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.

10 files changed, 395 insertions, 77 deletions

diff --git a/crates/ra_hir_ty/src/db.rs b/crates/ra_hir_ty/src/db.rs
index 0a8bb24ac..bf71d38d6 100644
--- a/crates/ra_hir_ty/src/db.rs
+++ b/crates/ra_hir_ty/src/db.rs
@@ -3,8 +3,8 @@
 use std::sync::Arc;
 
 use hir_def::{
-    db::DefDatabase, DefWithBodyId, GenericDefId, ImplId, LocalFieldId, TraitId, TypeParamId,
-    VariantId,
+    db::DefDatabase, DefWithBodyId, FunctionId, GenericDefId, ImplId, LocalFieldId, TraitId,
+    TypeParamId, VariantId,
 };
 use ra_arena::map::ArenaMap;
 use ra_db::{impl_intern_key, salsa, CrateId, Upcast};
@@ -13,8 +13,8 @@ use ra_prof::profile;
 use crate::{
     method_resolution::{CrateImplDefs, TyFingerprint},
     traits::{chalk, AssocTyValue, Impl},
-    Binders, CallableDef, GenericPredicate, InferenceResult, PolyFnSig, Substs, TraitRef, Ty,
-    TyDefId, TypeCtor, ValueTyDefId,
+    Binders, CallableDef, GenericPredicate, InferenceResult, OpaqueTyId, PolyFnSig,
+    ReturnTypeImplTraits, Substs, TraitRef, Ty, TyDefId, TypeCtor, ValueTyDefId,
 };
 use hir_expand::name::Name;
 
@@ -48,6 +48,12 @@ pub trait HirDatabase: DefDatabase + Upcast<dyn DefDatabase> {
     #[salsa::invoke(crate::callable_item_sig)]
     fn callable_item_signature(&self, def: CallableDef) -> PolyFnSig;
 
