Implement return position impl trait / opaque type support

This is working, but I'm not that happy with how the lowering works. We might
need an additional representation between `TypeRef` and `Ty` where names are
resolved and `impl Trait` bounds are separated out, but things like inference
variables don't exist and `impl Trait` is always represented the same
way.

Also note that this doesn't implement correct handling of RPIT *inside* the
function (which involves turning the `impl Trait`s into variables and creating
obligations for them). That intermediate representation might help there as
well.

4 files changed, 89 insertions, 15 deletions

diff --git a/crates/ra_hir_ty/src/traits/chalk.rs b/crates/ra_hir_ty/src/traits/chalk.rs
index 61de3cc30..a72a82f5a 100644
--- a/crates/ra_hir_ty/src/traits/chalk.rs
+++ b/crates/ra_hir_ty/src/traits/chalk.rs
@@ -4,7 +4,7 @@ use std::sync::Arc;
 use log::debug;
 
 use chalk_ir::{fold::shift::Shift, GenericArg, TypeName};
-use chalk_solve::rust_ir::{self, WellKnownTrait};
+use chalk_solve::rust_ir::{self, OpaqueTyDatumBound, WellKnownTrait};
 
 use hir_def::{
     lang_item::{lang_attr, LangItemTarget},
@@ -100,6 +100,7 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
     fn associated_ty_value(&self, id: AssociatedTyValueId) -> Arc<AssociatedTyValue> {
         self.db.associated_ty_value(self.krate, id)
     }
+
     fn custom_clauses(&self) -> Vec<chalk_ir::ProgramClause<Interner>> {
         vec![]
     }
@@ -130,11 +131,34 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
         self.db.program_clauses_for_chalk_env(self.krate, environment.clone())
     }
 
-    fn opaque_ty_data(
-        &self,
-        _id: chalk_ir::OpaqueTyId<Interner>,
-    ) -> Arc<rust_ir::OpaqueTyDatum<Interner>> {
-        unimplemented!()
+    fn opaque_ty_data(&self, id: chalk_ir::OpaqueTyId<Interner>) -> Arc<OpaqueTyDatum> {
+        let interned_id = crate::db::InternedOpaqueTyId::from(id);
+        let full_id = self.db.lookup_intern_impl_trait_id(interned_id);
+        let (func, idx) = match full_id {
+            crate::OpaqueTyId::ReturnTypeImplTrait(func, idx) => (func, idx),
+        };
+        let datas =
+            self.db.return_type_impl_traits(func).expect("impl trait id without impl traits");
+        let data = &datas.value.impl_traits[idx as usize];
+        let bound = OpaqueTyDatumBound {
+            bounds: make_binders(
+                data.bounds
+                    .value
+                    .iter()
+                    .cloned()
+                    .filter(|b| !b.is_error())
+                    .map(|b| b.to_chalk(self.db))
+                    .collect(),
+                1,
+            ),
+        };
+        let num_vars = datas.num_binders;
+        Arc::new(OpaqueTyDatum { opaque_ty_id: id, bound: make_binders(bound, num_vars) })
+    }
+
+    fn hidden_opaque_type(&self, _id: chalk_ir::OpaqueTyId<Interner>) -> chalk_ir::Ty<Interner> {
+        // FIXME: actually provide the hidden type; it is relevant for auto traits
+        Ty::Unknown.to_chalk(self.db)
     }
 
     fn force_impl_for(
@@ -150,10 +174,6 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
         // FIXME: implement actual object safety
         true
     }
-
-    fn hidden_opaque_type(&self, _id: chalk_ir::OpaqueTyId<Interner>) -> chalk_ir::Ty<Interner> {
-        Ty::Unknown.to_chalk(self.db)
-    }
 }
 
 pub(crate) fn program_clauses_for_chalk_env_query(
@@ -460,6 +480,18 @@ impl From<crate::traits::GlobalImplId> for ImplId {
     }
 }
 
