1 files changed, 241 insertions, 88 deletions

diff --git a/crates/ra_hir_ty/src/lower.rs b/crates/ra_hir_ty/src/lower.rs
index 35ac86a46..f274579ea 100644
--- a/crates/ra_hir_ty/src/lower.rs
+++ b/crates/ra_hir_ty/src/lower.rs
@@ -5,10 +5,7 @@
 //!  - Building the type for an item: This happens through the `type_for_def` query.
 //!
 //! This usually involves resolving names, collecting generic arguments etc.
-use std::iter;
-use std::sync::Arc;
-
-use smallvec::SmallVec;
+use std::{iter, sync::Arc};
 
 use hir_def::{
     adt::StructKind,
@@ -21,8 +18,12 @@ use hir_def::{
     HasModule, ImplId, LocalFieldId, Lookup, StaticId, StructId, TraitId, TypeAliasId, TypeParamId,
     UnionId, VariantId,
 };
+use hir_expand::name::Name;
 use ra_arena::map::ArenaMap;
 use ra_db::CrateId;
+use smallvec::SmallVec;
+use stdx::impl_from;
+use test_utils::mark;
 
 use crate::{
     db::HirDatabase,
@@ -31,10 +32,10 @@ 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;
 
 #[derive(Debug)]
 pub struct TyLoweringContext<'a> {
@@ -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 {
@@ -150,9 +176,12 @@ impl Ty {
                 Ty::apply_one(TypeCtor::Ref(*mutability), inner_ty)
             }
             TypeRef::Placeholder => Ty::Unknown,
-            TypeRef::Fn(params) => {
+            TypeRef::Fn(params, is_varargs) => {
                 let sig = Substs(params.iter().map(|tr| Ty::from_hir(ctx, tr)).collect());
-                Ty::apply(TypeCtor::FnPtr { num_args: sig.len() as u16 - 1 }, sig)
+                Ty::apply(
+                    TypeCtor::FnPtr { num_args: sig.len() as u16 - 1, is_varargs: *is_varargs },
+                    sig,
+                )
             }
             TypeRef::DynTrait(bounds) => {
                 let self_ty = Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, 0));
@@ -167,20 +196,44 @@ 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,
+                            _ => panic!("opaque impl trait lowering in non-function"),
+                        };
+                        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);
@@ -271,6 +325,7 @@ impl Ty {
         resolution: TypeNs,
         resolved_segment: PathSegment<'_>,
         remaining_segments: PathSegments<'_>,
+        infer_args: bool,
     ) -> (Ty, Option<TypeNs>) {
         let ty = match resolution {
             TypeNs::TraitId(trait_) => {
@@ -284,17 +339,17 @@ impl Ty {
                     TraitRef::from_resolved_path(ctx, trait_, resolved_segment, self_ty);
                 let ty = if remaining_segments.len() == 1 {
                     let segment = remaining_segments.first().unwrap();
-                    let associated_ty = associated_type_by_name_including_super_traits(
-                        ctx.db.upcast(),
-                        trait_ref.trait_,
+                    let found = associated_type_by_name_including_super_traits(
+                        ctx.db,
+                        trait_ref.clone(),
                         &segment.name,
                     );
-                    match associated_ty {
-                        Some(associated_ty) => {
+                    match found {
+                        Some((super_trait_ref, associated_ty)) => {
                             // FIXME handle type parameters on the segment
                             Ty::Projection(ProjectionTy {
                                 associated_ty,
-                                parameters: trait_ref.substs,
+                                parameters: super_trait_ref.substs,
                             })
                         }
                         None => {
@@ -348,9 +403,15 @@ impl Ty {
                 ctx.db.ty(adt.into()).subst(&substs)
             }
 
-            TypeNs::AdtId(it) => Ty::from_hir_path_inner(ctx, resolved_segment, it.into()),
-            TypeNs::BuiltinType(it) => Ty::from_hir_path_inner(ctx, resolved_segment, it.into()),
-            TypeNs::TypeAliasId(it) => Ty::from_hir_path_inner(ctx, resolved_segment, it.into()),
+            TypeNs::AdtId(it) => {
+                Ty::from_hir_path_inner(ctx, resolved_segment, it.into(), infer_args)
+            }
+            TypeNs::BuiltinType(it) => {
+                Ty::from_hir_path_inner(ctx, resolved_segment, it.into(), infer_args)
+            }
+            TypeNs::TypeAliasId(it) => {
+                Ty::from_hir_path_inner(ctx, resolved_segment, it.into(), infer_args)
+            }
             // FIXME: report error
             TypeNs::EnumVariantId(_) => return (Ty::Unknown, None),
         };
@@ -376,7 +437,13 @@ impl Ty {
             ),
             Some(i) => (path.segments().get(i - 1).unwrap(), path.segments().skip(i)),
         };
-        Ty::from_partly_resolved_hir_path(ctx, resolution, resolved_segment, remaining_segments)
+        Ty::from_partly_resolved_hir_path(
+            ctx,
+            resolution,
+            resolved_segment,
+            remaining_segments,
+            false,
+        )
     }
 