+    #[salsa::invoke(crate::lower::return_type_impl_traits)]
+    fn return_type_impl_traits(
+        &self,
+        def: FunctionId,
+    ) -> Option<Arc<Binders<ReturnTypeImplTraits>>>;
+
     #[salsa::invoke(crate::lower::generic_predicates_for_param_query)]
     #[salsa::cycle(crate::lower::generic_predicates_for_param_recover)]
     fn generic_predicates_for_param(
@@ -80,6 +86,8 @@ pub trait HirDatabase: DefDatabase + Upcast<dyn DefDatabase> {
     #[salsa::interned]
     fn intern_type_param_id(&self, param_id: TypeParamId) -> GlobalTypeParamId;
     #[salsa::interned]
+    fn intern_impl_trait_id(&self, id: OpaqueTyId) -> InternedOpaqueTyId;
+    #[salsa::interned]
     fn intern_chalk_impl(&self, impl_: Impl) -> crate::traits::GlobalImplId;
     #[salsa::interned]
     fn intern_assoc_ty_value(&self, assoc_ty_value: AssocTyValue) -> crate::traits::AssocTyValueId;
@@ -142,3 +150,7 @@ fn hir_database_is_object_safe() {
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct GlobalTypeParamId(salsa::InternId);
 impl_intern_key!(GlobalTypeParamId);
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct InternedOpaqueTyId(salsa::InternId);
+impl_intern_key!(InternedOpaqueTyId);
diff --git a/crates/ra_hir_ty/src/display.rs b/crates/ra_hir_ty/src/display.rs
index b9c4d2e89..3e63a2415 100644
--- a/crates/ra_hir_ty/src/display.rs
+++ b/crates/ra_hir_ty/src/display.rs
@@ -359,6 +359,21 @@ impl HirDisplay for ApplicationTy {
                     write!(f, ">")?;
                 }
             }
+            TypeCtor::OpaqueType(opaque_ty_id) => {
+                let bounds = match opaque_ty_id {
+                    crate::OpaqueTyId::ReturnTypeImplTrait(func, idx) => {
+                        let datas =
+                            f.db.return_type_impl_traits(func).expect("impl trait id without data");
+                        let data = (*datas)
+                            .as_ref()
+                            .map(|rpit| rpit.impl_traits[idx as usize].bounds.clone());
+                        data.clone().subst(&self.parameters)
+                    }
+                };
+                write!(f, "impl ")?;
+                write_bounds_like_dyn_trait(&bounds.value, f)?;
+                // FIXME: it would maybe be good to distinguish this from the alias type (when debug printing), and to show the substitution
+            }
             TypeCtor::Closure { .. } => {
                 let sig = self.parameters[0].callable_sig(f.db);
                 if let Some(sig) = sig {
@@ -427,14 +442,24 @@ impl HirDisplay for Ty {
                 }
             }
             Ty::Bound(idx) => write!(f, "?{}.{}", idx.debruijn.depth(), idx.index)?,
-            Ty::Dyn(predicates) | Ty::Opaque(predicates) => {
-                match self {
-                    Ty::Dyn(_) => write!(f, "dyn ")?,
-                    Ty::Opaque(_) => write!(f, "impl ")?,
-                    _ => unreachable!(),
-                };
+            Ty::Dyn(predicates) => {
+                write!(f, "dyn ")?;
                 write_bounds_like_dyn_trait(predicates, f)?;
             }
+            Ty::Opaque(opaque_ty) => {
+                let bounds = match opaque_ty.opaque_ty_id {
+                    crate::OpaqueTyId::ReturnTypeImplTrait(func, idx) => {
+                        let datas =
+                            f.db.return_type_impl_traits(func).expect("impl trait id without data");
+                        let data = (*datas)
+                            .as_ref()
+                            .map(|rpit| rpit.impl_traits[idx as usize].bounds.clone());
+                        data.clone().subst(&opaque_ty.parameters)
+                    }
+                };
+                write!(f, "impl ")?;
+                write_bounds_like_dyn_trait(&bounds.value, f)?;
+            }
             Ty::Unknown => write!(f, "{{unknown}}")?,
             Ty::Infer(..) => write!(f, "_")?,
         }
diff --git a/crates/ra_hir_ty/src/infer.rs b/crates/ra_hir_ty/src/infer.rs
index dc77e88e5..d9bf3c2f0 100644
--- a/crates/ra_hir_ty/src/infer.rs
+++ b/crates/ra_hir_ty/src/infer.rs
@@ -39,8 +39,8 @@ use ra_syntax::SmolStr;
 use super::{
     primitive::{FloatTy, IntTy},
     traits::{Guidance, Obligation, ProjectionPredicate, Solution},
-    ApplicationTy, GenericPredicate, InEnvironment, ProjectionTy, Substs, TraitEnvironment,
-    TraitRef, Ty, TypeCtor, TypeWalk, Uncertain,
+    ApplicationTy, InEnvironment, ProjectionTy, Substs, TraitEnvironment, TraitRef, Ty, TypeCtor,
+    TypeWalk, Uncertain,
 };
 use crate::{
     db::HirDatabase, infer::diagnostics::InferenceDiagnostic, lower::ImplTraitLoweringMode,
@@ -383,25 +383,6 @@ impl<'a> InferenceContext<'a> {
     ) -> Ty {
         match assoc_ty {
             Some(res_assoc_ty) => {
-                // FIXME:
-                // Check if inner_ty is is `impl Trait` and contained input TypeAlias id
-                // this is a workaround while Chalk assoc type projection doesn't always work yet,
-                // but once that is fixed I don't think we should keep this
-                // (we'll probably change how associated types are resolved anyway)
-                if let Ty::Opaque(ref predicates) = inner_ty {
-                    for p in predicates.iter() {
-                        if let GenericPredicate::Projection(projection) = p {
-                            if projection.projection_ty.associated_ty == res_assoc_ty {
-                                if let ty_app!(_, params) = &projection.ty {
-                                    if params.len() == 0 {
-                                        return projection.ty.clone();
-                                    }
-                                }
-                            }
-                        }
-                    }
-                }
-
                 let ty = self.table.new_type_var();
                 let builder = Substs::build_for_def(self.db, res_assoc_ty)
                     .push(inner_ty)
diff --git a/crates/ra_hir_ty/src/lib.rs b/crates/ra_hir_ty/src/lib.rs
index 9fa8d3bdc..135976fcd 100644
--- a/crates/ra_hir_ty/src/lib.rs
+++ b/crates/ra_hir_ty/src/lib.rs
@@ -147,6 +147,12 @@ pub enum TypeCtor {
     /// an **application type** like `(Iterator::Item)<T>`.
     AssociatedType(TypeAliasId),
 