+impl From<OpaqueTyId> for crate::db::InternedOpaqueTyId {
+    fn from(id: OpaqueTyId) -> Self {
+        InternKey::from_intern_id(id.0)
+    }
+}
+
+impl From<crate::db::InternedOpaqueTyId> for OpaqueTyId {
+    fn from(id: crate::db::InternedOpaqueTyId) -> Self {
+        chalk_ir::OpaqueTyId(id.as_intern_id())
+    }
+}
+
 impl From<rust_ir::AssociatedTyValueId<Interner>> for crate::traits::AssocTyValueId {
     fn from(id: rust_ir::AssociatedTyValueId<Interner>) -> Self {
         Self::from_intern_id(id.0)
diff --git a/crates/ra_hir_ty/src/traits/chalk/interner.rs b/crates/ra_hir_ty/src/traits/chalk/interner.rs
index e27074ba6..56aab640c 100644
--- a/crates/ra_hir_ty/src/traits/chalk/interner.rs
+++ b/crates/ra_hir_ty/src/traits/chalk/interner.rs
@@ -22,6 +22,8 @@ pub type AssociatedTyValueId = chalk_solve::rust_ir::AssociatedTyValueId<Interne
 pub type AssociatedTyValue = chalk_solve::rust_ir::AssociatedTyValue<Interner>;
 pub type FnDefId = chalk_ir::FnDefId<Interner>;
 pub type FnDefDatum = chalk_solve::rust_ir::FnDefDatum<Interner>;
+pub type OpaqueTyId = chalk_ir::OpaqueTyId<Interner>;
+pub type OpaqueTyDatum = chalk_solve::rust_ir::OpaqueTyDatum<Interner>;
 
 impl chalk_ir::interner::Interner for Interner {
     type InternedType = Box<chalk_ir::TyData<Self>>; // FIXME use Arc?
diff --git a/crates/ra_hir_ty/src/traits/chalk/mapping.rs b/crates/ra_hir_ty/src/traits/chalk/mapping.rs
index 5f6daf842..834360430 100644
--- a/crates/ra_hir_ty/src/traits/chalk/mapping.rs
+++ b/crates/ra_hir_ty/src/traits/chalk/mapping.rs
@@ -16,8 +16,8 @@ use crate::{
     db::HirDatabase,
     primitive::{FloatBitness, FloatTy, IntBitness, IntTy, Signedness, Uncertain},
     traits::{builtin, AssocTyValue, Canonical, Impl, Obligation},
-    ApplicationTy, CallableDef, GenericPredicate, InEnvironment, ProjectionPredicate, ProjectionTy,
-    Substs, TraitEnvironment, TraitRef, Ty, TypeCtor,
+    ApplicationTy, CallableDef, GenericPredicate, InEnvironment, OpaqueTy, OpaqueTyId,
+    ProjectionPredicate, ProjectionTy, Substs, TraitEnvironment, TraitRef, Ty, TypeCtor,
 };
 
 use super::interner::*;
@@ -68,7 +68,16 @@ impl ToChalk for Ty {
                 let bounded_ty = chalk_ir::DynTy { bounds: make_binders(where_clauses, 1) };
                 chalk_ir::TyData::Dyn(bounded_ty).intern(&Interner)
             }
-            Ty::Opaque(_) | Ty::Unknown => {
+            Ty::Opaque(opaque_ty) => {
+                let opaque_ty_id = opaque_ty.opaque_ty_id.to_chalk(db);
+                let substitution = opaque_ty.parameters.to_chalk(db);
+                chalk_ir::TyData::Alias(chalk_ir::AliasTy::Opaque(chalk_ir::OpaqueTy {
+                    opaque_ty_id,
+                    substitution,
+                }))
+                .intern(&Interner)
+            }
+            Ty::Unknown => {
                 let substitution = chalk_ir::Substitution::empty(&Interner);
                 let name = TypeName::Error;
                 chalk_ir::ApplicationTy { name, substitution }.cast(&Interner).intern(&Interner)
@@ -98,7 +107,11 @@ impl ToChalk for Ty {
                 let parameters = from_chalk(db, proj.substitution);
                 Ty::Projection(ProjectionTy { associated_ty, parameters })
             }
-            chalk_ir::TyData::Alias(chalk_ir::AliasTy::Opaque(_)) => unimplemented!(),
+            chalk_ir::TyData::Alias(chalk_ir::AliasTy::Opaque(opaque_ty)) => {
+                let impl_trait_id = from_chalk(db, opaque_ty.opaque_ty_id);
+                let parameters = from_chalk(db, opaque_ty.substitution);
+                Ty::Opaque(OpaqueTy { opaque_ty_id: impl_trait_id, parameters })
+            }
             chalk_ir::TyData::Function(chalk_ir::Fn { num_binders: _, substitution }) => {
                 let parameters: Substs = from_chalk(db, substitution);
                 Ty::Apply(ApplicationTy {
@@ -204,6 +217,21 @@ impl ToChalk for hir_def::TraitId {
     }
 }
 