     fn select_associated_type(
@@ -402,6 +469,9 @@ impl Ty {
                             }
                             TypeParamLoweringMode::Variable => t.substs.clone(),
                         };
+                        // We need to shift in the bound vars, since
+                        // associated_type_shorthand_candidates does not do that
+                        let substs = substs.shift_bound_vars(ctx.in_binders);
                         // FIXME handle type parameters on the segment
                         return Some(Ty::Projection(ProjectionTy {
                             associated_ty,
@@ -422,13 +492,14 @@ impl Ty {
         ctx: &TyLoweringContext<'_>,
         segment: PathSegment<'_>,
         typable: TyDefId,
+        infer_args: bool,
     ) -> Ty {
         let generic_def = match typable {
             TyDefId::BuiltinType(_) => None,
             TyDefId::AdtId(it) => Some(it.into()),
             TyDefId::TypeAliasId(it) => Some(it.into()),
         };
-        let substs = substs_from_path_segment(ctx, segment, generic_def, false);
+        let substs = substs_from_path_segment(ctx, segment, generic_def, infer_args);
         ctx.db.ty(typable).subst(&substs)
     }
 
@@ -441,6 +512,7 @@ impl Ty {
         // `ValueTyDefId` is just a convenient way to pass generics and
         // special-case enum variants
         resolved: ValueTyDefId,
+        infer_args: bool,
     ) -> Substs {
         let last = path.segments().last().expect("path should have at least one segment");
         let (segment, generic_def) = match resolved {
@@ -463,22 +535,27 @@ impl Ty {
                 (segment, Some(var.parent.into()))
             }
         };
-        substs_from_path_segment(ctx, segment, generic_def, false)
+        substs_from_path_segment(ctx, segment, generic_def, infer_args)
     }
 }
 
-pub(super) fn substs_from_path_segment(
+fn substs_from_path_segment(
     ctx: &TyLoweringContext<'_>,
     segment: PathSegment<'_>,
     def_generic: Option<GenericDefId>,
-    _add_self_param: bool,
+    infer_args: bool,
 ) -> Substs {
     let mut substs = Vec::new();
     let def_generics = def_generic.map(|def| generics(ctx.db.upcast(), def));
 
     let (parent_params, self_params, type_params, impl_trait_params) =
         def_generics.map_or((0, 0, 0, 0), |g| g.provenance_split());
+    let total_len = parent_params + self_params + type_params + impl_trait_params;
+
     substs.extend(iter::repeat(Ty::Unknown).take(parent_params));
+
+    let mut had_explicit_args = false;
+
     if let Some(generic_args) = &segment.args_and_bindings {
         if !generic_args.has_self_type {
             substs.extend(iter::repeat(Ty::Unknown).take(self_params));
@@ -490,31 +567,37 @@ pub(super) fn substs_from_path_segment(
         for arg in generic_args.args.iter().skip(skip).take(expected_num) {
             match arg {
                 GenericArg::Type(type_ref) => {
+                    had_explicit_args = true;
                     let ty = Ty::from_hir(ctx, type_ref);
                     substs.push(ty);
                 }
             }
         }
     }
-    let total_len = parent_params + self_params + type_params + impl_trait_params;
+
+    // handle defaults. In expression or pattern path segments without
+    // explicitly specified type arguments, missing type arguments are inferred
+    // (i.e. defaults aren't used).
+    if !infer_args || had_explicit_args {
+        if let Some(def_generic) = def_generic {
+            let defaults = ctx.db.generic_defaults(def_generic);
+            assert_eq!(total_len, defaults.len());
+
+            for default_ty in defaults.iter().skip(substs.len()) {
+                // each default can depend on the previous parameters
+                let substs_so_far = Substs(substs.clone().into());
+                substs.push(default_ty.clone().subst(&substs_so_far));
+            }
+        }
+    }
+
     // add placeholders for args that were not provided
+    // FIXME: emit diagnostics in contexts where this is not allowed
     for _ in substs.len()..total_len {
         substs.push(Ty::Unknown);
     }
     assert_eq!(substs.len(), total_len);
 