+    /// This represents a placeholder for an opaque type in situations where we
+    /// don't know the hidden type (i.e. currently almost always). This is
+    /// analogous to the `AssociatedType` type constructor. As with that one,
+    /// these are only produced by Chalk.
+    OpaqueType(OpaqueTyId),
+
     /// The type of a specific closure.
     ///
     /// The closure signature is stored in a `FnPtr` type in the first type
@@ -194,6 +200,14 @@ impl TypeCtor {
                 let generic_params = generics(db.upcast(), type_alias.into());
                 generic_params.len()
             }
+            TypeCtor::OpaqueType(opaque_ty_id) => {
+                match opaque_ty_id {
+                    OpaqueTyId::ReturnTypeImplTrait(func, _) => {
+                        let generic_params = generics(db.upcast(), func.into());
+                        generic_params.len()
+                    }
+                }
+            }
             TypeCtor::FnPtr { num_args } => num_args as usize + 1,
             TypeCtor::Tuple { cardinality } => cardinality as usize,
         }
@@ -220,6 +234,11 @@ impl TypeCtor {
             TypeCtor::AssociatedType(type_alias) => {
                 Some(type_alias.lookup(db.upcast()).module(db.upcast()).krate)
             }
+            TypeCtor::OpaqueType(opaque_ty_id) => match opaque_ty_id {
+                OpaqueTyId::ReturnTypeImplTrait(func, _) => {
+                    Some(func.lookup(db.upcast()).module(db.upcast()).krate)
+                }
+            },
         }
     }
 
@@ -241,6 +260,7 @@ impl TypeCtor {
             TypeCtor::Adt(adt) => Some(adt.into()),
             TypeCtor::FnDef(callable) => Some(callable.into()),
             TypeCtor::AssociatedType(type_alias) => Some(type_alias.into()),
+            TypeCtor::OpaqueType(_impl_trait_id) => None,
         }
     }
 }
@@ -254,6 +274,12 @@ pub struct ApplicationTy {
     pub parameters: Substs,
 }
 
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
+pub struct OpaqueTy {
+    pub opaque_ty_id: OpaqueTyId,
+    pub parameters: Substs,
+}
+
 /// A "projection" type corresponds to an (unnormalized)
 /// projection like `<P0 as Trait<P1..Pn>>::Foo`. Note that the
 /// trait and all its parameters are fully known.
@@ -308,6 +334,12 @@ pub enum Ty {
     /// trait and all its parameters are fully known.
     Projection(ProjectionTy),
 
+    /// An opaque type (`impl Trait`).
+    ///
+    /// This is currently only used for return type impl trait; each instance of
+    /// `impl Trait` in a return type gets its own ID.
+    Opaque(OpaqueTy),
+
     /// A placeholder for a type parameter; for example, `T` in `fn f<T>(x: T)
     /// {}` when we're type-checking the body of that function. In this
     /// situation, we know this stands for *some* type, but don't know the exact
@@ -332,12 +364,6 @@ pub enum Ty {
     /// didn't seem worth the overhead yet.
     Dyn(Arc<[GenericPredicate]>),
 
-    /// An opaque type (`impl Trait`).
-    ///
-    /// The predicates are quantified over the `Self` type; see `Ty::Dyn` for
-    /// more.
-    Opaque(Arc<[GenericPredicate]>),
-
     /// A placeholder for a type which could not be computed; this is propagated
     /// to avoid useless error messages. Doubles as a placeholder where type
     /// variables are inserted before type checking, since we want to try to
@@ -490,7 +516,7 @@ impl Deref for Substs {
     }
 }
 
-#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
 pub struct Binders<T> {
     pub num_binders: usize,
     pub value: T,
@@ -534,6 +560,20 @@ impl<T: TypeWalk> Binders<T> {
     }
 }
 
