTraitRef::Positive(ref tr) |
+            PolarizedTraitRef::Negative(ref tr) => tr
+        }
+    }
+}
+
 /// A "domain goal" is a goal that is directly about Rust, rather than a pure
 /// logical statement. As much as possible, the Chalk solver should avoid
 /// decomposing this enum, and instead treat its values opaquely.
@@ -374,9 +414,8 @@ impl DomainGoal {
     pub fn expanded(self, program: &Program) -> impl Iterator<Item = DomainGoal> {
         let mut expanded = vec![];
         match self {
-            DomainGoal::Implemented(ref trait_ref) => {
-                expanded.push(WellFormed::TraitRef(trait_ref.clone()).cast());
-            }
+            DomainGoal::Implemented(ref trait_ref) =>
+                expanded.push(WellFormed::TraitRef(trait_ref.clone()).cast()),
             DomainGoal::Normalize(Normalize { ref projection, .. }) => {
                 let (associated_ty_data, trait_params, _) = program.split_projection(&projection);
                 let trait_ref = TraitRef {
@@ -500,6 +539,31 @@ pub enum FullyReducedGoal {
     DomainGoal(Canonical<InEnvironment<DomainGoal>>),
 }
 
+impl FullyReducedGoal {
+    pub fn into_binders(self) -> Vec<ParameterKind<UniverseIndex>> {
+        match self {
+            FullyReducedGoal::EqGoal(Canonical { binders, .. }) |
+            FullyReducedGoal::DomainGoal(Canonical { binders, ..}) => binders,
+        }
+    }
+
+    /// A goal has coinductive semantics if it is of the form `T: AutoTrait`.
+    pub fn is_coinductive(&self, program: &ProgramEnvironment) -> bool {
+        if let FullyReducedGoal::DomainGoal(Canonical {
+                value: InEnvironment {
+                    goal: DomainGoal::Implemented(ref tr),
+                    ..
+                },
+                ..
+        }) = *self {
+            let trait_datum = &program.trait_data[&tr.trait_id];
+            return trait_datum.binders.value.auto;
+        }
+
+        false
+    }
+}
+
 impl<T> Canonical<T> {
     pub fn map<OP, U>(self, op: OP) -> Canonical<U>
         where OP: FnOnce(T) -> U

src/lower/default.rs

@@ -0,0 +1,80 @@
+use ir::*;
+use solve::infer::InferenceTable;
+use cast::Cast;
+
+impl Program {
+    pub(super) fn add_default_impls(&mut self) {
+        // For each auto trait `MyAutoTrait` and for each struct/type `MyStruct`
+        for auto_trait in self.trait_data.values().filter(|t| t.binders.value.auto) {
+            for struct_datum in self.struct_data.values() {
+
+                // `MyStruct: MyAutoTrait`
+                let trait_ref = TraitRef {
+                    trait_id: auto_trait.binders.value.trait_ref.trait_id,
+                    parameters: vec![
+                        ParameterKind::Ty(Ty::Apply(struct_datum.binders.value.self_ty.clone()))
+                    ]
+                };
+
+                // If a positive or negative impl is already provided for a type family
+                // which includes `MyStruct`, we do not generate a default impl.
+                if self.provide_impl_for(trait_ref.clone(), struct_datum) {
+                    continue;
+                }
+
+                self.default_impl_data.push(DefaultImplDatum {
+                    binders: Binders {
+                        binders: struct_datum.binders.binders.clone(),
+                        value: DefaultImplDatumBound {
+                            trait_ref,
+                            accessible_tys: struct_datum.binders.value.fields.clone(),
+                        }
+                    }
+                });
+            }
+        }
+    }
+
+    fn provide_impl_for(&self, trait_ref: TraitRef, struct_datum: &StructDatum) -> bool {
+        let goal: DomainGoal = trait_ref.cast();
+
+        let env = Environment::new();
+        let mut infer = InferenceTable::new();
+
+        let goal = infer.instantiate_in(env.universe, struct_datum.binders.binders.clone(), &goal);
+
+        for impl_datum in self.impl_data.values() {
+            // We retrieve the trait ref given by the positive impl (even if the actual impl is negative)
+            let impl_goal: DomainGoal = impl_datum.binders.value.trait_ref.trait_ref().clone().cast();
+
+            let impl_goal = infer.instantiate_in(env.universe, impl_datum.binders.binders.clone(), &impl_goal);
+
+            // We check whether the impl `MyStruct: (!)MyAutoTrait` unifies with an existing impl.
+            // Examples:
+            //
+            // ```
+            // struct MyStruct;
+            // impl<T> Send for T where T: Foo { }
+            // ```
+            // `MyStruct: Send` unifies with `T: Send` so no default impl is generated for `MyStruct`.
+            //
+            // ```
+            // struct MyStruct;
+            // impl<T> Send for Vec<T> where T: Foo { }
+            // ```
+            // `Vec<i32>: Send` unifies with `Vec<T>: Send` so no default impl is generated for `Vec<i32>`.
+            // But a default impl is generated for `MyStruct`.
+            //
+            // ```
+            // struct MyStruct;
+            // impl<T> !Send for T where T: Foo { }
+            // ```
+            // `MyStruct: !Send` unifies with `T: !Send` so no default impl is generated for `MyStruct`.
+            if infer.unify(&Environment::new(), &goal, &impl_goal).is_ok() {
+                return true;
+            }
+        }
+
+        false
+    }
+}