+impl ToChalk for OpaqueTyId {
+    type Chalk = chalk_ir::OpaqueTyId<Interner>;
+
+    fn to_chalk(self, db: &dyn HirDatabase) -> chalk_ir::OpaqueTyId<Interner> {
+        db.intern_impl_trait_id(self).into()
+    }
+
+    fn from_chalk(
+        db: &dyn HirDatabase,
+        opaque_ty_id: chalk_ir::OpaqueTyId<Interner>,
+    ) -> OpaqueTyId {
+        db.lookup_intern_impl_trait_id(opaque_ty_id.into())
+    }
+}
+
 impl ToChalk for TypeCtor {
     type Chalk = TypeName<Interner>;
 
@@ -214,6 +242,11 @@ impl ToChalk for TypeCtor {
                 TypeName::AssociatedType(type_id)
             }
 
+            TypeCtor::OpaqueType(impl_trait_id) => {
+                let id = impl_trait_id.to_chalk(db);
+                TypeName::OpaqueType(id)
+            }
+
             TypeCtor::Bool => TypeName::Scalar(Scalar::Bool),
             TypeCtor::Char => TypeName::Scalar(Scalar::Char),
             TypeCtor::Int(Uncertain::Known(int_ty)) => TypeName::Scalar(int_ty_to_chalk(int_ty)),
@@ -252,7 +285,9 @@ impl ToChalk for TypeCtor {
         match type_name {
             TypeName::Adt(struct_id) => db.lookup_intern_type_ctor(struct_id.into()),
             TypeName::AssociatedType(type_id) => TypeCtor::AssociatedType(from_chalk(db, type_id)),
-            TypeName::OpaqueType(_) => unreachable!(),
+            TypeName::OpaqueType(opaque_type_id) => {
+                TypeCtor::OpaqueType(from_chalk(db, opaque_type_id))
+            }
 
             TypeName::Scalar(Scalar::Bool) => TypeCtor::Bool,
             TypeName::Scalar(Scalar::Char) => TypeCtor::Char,
diff --git a/crates/ra_hir_ty/src/traits/chalk/tls.rs b/crates/ra_hir_ty/src/traits/chalk/tls.rs
index d88828c7c..556af7098 100644
--- a/crates/ra_hir_ty/src/traits/chalk/tls.rs
+++ b/crates/ra_hir_ty/src/traits/chalk/tls.rs
@@ -69,6 +69,11 @@ impl DebugContext<'_> {
                 let name = self.0.type_alias_data(type_alias).name.clone();
                 write!(f, "{}::{}", trait_name, name)?;
             }
+            TypeCtor::OpaqueType(opaque_ty_id) => match opaque_ty_id {
+                crate::OpaqueTyId::ReturnTypeImplTrait(func, idx) => {
+                    write!(f, "{{impl trait {} of {:?}}}", idx, func)?;
+                }
+            },
             TypeCtor::Closure { def, expr } => {
                 write!(f, "{{closure {:?} in ", expr.into_raw())?;
                 match def {