+impl<T: TypeWalk> TypeWalk for Binders<T> {
+    fn walk(&self, f: &mut impl FnMut(&Ty)) {
+        self.value.walk(f);
+    }
+
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    ) {
+        self.value.walk_mut_binders(f, binders.shifted_in())
+    }
+}
+
 /// A trait with type parameters. This includes the `Self`, so this represents a concrete type implementing the trait.
 /// Name to be bikeshedded: TraitBound? TraitImplements?
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
@@ -947,11 +987,16 @@ impl TypeWalk for Ty {
                     t.walk(f);
                 }
             }
-            Ty::Dyn(predicates) | Ty::Opaque(predicates) => {
+            Ty::Dyn(predicates) => {
                 for p in predicates.iter() {
                     p.walk(f);
                 }
             }
+            Ty::Opaque(o_ty) => {
+                for t in o_ty.parameters.iter() {
+                    t.walk(f);
+                }
+            }
             Ty::Placeholder { .. } | Ty::Bound(_) | Ty::Infer(_) | Ty::Unknown => {}
         }
         f(self);
@@ -969,13 +1014,48 @@ impl TypeWalk for Ty {
             Ty::Projection(p_ty) => {
                 p_ty.parameters.walk_mut_binders(f, binders);
             }
-            Ty::Dyn(predicates) | Ty::Opaque(predicates) => {
+            Ty::Dyn(predicates) => {
                 for p in make_mut_slice(predicates) {
                     p.walk_mut_binders(f, binders.shifted_in());
                 }
             }
+            Ty::Opaque(o_ty) => {
+                o_ty.parameters.walk_mut_binders(f, binders);
+            }
             Ty::Placeholder { .. } | Ty::Bound(_) | Ty::Infer(_) | Ty::Unknown => {}
         }
         f(self, binders);
     }
 }
+
+impl<T: TypeWalk> TypeWalk for Vec<T> {
+    fn walk(&self, f: &mut impl FnMut(&Ty)) {
+        for t in self {
+            t.walk(f);
+        }
+    }
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    ) {
+        for t in self {
+            t.walk_mut_binders(f, binders);
+        }
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
+pub enum OpaqueTyId {
+    ReturnTypeImplTrait(hir_def::FunctionId, u16),
+}
+
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
+pub struct ReturnTypeImplTraits {
+    pub(crate) impl_traits: Vec<ReturnTypeImplTrait>,
+}
+
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
+pub(crate) struct ReturnTypeImplTrait {
+    pub(crate) bounds: Binders<Vec<GenericPredicate>>,
+}
diff --git a/crates/ra_hir_ty/src/lower.rs b/crates/ra_hir_ty/src/lower.rs
index 35ac86a46..dfc018b0b 100644
--- a/crates/ra_hir_ty/src/lower.rs
+++ b/crates/ra_hir_ty/src/lower.rs
@@ -31,8 +31,9 @@ use crate::{
         all_super_trait_refs, associated_type_by_name_including_super_traits, generics,
         make_mut_slice, variant_data,
     },
-    Binders, BoundVar, DebruijnIndex, FnSig, GenericPredicate, PolyFnSig, ProjectionPredicate,
-    ProjectionTy, Substs, TraitEnvironment, TraitRef, Ty, TypeCtor, TypeWalk,
+    Binders, BoundVar, DebruijnIndex, FnSig, GenericPredicate, OpaqueTy, OpaqueTyId, PolyFnSig,
+    ProjectionPredicate, ProjectionTy, ReturnTypeImplTrait, ReturnTypeImplTraits, Substs,
+    TraitEnvironment, TraitRef, Ty, TypeCtor, TypeWalk,
 };
 use hir_expand::name::Name;
 