-    // handle defaults
-    if let Some(def_generic) = def_generic {
-        let default_substs = ctx.db.generic_defaults(def_generic);
-        assert_eq!(substs.len(), default_substs.len());
-
-        for (i, default_ty) in default_substs.iter().enumerate() {
-            if substs[i] == Ty::Unknown {
-                substs[i] = default_ty.clone();
-            }
-        }
-    }
-
     Substs(substs.into())
 }
 
@@ -563,9 +646,7 @@ impl TraitRef {
         segment: PathSegment<'_>,
         resolved: TraitId,
     ) -> Substs {
-        let has_self_param =
-            segment.args_and_bindings.as_ref().map(|a| a.has_self_type).unwrap_or(false);
-        substs_from_path_segment(ctx, segment, Some(resolved.into()), !has_self_param)
+        substs_from_path_segment(ctx, segment, Some(resolved.into()), false)
     }
 
     pub(crate) fn from_type_bound(
@@ -632,17 +713,16 @@ fn assoc_type_bindings_from_type_bound<'a>(
         .flat_map(|segment| segment.args_and_bindings.into_iter())
         .flat_map(|args_and_bindings| args_and_bindings.bindings.iter())
         .flat_map(move |binding| {
-            let associated_ty = associated_type_by_name_including_super_traits(
-                ctx.db.upcast(),
-                trait_ref.trait_,
+            let found = associated_type_by_name_including_super_traits(
+                ctx.db,
+                trait_ref.clone(),
                 &binding.name,
             );
-            let associated_ty = match associated_ty {
+            let (super_trait_ref, associated_ty) = match found {
                 None => return SmallVec::<[GenericPredicate; 1]>::new(),
                 Some(t) => t,
             };
-            let projection_ty =
-                ProjectionTy { associated_ty, parameters: trait_ref.substs.clone() };
+            let projection_ty = ProjectionTy { associated_ty, parameters: super_trait_ref.substs };
             let mut preds = SmallVec::with_capacity(
                 binding.type_ref.as_ref().map_or(0, |_| 1) + binding.bounds.len(),
             );
@@ -663,12 +743,36 @@ fn assoc_type_bindings_from_type_bound<'a>(
         })
 }
 
+impl ReturnTypeImplTrait {
+    fn from_hir(ctx: &TyLoweringContext, bounds: &[TypeBound]) -> Self {
+        mark::hit!(lower_rpit);
+        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 {
+pub fn callable_item_sig(db: &dyn HirDatabase, def: CallableDefId) -> PolyFnSig {
     match def {
-        CallableDef::FunctionId(f) => fn_sig_for_fn(db, f),
-        CallableDef::StructId(s) => fn_sig_for_struct_constructor(db, s),
-        CallableDef::EnumVariantId(e) => fn_sig_for_enum_variant_constructor(db, e),
+        CallableDefId::FunctionId(f) => fn_sig_for_fn(db, f),
+        CallableDefId::StructId(s) => fn_sig_for_struct_constructor(db, s),
+        CallableDefId::EnumVariantId(e) => fn_sig_for_enum_variant_constructor(db, e),
     }
 }
 
@@ -844,17 +948,42 @@ pub(crate) fn generic_predicates_query(
 }
 
 /// Resolve the default type params from generics
-pub(crate) fn generic_defaults_query(db: &dyn HirDatabase, def: GenericDefId) -> Substs {
+pub(crate) fn generic_defaults_query(
+    db: &dyn HirDatabase,
+    def: GenericDefId,
+) -> Arc<[Binders<Ty>]> {
     let resolver = def.resolver(db.upcast());
-    let ctx = TyLoweringContext::new(db, &resolver);
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
     let generic_params = generics(db.upcast(), def);
 
     let defaults = generic_params
         .iter()
-        .map(|(_idx, p)| p.default.as_ref().map_or(Ty::Unknown, |t| Ty::from_hir(&ctx, t)))
+        .enumerate()
+        .map(|(idx, (_, p))| {
+            let mut ty = p.default.as_ref().map_or(Ty::Unknown, |t| Ty::from_hir(&ctx, t));
+
+            // Each default can only refer to previous parameters.
+            ty.walk_mut_binders(
+                &mut |ty, binders| match ty {
+                    Ty::Bound(BoundVar { debruijn, index }) if *debruijn == binders => {
+                        if *index >= idx {
+                            // type variable default referring to parameter coming
+                            // after it. This is forbidden (FIXME: report
+                            // diagnostic)
+                            *ty = Ty::Unknown;
+                        }
+                    }
+                    _ => {}
+                },
+                DebruijnIndex::INNERMOST,
+            );
+
+            Binders::new(idx, ty)
+        })
         .collect();
 
