3 files changed, 56 insertions, 10 deletions

diff --git a/crates/ra_hir_def/src/generics.rs b/crates/ra_hir_def/src/generics.rs
index 7553d8a87..e4e616519 100644
--- a/crates/ra_hir_def/src/generics.rs
+++ b/crates/ra_hir_def/src/generics.rs
@@ -53,10 +53,17 @@ pub struct GenericParams {
 /// associated type bindings like `Iterator<Item = u32>`.
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct WherePredicate {
-    pub type_ref: TypeRef,
+    pub target: WherePredicateTarget,
     pub bound: TypeBound,
 }
 
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub enum WherePredicateTarget {
+    TypeRef(TypeRef),
+    /// For desugared where predicates that can directly refer to a type param.
+    TypeParam(LocalTypeParamId)
+}
+
 type SourceMap = ArenaMap<LocalTypeParamId, Either<ast::TraitDef, ast::TypeParam>>;
 
 impl GenericParams {
@@ -190,18 +197,24 @@ impl GenericParams {
             return;
         }
         let bound = TypeBound::from_ast(bound);
-        self.where_predicates.push(WherePredicate { type_ref, bound });
+        self.where_predicates.push(WherePredicate { target: WherePredicateTarget::TypeRef(type_ref), bound });
     }
 
     fn fill_implicit_impl_trait_args(&mut self, type_ref: &TypeRef) {
         type_ref.walk(&mut |type_ref| {
-            if let TypeRef::ImplTrait(_) = type_ref {
+            if let TypeRef::ImplTrait(bounds) = type_ref {
                 let param = TypeParamData {
                     name: None,
                     default: None,
                     provenance: TypeParamProvenance::ArgumentImplTrait,
                 };
-                let _param_id = self.types.alloc(param);
+                let param_id = self.types.alloc(param);
+                for bound in bounds {
+                    self.where_predicates.push(WherePredicate {
+                        target: WherePredicateTarget::TypeParam(param_id),
+                        bound: bound.clone()
+                    });
+                }
             }
         });
     }
@@ -211,6 +224,12 @@ impl GenericParams {
             .iter()
             .find_map(|(id, p)| if p.name.as_ref() == Some(name) { Some(id) } else { None })
     }
+
+    pub fn find_trait_self_param(&self) -> Option<LocalTypeParamId> {
+        self.types
+            .iter()
+            .find_map(|(id, p)| if p.provenance == TypeParamProvenance::TraitSelf { Some(id) } else { None })
+    }
 }
 
 impl HasChildSource for GenericDefId {
diff --git a/crates/ra_hir_ty/src/lower.rs b/crates/ra_hir_ty/src/lower.rs
index 459b96280..f1a11e073 100644
--- a/crates/ra_hir_ty/src/lower.rs
+++ b/crates/ra_hir_ty/src/lower.rs
@@ -10,7 +10,7 @@ use std::sync::Arc;
 
 use hir_def::{
     builtin_type::BuiltinType,
-    generics::WherePredicate,
+    generics::{WherePredicateTarget, WherePredicate},
     path::{GenericArg, Path, PathSegment, PathSegments},
     resolver::{HasResolver, Resolver, TypeNs},
     type_ref::{TypeBound, TypeRef},
@@ -505,7 +505,22 @@ impl GenericPredicate {
         ctx: &'a TyLoweringContext<'a, impl HirDatabase>,
         where_predicate: &'a WherePredicate,
     ) -> impl Iterator<Item = GenericPredicate> + 'a {
-        let self_ty = Ty::from_hir(ctx, &where_predicate.type_ref);
+        let self_ty = match &where_predicate.target {
+            WherePredicateTarget::TypeRef(type_ref) => Ty::from_hir(ctx, type_ref),
+            WherePredicateTarget::TypeParam(param_id) => {
+                let generic_def = ctx.resolver.generic_def().expect("generics in scope");
+                let generics = generics(ctx.db, generic_def);
+                let param_id = hir_def::TypeParamId { parent: generic_def, local_id: *param_id };
+                let idx = generics.param_idx(param_id);
+                match ctx.type_param_mode {
+                    TypeParamLoweringMode::Placeholder => {
+                        let name = generics.param_name(param_id);
+                        Ty::Param { idx, name }
+                    },
+                    TypeParamLoweringMode::Variable => Ty::Bound(idx),
+                }
+            },
+        };
         GenericPredicate::from_type_bound(ctx, &where_predicate.bound, self_ty)
     }
 
@@ -595,10 +610,18 @@ pub(crate) fn generic_predicates_for_param_query(
 ) -> Arc<[GenericPredicate]> {
     let resolver = def.resolver(db);
     let ctx = TyLoweringContext::new(db, &resolver);
+    let generics = generics(db, def);
     resolver
         .where_predicates_in_scope()
         // we have to filter out all other predicates *first*, before attempting to lower them
-        .filter(|pred| Ty::from_hir_only_param(&ctx, &pred.type_ref) == Some(param_idx))
+        .filter(|pred| match &pred.target {
+            WherePredicateTarget::TypeRef(type_ref) => Ty::from_hir_only_param(&ctx, type_ref) == Some(param_idx),
+            WherePredicateTarget::TypeParam(local_id) => {
+                let param_id = hir_def::TypeParamId { parent: def, local_id: *local_id };
+                let idx = generics.param_idx(param_id);
+                idx == param_idx
+            }
+        })
         .flat_map(|pred| GenericPredicate::from_where_predicate(&ctx, pred))
         .collect()
 }
diff --git a/crates/ra_hir_ty/src/utils.rs b/crates/ra_hir_ty/src/utils.rs
index f116b95e7..77b7de729 100644
--- a/crates/ra_hir_ty/src/utils.rs
+++ b/crates/ra_hir_ty/src/utils.rs
@@ -12,6 +12,7 @@ use hir_def::{
     AssocContainerId, GenericDefId, Lookup, TraitId, TypeAliasId, TypeParamId, VariantId,
 };
 use hir_expand::name::{name, Name};
+use hir_def::generics::WherePredicateTarget;
 
 fn direct_super_traits(db: &impl DefDatabase, trait_: TraitId) -> Vec<TraitId> {
     let resolver = trait_.resolver(db);
@@ -19,11 +20,14 @@ fn direct_super_traits(db: &impl DefDatabase, trait_: TraitId) -> Vec<TraitId> {
     // lifetime problems, but since there usually shouldn't be more than a
     // few direct traits this should be fine (we could even use some kind of
     // SmallVec if performance is a concern)
-    db.generic_params(trait_.into())
+    let generic_params = db.generic_params(trait_.into());
+    let trait_self = generic_params.find_trait_self_param();
+    generic_params
         .where_predicates
         .iter()
-        .filter_map(|pred| match &pred.type_ref {
-            TypeRef::Path(p) if p == &Path::from(name![Self]) => pred.bound.as_path(),
+        .filter_map(|pred| match &pred.target {
+            WherePredicateTarget::TypeRef(TypeRef::Path(p)) if p == &Path::from(name![Self]) => pred.bound.as_path(),
+            WherePredicateTarget::TypeParam(local_id) if Some(*local_id) == trait_self => pred.bound.as_path(),
             _ => None,
         })
         .filter_map(|path| match resolver.resolve_path_in_type_ns_fully(db, path.mod_path()) {