@@ -47,7 +48,16 @@ pub struct TyLoweringContext<'a> {
     /// possible currently, so this should be fine for now.
     pub type_param_mode: TypeParamLoweringMode,
     pub impl_trait_mode: ImplTraitLoweringMode,
-    pub impl_trait_counter: std::cell::Cell<u16>,
+    impl_trait_counter: std::cell::Cell<u16>,
+    /// When turning `impl Trait` into opaque types, we have to collect the
+    /// bounds at the same time to get the IDs correct (without becoming too
+    /// complicated). I don't like using interior mutability (as for the
+    /// counter), but I've tried and failed to make the lifetimes work for
+    /// passing around a `&mut TyLoweringContext`. The core problem is that
+    /// we're grouping the mutable data (the counter and this field) together
+    /// with the immutable context (the references to the DB and resolver).
+    /// Splitting this up would be a possible fix.
+    opaque_type_data: std::cell::RefCell<Vec<ReturnTypeImplTrait>>,
 }
 
 impl<'a> TyLoweringContext<'a> {
@@ -56,26 +66,42 @@ impl<'a> TyLoweringContext<'a> {
         let impl_trait_mode = ImplTraitLoweringMode::Disallowed;
         let type_param_mode = TypeParamLoweringMode::Placeholder;
         let in_binders = DebruijnIndex::INNERMOST;
-        Self { db, resolver, in_binders, impl_trait_mode, impl_trait_counter, type_param_mode }
+        let opaque_type_data = std::cell::RefCell::new(Vec::new());
+        Self {
+            db,
+            resolver,
+            in_binders,
+            impl_trait_mode,
+            impl_trait_counter,
+            type_param_mode,
+            opaque_type_data,
+        }
     }
 
-    pub fn with_shifted_in<T>(
+    pub fn with_debruijn<T>(
         &self,
         debruijn: DebruijnIndex,
         f: impl FnOnce(&TyLoweringContext) -> T,
     ) -> T {
+        let opaque_ty_data_vec = self.opaque_type_data.replace(Vec::new());
         let new_ctx = Self {
-            in_binders: self.in_binders.shifted_in_from(debruijn),
+            in_binders: debruijn,
             impl_trait_counter: std::cell::Cell::new(self.impl_trait_counter.get()),
+            opaque_type_data: std::cell::RefCell::new(opaque_ty_data_vec),
             ..*self
         };
         let result = f(&new_ctx);
         self.impl_trait_counter.set(new_ctx.impl_trait_counter.get());
+        self.opaque_type_data.replace(new_ctx.opaque_type_data.into_inner());
         result
     }
 
-    pub fn shifted_in(self, debruijn: DebruijnIndex) -> Self {
-        Self { in_binders: self.in_binders.shifted_in_from(debruijn), ..self }
+    pub fn with_shifted_in<T>(
+        &self,
+        debruijn: DebruijnIndex,
+        f: impl FnOnce(&TyLoweringContext) -> T,
+    ) -> T {
+        self.with_debruijn(self.in_binders.shifted_in_from(debruijn), f)
     }
 
