7 files changed, 112 insertions, 5 deletions

diff --git a/crates/ra_hir_def/src/data.rs b/crates/ra_hir_def/src/data.rs
index 1aa9a9b7d..0a282f31b 100644
--- a/crates/ra_hir_def/src/data.rs
+++ b/crates/ra_hir_def/src/data.rs
@@ -6,12 +6,15 @@ use hir_expand::{
     name::{name, AsName, Name},
     AstId, InFile,
 };
-use ra_syntax::ast::{self, AstNode, ImplItem, ModuleItemOwner, NameOwner, TypeAscriptionOwner};
+use ra_syntax::ast::{
+    self, AstNode, AsyncOwner, ImplItem, ModuleItemOwner, NameOwner, TypeAscriptionOwner,
+};
 
 use crate::{
     db::DefDatabase,
+    path::{path, GenericArgs, Path},
     src::HasSource,
-    type_ref::{Mutability, TypeRef},
+    type_ref::{Mutability, TypeBound, TypeRef},
     AssocContainerId, AssocItemId, ConstId, ConstLoc, Expander, FunctionId, FunctionLoc, HasModule,
     ImplId, Intern, Lookup, ModuleId, StaticId, TraitId, TypeAliasId, TypeAliasLoc,
 };
@@ -62,11 +65,31 @@ impl FunctionData {
             TypeRef::unit()
         };
 
+        let ret_type = if src.value.is_async() {
+            let future_impl = desugar_future_path(ret_type);
+            let ty_bound = TypeBound::Path(future_impl);
+            TypeRef::ImplTrait(vec![ty_bound])
+        } else {
+            ret_type
+        };
+
         let sig = FunctionData { name, params, ret_type, has_self_param };
         Arc::new(sig)
     }
 }
 
+fn desugar_future_path(orig: TypeRef) -> Path {
+    let path = path![std::future::Future];
+
+    let mut generic_args: Vec<_> = std::iter::repeat(None).take(path.segments.len() - 1).collect();
+
+    let mut last = GenericArgs::empty();
+    last.bindings.push((name![Output], orig));
+    generic_args.push(Some(Arc::new(last)));
+
+    Path::from_known_path(path, generic_args)
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct TypeAliasData {
     pub name: Name,
diff --git a/crates/ra_hir_def/src/path.rs b/crates/ra_hir_def/src/path.rs
index 8e1294201..bf401df35 100644
--- a/crates/ra_hir_def/src/path.rs
+++ b/crates/ra_hir_def/src/path.rs
@@ -130,6 +130,11 @@ impl Path {
         Path { type_anchor: None, mod_path: name_ref.as_name().into(), generic_args: vec![None] }
     }
 
+    /// Converts a known mod path to `Path`.
+    pub(crate) fn from_known_path(path: ModPath, generic_args: Vec<Option<Arc<GenericArgs>>>) -> Path {
+        Path { type_anchor: None, mod_path: path, generic_args }
+    }
+
     pub fn kind(&self) -> &PathKind {
         &self.mod_path.kind
     }
diff --git a/crates/ra_hir_ty/src/infer.rs b/crates/ra_hir_ty/src/infer.rs
index e97b81473..08c220cfe 100644
--- a/crates/ra_hir_ty/src/infer.rs
+++ b/crates/ra_hir_ty/src/infer.rs
@@ -37,8 +37,8 @@ use test_utils::tested_by;
 use super::{
     primitive::{FloatTy, IntTy},
     traits::{Guidance, Obligation, ProjectionPredicate, Solution},
-    ApplicationTy, InEnvironment, ProjectionTy, Substs, TraitEnvironment, TraitRef, Ty, TypeCtor,
-    TypeWalk, Uncertain,
+    ApplicationTy, GenericPredicate, InEnvironment, ProjectionTy, Substs, TraitEnvironment,
+    TraitRef, Ty, TypeCtor, TypeWalk, Uncertain,
 };
 use crate::{db::HirDatabase, infer::diagnostics::InferenceDiagnostic};
 
