3 files changed, 104 insertions, 10 deletions

diff --git a/crates/ra_hir/src/source_binder.rs b/crates/ra_hir/src/source_binder.rs
index c2c6921cb..e86716d74 100644
--- a/crates/ra_hir/src/source_binder.rs
+++ b/crates/ra_hir/src/source_binder.rs
@@ -18,14 +18,18 @@ use ra_syntax::{
 use rustc_hash::{FxHashMap, FxHashSet};
 
 use crate::{
-    expr,
     expr::{
+        self,
         scope::{ExprScopes, ScopeId},
         BodySourceMap,
     },
     ids::LocationCtx,
+    name,
+    path::{PathKind, PathSegment},
+    ty::method_resolution::implements_trait,
     AsName, AstId, Const, Crate, DefWithBody, Either, Enum, Function, HirDatabase, HirFileId,
-    MacroDef, Module, Name, Path, PerNs, Resolver, Static, Struct, Trait, Ty,
+    MacroDef, Module, ModuleDef, Name, Path, PerNs, Resolution, Resolver, Static, Struct, Trait,
+    Ty,
 };
 
 /// Locates the module by `FileId`. Picks topmost module in the file.
@@ -409,6 +413,33 @@ impl SourceAnalyzer {
         crate::ty::autoderef(db, &self.resolver, canonical).map(|canonical| canonical.value)
     }
 
+    /// Checks that particular type `ty` implements `std::future::Future`.
+    /// This function is used in `.await` syntax completion.
+    pub fn impls_future(&self, db: &impl HirDatabase, ty: Ty) -> bool {
+        let std_future_path = Path {
+            kind: PathKind::Abs,
+            segments: vec![
+                PathSegment { name: name::STD, args_and_bindings: None },
+                PathSegment { name: name::FUTURE_MOD, args_and_bindings: None },
+                PathSegment { name: name::FUTURE_TYPE, args_and_bindings: None },
+            ],
+        };
+
+        let std_future_trait =
+            match self.resolver.resolve_path_segments(db, &std_future_path).into_fully_resolved() {
+                PerNs { types: Some(Resolution::Def(ModuleDef::Trait(trait_))), .. } => trait_,
+                _ => return false,
+            };
+
+        let krate = match self.resolver.krate() {
+            Some(krate) => krate,
+            _ => return false,
+        };
+
+        let canonical_ty = crate::ty::Canonical { value: ty, num_vars: 0 };
+        implements_trait(&canonical_ty, db, &self.resolver, krate, std_future_trait)
+    }
+
     #[cfg(test)]
     pub(crate) fn body_source_map(&self) -> Arc<BodySourceMap> {
         self.body_source_map.clone().unwrap()
diff --git a/crates/ra_hir/src/ty/method_resolution.rs b/crates/ra_hir/src/ty/method_resolution.rs
index d421bf9ef..88d012a74 100644
--- a/crates/ra_hir/src/ty/method_resolution.rs
+++ b/crates/ra_hir/src/ty/method_resolution.rs
@@ -255,6 +255,20 @@ fn iterate_inherent_methods<T>(
     None
 }
 
+pub(crate) fn implements_trait(
+    ty: &Canonical<Ty>,
+    db: &impl HirDatabase,
+    resolver: &Resolver,
+    krate: Crate,
+    trait_: Trait,
+) -> bool {
+    let env = lower::trait_env(db, resolver);
+    let goal = generic_implements_goal(db, env.clone(), trait_, ty.clone());
+    let solution = db.trait_solve(krate, goal);
+
+    solution.is_some()
+}
+
 impl Ty {
     // This would be nicer if it just returned an iterator, but that runs into
     // lifetime problems, because we need to borrow temp `CrateImplBlocks`.
diff --git a/crates/ra_ide_api/src/completion/complete_dot.rs b/crates/ra_ide_api/src/completion/complete_dot.rs
index 536ba36df..d43ff2eec 100644
--- a/crates/ra_ide_api/src/completion/complete_dot.rs
+++ b/crates/ra_ide_api/src/completion/complete_dot.rs
@@ -1,19 +1,36 @@
 use hir::{AdtDef, Ty, TypeCtor};
 
-use crate::completion::{CompletionContext, Completions};
+use crate::completion::completion_item::CompletionKind;
+use crate::{
+    completion::{completion_context::CompletionContext, completion_item::Completions},
+    CompletionItem,
+};
 use rustc_hash::FxHashSet;
 
-/// Complete dot accesses, i.e. fields or methods (currently only fields).
+/// Complete dot accesses, i.e. fields or methods (and .await syntax).
 pub(super) fn complete_dot(acc: &mut Completions, ctx: &CompletionContext) {
-    let receiver_ty =
-        match ctx.dot_receiver.as_ref().and_then(|it| ctx.analyzer.type_of(ctx.db, it)) {
-            Some(it) => it,
-            None => return,
-        };
+    let dot_receiver = match &ctx.dot_receiver {
+        Some(expr) => expr,
+        _ => return,
+    };
+
+    let receiver_ty = match ctx.analyzer.type_of(ctx.db, &dot_receiver) {
+        Some(ty) => ty,
+        _ => return,
+    };
+
     if !ctx.is_call {
         complete_fields(acc, ctx, receiver_ty.clone());
     }
-    complete_methods(acc, ctx, receiver_ty);
+    complete_methods(acc, ctx, receiver_ty.clone());
+
+    // Suggest .await syntax for types that implement Future trait
+    if ctx.analyzer.impls_future(ctx.db, receiver_ty) {
+        CompletionItem::new(CompletionKind::Keyword, ctx.source_range(), "await")
+            .detail("expr.await")
+            .insert_text("await")
+            .add_to(acc);
+    }
 }
 
 fn complete_fields(acc: &mut Completions, ctx: &CompletionContext, receiver: Ty) {
@@ -406,4 +423,36 @@ mod tests {
         "###
         );
     }
+
+    #[test]
+    fn test_completion_await_impls_future() {
+        assert_debug_snapshot_matches!(
+        do_completion(
+            r###"
+            //- /main.rs
+            use std::future::*;
+            struct A {}
+            impl Future for A {}
+            fn foo(a: A) {
+                a.<|>
+            }
+
+            //- /std/lib.rs
+            pub mod future {
+                pub trait Future {}
+            }
+            "###, CompletionKind::Keyword),
+        @r###"
+       ⋮[
+       ⋮    CompletionItem {
+       ⋮        label: "await",
+       ⋮        source_range: [74; 74),
+       ⋮        delete: [74; 74),
+       ⋮        insert: "await",
+       ⋮        detail: "expr.await",
+       ⋮    },
+       ⋮]
+        "###
+        )
+    }
 }