     pub fn with_impl_trait_mode(self, impl_trait_mode: ImplTraitLoweringMode) -> Self {
@@ -167,20 +193,47 @@ impl Ty {
             TypeRef::ImplTrait(bounds) => {
                 match ctx.impl_trait_mode {
                     ImplTraitLoweringMode::Opaque => {
-                        let self_ty = Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, 0));
-                        let predicates = ctx.with_shifted_in(DebruijnIndex::ONE, |ctx| {
-                            bounds
-                                .iter()
-                                .flat_map(|b| {
-                                    GenericPredicate::from_type_bound(ctx, b, self_ty.clone())
-                                })
-                                .collect()
-                        });
-                        Ty::Opaque(predicates)
+                        let idx = ctx.impl_trait_counter.get();
+                        ctx.impl_trait_counter.set(idx + 1);
+
+                        assert!(idx as usize == ctx.opaque_type_data.borrow().len());
+                        // this dance is to make sure the data is in the right
+                        // place even if we encounter more opaque types while
+                        // lowering the bounds
+                        ctx.opaque_type_data
+                            .borrow_mut()
+                            .push(ReturnTypeImplTrait { bounds: Binders::new(1, Vec::new()) });
+                        // We don't want to lower the bounds inside the binders
+                        // we're currently in, because they don't end up inside
+                        // those binders. E.g. when we have `impl Trait<impl
+                        // OtherTrait<T>>`, the `impl OtherTrait<T>` can't refer
+                        // to the self parameter from `impl Trait`, and the
+                        // bounds aren't actually stored nested within each
+                        // other, but separately. So if the `T` refers to a type
+                        // parameter of the outer function, it's just one binder
+                        // away instead of two.
+                        let actual_opaque_type_data = ctx
+                            .with_debruijn(DebruijnIndex::INNERMOST, |ctx| {
+                                ReturnTypeImplTrait::from_hir(ctx, &bounds)
+                            });
+                        ctx.opaque_type_data.borrow_mut()[idx as usize] = actual_opaque_type_data;
+
+                        let func = match ctx.resolver.generic_def() {
+                            Some(GenericDefId::FunctionId(f)) => f,
+                            _ => {
+                                // this shouldn't happen
+                                return (Ty::Unknown, None);
+                            }
+                        };
+                        let impl_trait_id = OpaqueTyId::ReturnTypeImplTrait(func, idx);
+                        let generics = generics(ctx.db.upcast(), func.into());
+                        let parameters = Substs::bound_vars(&generics, ctx.in_binders);
+                        Ty::Opaque(OpaqueTy { opaque_ty_id: impl_trait_id, parameters })
                     }
                     ImplTraitLoweringMode::Param => {
                         let idx = ctx.impl_trait_counter.get();
-                        ctx.impl_trait_counter.set(idx + 1);
+                        // FIXME we're probably doing something wrong here
+                        ctx.impl_trait_counter.set(idx + count_impl_traits(type_ref) as u16);
                         if let Some(def) = ctx.resolver.generic_def() {
                             let generics = generics(ctx.db.upcast(), def);
                             let param = generics
@@ -197,7 +250,8 @@ impl Ty {
                     }
                     ImplTraitLoweringMode::Variable => {
                         let idx = ctx.impl_trait_counter.get();
-                        ctx.impl_trait_counter.set(idx + 1);
+                        // FIXME we're probably doing something wrong here
+                        ctx.impl_trait_counter.set(idx + count_impl_traits(type_ref) as u16);
                         let (parent_params, self_params, list_params, _impl_trait_params) =
                             if let Some(def) = ctx.resolver.generic_def() {
                                 let generics = generics(ctx.db.upcast(), def);
@@ -663,6 +717,29 @@ fn assoc_type_bindings_from_type_bound<'a>(
         })
 }
 