-    Substs(defaults)
+    defaults
 }
 
 fn fn_sig_for_fn(db: &dyn HirDatabase, def: FunctionId) -> PolyFnSig {
@@ -864,11 +993,13 @@ 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();
-    Binders::new(num_binders, FnSig::from_params_and_return(params, ret))
+    Binders::new(num_binders, FnSig::from_params_and_return(params, ret, data.is_varargs))
 }
 
 /// Build the declared type of a function. This should not need to look at the
@@ -919,7 +1050,7 @@ fn fn_sig_for_struct_constructor(db: &dyn HirDatabase, def: StructId) -> PolyFnS
     let params =
         fields.iter().map(|(_, field)| Ty::from_hir(&ctx, &field.type_ref)).collect::<Vec<_>>();
     let ret = type_for_adt(db, def.into());
-    Binders::new(ret.num_binders, FnSig::from_params_and_return(params, ret.value))
+    Binders::new(ret.num_binders, FnSig::from_params_and_return(params, ret.value, false))
 }
 
 /// Build the type of a tuple struct constructor.
@@ -943,7 +1074,7 @@ fn fn_sig_for_enum_variant_constructor(db: &dyn HirDatabase, def: EnumVariantId)
     let params =
         fields.iter().map(|(_, field)| Ty::from_hir(&ctx, &field.type_ref)).collect::<Vec<_>>();
     let ret = type_for_adt(db, def.parent.into());
-    Binders::new(ret.num_binders, FnSig::from_params_and_return(params, ret.value))
+    Binders::new(ret.num_binders, FnSig::from_params_and_return(params, ret.value, false))
 }
 
 /// Build the type of a tuple enum variant constructor.
@@ -976,31 +1107,31 @@ fn type_for_type_alias(db: &dyn HirDatabase, t: TypeAliasId) -> Binders<Ty> {
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
-pub enum CallableDef {
+pub enum CallableDefId {
     FunctionId(FunctionId),
     StructId(StructId),
     EnumVariantId(EnumVariantId),
 }
-impl_froms!(CallableDef: FunctionId, StructId, EnumVariantId);
+impl_from!(FunctionId, StructId, EnumVariantId for CallableDefId);
 
-impl CallableDef {
+impl CallableDefId {
     pub fn krate(self, db: &dyn HirDatabase) -> CrateId {
         let db = db.upcast();
         match self {
-            CallableDef::FunctionId(f) => f.lookup(db).module(db),
-            CallableDef::StructId(s) => s.lookup(db).container.module(db),
-            CallableDef::EnumVariantId(e) => e.parent.lookup(db).container.module(db),
+            CallableDefId::FunctionId(f) => f.lookup(db).module(db),
+            CallableDefId::StructId(s) => s.lookup(db).container.module(db),
+            CallableDefId::EnumVariantId(e) => e.parent.lookup(db).container.module(db),
         }
         .krate
     }
 }
 
-impl From<CallableDef> for GenericDefId {
-    fn from(def: CallableDef) -> GenericDefId {
+impl From<CallableDefId> for GenericDefId {
+    fn from(def: CallableDefId) -> GenericDefId {
         match def {
-            CallableDef::FunctionId(f) => f.into(),
-            CallableDef::StructId(s) => s.into(),
-            CallableDef::EnumVariantId(e) => e.into(),
+            CallableDefId::FunctionId(f) => f.into(),
+            CallableDefId::StructId(s) => s.into(),
+            CallableDefId::EnumVariantId(e) => e.into(),
         }
     }
 }
@@ -1011,7 +1142,7 @@ pub enum TyDefId {
     AdtId(AdtId),
     TypeAliasId(TypeAliasId),
 }
-impl_froms!(TyDefId: BuiltinType, AdtId(StructId, EnumId, UnionId), TypeAliasId);
+impl_from!(BuiltinType, AdtId(StructId, EnumId, UnionId), TypeAliasId for TyDefId);
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub enum ValueTyDefId {
@@ -1021,7 +1152,7 @@ pub enum ValueTyDefId {
     ConstId(ConstId),
     StaticId(StaticId),
 }
-impl_froms!(ValueTyDefId: FunctionId, StructId, EnumVariantId, ConstId, StaticId);
+impl_from!(FunctionId, StructId, EnumVariantId, ConstId, StaticId for ValueTyDefId);
 
 /// Build the declared type of an item. This depends on the namespace; e.g. for
 /// `struct Foo(usize)`, we have two types: The type of the struct itself, and
@@ -1084,3 +1215,25 @@ 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: &dyn 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)))
+    }
+}