@@ -379,6 +379,19 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     ) -> Ty {
         match assoc_ty {
             Some(res_assoc_ty) => {
+                // Fast path: Check if inner_ty is is `impl Trait` and contained input TypeAlias id
+                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
+                                && projection.ty != Ty::Unknown
+                            {
+                                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/tests/traits.rs b/crates/ra_hir_ty/src/tests/traits.rs
index 76e2198b6..08d1bf044 100644
--- a/crates/ra_hir_ty/src/tests/traits.rs
+++ b/crates/ra_hir_ty/src/tests/traits.rs
@@ -38,6 +38,63 @@ mod future {
 }
 
 #[test]
+fn infer_async() {
+    let (db, pos) = TestDB::with_position(
+        r#"
+//- /main.rs crate:main deps:std
+
+async fn foo() -> u64 {
+    128
+}
+
+fn test() {
+    let r = foo();
+    let v = r.await;
+    v<|>;
+}
+
+//- /std.rs crate:std
+#[prelude_import] use future::*;
+mod future {
+    trait Future {
+        type Output;
+    }
+}
+
+"#,
+    );
+    assert_eq!("u64", type_at_pos(&db, pos));
+}
+
+#[test]
+fn infer_desugar_async() {
+    let (db, pos) = TestDB::with_position(
+        r#"
+//- /main.rs crate:main deps:std
+
+async fn foo() -> u64 {
+    128
+}
+
+fn test() {
+    let r = foo();
+    r<|>;
+}
+
+//- /std.rs crate:std
+#[prelude_import] use future::*;
+mod future {
+    trait Future {
+        type Output;
+    }
+}
+
+"#,
+    );
+    assert_eq!("impl Future<Output = u64>", type_at_pos(&db, pos));
+}
+
+#[test]
 fn infer_try() {
     let (db, pos) = TestDB::with_position(
         r#"
diff --git a/crates/ra_syntax/src/ast/generated.rs b/crates/ra_syntax/src/ast/generated.rs
index 9f9d6e63c..73e1c407c 100644
--- a/crates/ra_syntax/src/ast/generated.rs
+++ b/crates/ra_syntax/src/ast/generated.rs
@@ -1129,6 +1129,7 @@ impl ast::NameOwner for FnDef {}
 impl ast::TypeParamsOwner for FnDef {}
 impl ast::AttrsOwner for FnDef {}
 impl ast::DocCommentsOwner for FnDef {}
+impl ast::AsyncOwner for FnDef {}
 impl FnDef {
     pub fn param_list(&self) -> Option<ParamList> {
         AstChildren::new(&self.syntax).next()
diff --git a/crates/ra_syntax/src/ast/traits.rs b/crates/ra_syntax/src/ast/traits.rs
index f99984fe0..8bf6aa2f0 100644
--- a/crates/ra_syntax/src/ast/traits.rs
+++ b/crates/ra_syntax/src/ast/traits.rs
@@ -8,6 +8,7 @@ use crate::{
     ast::{self, child_opt, children, AstChildren, AstNode, AstToken},
     match_ast,
     syntax_node::{SyntaxElementChildren, SyntaxNodeChildren},
+    SyntaxKind,
 };
 
 pub trait TypeAscriptionOwner: AstNode {
@@ -105,6 +106,12 @@ pub trait AttrsOwner: AstNode {
     }
 }
 
+pub trait AsyncOwner: AstNode {
+    fn is_async(&self) -> bool {
+        self.syntax().children_with_tokens().any(|t| t.kind() == SyntaxKind::ASYNC_KW)
+    }
+}
+
 pub trait DocCommentsOwner: AstNode {
     fn doc_comments(&self) -> CommentIter {
         CommentIter { iter: self.syntax().children_with_tokens() }
diff --git a/crates/ra_syntax/src/grammar.ron b/crates/ra_syntax/src/grammar.ron
index 08aafb610..7d11f0176 100644
--- a/crates/ra_syntax/src/grammar.ron
+++ b/crates/ra_syntax/src/grammar.ron
@@ -275,7 +275,8 @@ Grammar(
                 "NameOwner",
                 "TypeParamsOwner",
                 "AttrsOwner",
-                "DocCommentsOwner"
+                "DocCommentsOwner",
+                "AsyncOwner"
             ],
             options: [ "ParamList", ["body", "BlockExpr"], "RetType" ],
         ),