+impl ReturnTypeImplTrait {
+    fn from_hir(ctx: &TyLoweringContext, bounds: &[TypeBound]) -> Self {
+        let self_ty = Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, 0));
+        let predicates = ctx.with_shifted_in(DebruijnIndex::ONE, |ctx| {
+            bounds
+                .iter()
+                .flat_map(|b| GenericPredicate::from_type_bound(ctx, b, self_ty.clone()))
+                .collect()
+        });
+        ReturnTypeImplTrait { bounds: Binders::new(1, predicates) }
+    }
+}
+
+fn count_impl_traits(type_ref: &TypeRef) -> usize {
+    let mut count = 0;
+    type_ref.walk(&mut |type_ref| {
+        if matches!(type_ref, TypeRef::ImplTrait(_)) {
+            count += 1;
+        }
+    });
+    count
+}
+
 /// Build the signature of a callable item (function, struct or enum variant).
 pub fn callable_item_sig(db: &dyn HirDatabase, def: CallableDef) -> PolyFnSig {
     match def {
@@ -864,7 +941,9 @@ fn fn_sig_for_fn(db: &dyn HirDatabase, def: FunctionId) -> PolyFnSig {
         .with_impl_trait_mode(ImplTraitLoweringMode::Variable)
         .with_type_param_mode(TypeParamLoweringMode::Variable);
     let params = data.params.iter().map(|tr| Ty::from_hir(&ctx_params, tr)).collect::<Vec<_>>();
-    let ctx_ret = ctx_params.with_impl_trait_mode(ImplTraitLoweringMode::Opaque);
+    let ctx_ret = TyLoweringContext::new(db, &resolver)
+        .with_impl_trait_mode(ImplTraitLoweringMode::Opaque)
+        .with_type_param_mode(TypeParamLoweringMode::Variable);
     let ret = Ty::from_hir(&ctx_ret, &data.ret_type);
     let generics = generics(db.upcast(), def.into());
     let num_binders = generics.len();
@@ -1084,3 +1163,25 @@ pub(crate) fn impl_trait_query(db: &dyn HirDatabase, impl_id: ImplId) -> Option<
         TraitRef::from_hir(&ctx, target_trait, Some(self_ty.value))?,
     ))
 }
+
+pub(crate) fn return_type_impl_traits(
+    db: &impl HirDatabase,
+    def: hir_def::FunctionId,
+) -> Option<Arc<Binders<ReturnTypeImplTraits>>> {
+    // FIXME unify with fn_sig_for_fn instead of doing lowering twice, maybe
+    let data = db.function_data(def);
+    let resolver = def.resolver(db.upcast());
+    let ctx_ret = TyLoweringContext::new(db, &resolver)
+        .with_impl_trait_mode(ImplTraitLoweringMode::Opaque)
+        .with_type_param_mode(TypeParamLoweringMode::Variable);
+    let _ret = Ty::from_hir(&ctx_ret, &data.ret_type);
+    let generics = generics(db.upcast(), def.into());
+    let num_binders = generics.len();
+    let return_type_impl_traits =
+        ReturnTypeImplTraits { impl_traits: ctx_ret.opaque_type_data.into_inner() };
+    if return_type_impl_traits.impl_traits.is_empty() {
+        None
+    } else {
+        Some(Arc::new(Binders::new(num_binders, return_type_impl_traits)))
+    }
+}
diff --git a/crates/ra_hir_ty/src/tests/traits.rs b/crates/ra_hir_ty/src/tests/traits.rs
index e8778d419..d83dc6d79 100644
--- a/crates/ra_hir_ty/src/tests/traits.rs
+++ b/crates/ra_hir_ty/src/tests/traits.rs
@@ -1110,7 +1110,6 @@ fn test() {
 }
 
 #[test]
-#[ignore]
 fn impl_trait() {
     assert_snapshot!(
         infer(r#"
@@ -1161,6 +1160,52 @@ fn test(x: impl Trait<u64>, y: &impl Trait<u64>) {
 }
 
 #[test]
+fn return_pos_impl_trait() {
+    assert_snapshot!(
+        infer(r#"
+trait Iterator {
+    type Item;
+    fn next(&mut self) -> Self::Item;
+}
+trait Trait<T> {
+    fn foo(&self) -> T;
+}
+fn bar() -> (impl Iterator<Item = impl Trait<u32>>, impl Trait<u64>) { loop {} }
+fn baz<T>(t: T) -> (impl Iterator<Item = impl Trait<T>>, impl Trait<T>) { loop {} }
+
+fn test() {
+    // let (a, b) = bar();
+    // a.next().foo();
+    // b.foo();
+    let (c, d) = baz(1u128);
+    c.next();//.foo();
+    // d.foo();
+}
+"#),
+        @r###"
+    50..54 'self': &mut Self
+    102..106 'self': &Self
+    185..196 '{ loop {} }': ({unknown}, {unknown})
+    187..194 'loop {}': !
+    192..194 '{}': ()
+    207..208 't': T
+    269..280 '{ loop {} }': ({unknown}, {unknown})
+    271..278 'loop {}': !
+    276..278 '{}': ()
+    292..429 '{     ...o(); }': ()
+    368..374 '(c, d)': (impl Iterator<Item = impl Trait<u128>>, impl Trait<u128>)
+    369..370 'c': impl Iterator<Item = impl Trait<u128>>
+    372..373 'd': impl Trait<u128>
+    377..380 'baz': fn baz<u128>(u128) -> (impl Iterator<Item = impl Trait<u128>>, impl Trait<u128>)
+    377..387 'baz(1u128)': (impl Iterator<Item = impl Trait<u128>>, impl Trait<u128>)
+    381..386 '1u128': u128
+    393..394 'c': impl Iterator<Item = impl Trait<u128>>
+    393..401 'c.next()': impl Trait<u128>
+    "###
+    );
+}
+
+#[test]
 fn dyn_trait() {
     assert_snapshot!(
         infer(r#"
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 {