Check async in trait methods

src/libsyntax/feature_gate.rs

@@ -2068,6 +2068,9 @@ fn visit_trait_item(&mut self, ti: &'a ast::TraitItem) {
                 if block.is_none() {
                     self.check_abi(sig.header.abi, ti.span);
                 }
+                if sig.header.asyncness.node.is_async() {
+                    gate_feature_post!(&self, async_await, ti.span, "async fn is unstable");
+                }
                 if sig.decl.c_variadic {
                     gate_feature_post!(&self, c_variadic, ti.span,
                                        "C-variadic functions are unstable");

src/test/ui/feature-gates/feature-gate-async-await.rs

@@ -8,6 +8,11 @@ impl S {
     async fn foo() {} //~ ERROR async fn is unstable
 }
 
+trait T {
+    async fn foo(); //~ ERROR trait fns cannot be declared `async`
+    //~^ ERROR async fn is unstable
+}
+
 async fn foo() {} //~ ERROR async fn is unstable
 
 fn main() {

src/test/ui/feature-gates/feature-gate-async-await.stderr

+error[E0706]: trait fns cannot be declared `async`
+  --> $DIR/feature-gate-async-await.rs:12:5
+   |
+LL |     async fn foo();
+   |     ^^^^^^^^^^^^^^^
+
 error[E0658]: async fn is unstable
   --> $DIR/feature-gate-async-await.rs:8:5
    |
@@ -8,7 +14,16 @@ LL |     async fn foo() {}
    = help: add #![feature(async_await)] to the crate attributes to enable
 
 error[E0658]: async fn is unstable
-  --> $DIR/feature-gate-async-await.rs:11:1
+  --> $DIR/feature-gate-async-await.rs:12:5
+   |
+LL |     async fn foo();
+   |     ^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/50547
+   = help: add #![feature(async_await)] to the crate attributes to enable
+
+error[E0658]: async fn is unstable
+  --> $DIR/feature-gate-async-await.rs:16:1
    |
 LL | async fn foo() {}
    | ^^^^^^^^^^^^^^^^^
@@ -17,7 +32,7 @@ LL | async fn foo() {}
    = help: add #![feature(async_await)] to the crate attributes to enable
 
 error[E0658]: async blocks are unstable
-  --> $DIR/feature-gate-async-await.rs:14:13
+  --> $DIR/feature-gate-async-await.rs:19:13
    |
 LL |     let _ = async {};
    |             ^^^^^^^^
@@ -26,7 +41,7 @@ LL |     let _ = async {};
    = help: add #![feature(async_await)] to the crate attributes to enable
 
 error[E0658]: async closures are unstable
-  --> $DIR/feature-gate-async-await.rs:15:13
+  --> $DIR/feature-gate-async-await.rs:20:13
    |
 LL |     let _ = async || {};
    |             ^^^^^^^^^^^
@@ -34,6 +49,7 @@ LL |     let _ = async || {};
    = note: for more information, see https://github.com/rust-lang/rust/issues/50547
    = help: add #![feature(async_await)] to the crate attributes to enable
 
-error: aborting due to 4 previous errors
+error: aborting due to 6 previous errors
 
-For more information about this error, try `rustc --explain E0658`.
+Some errors occurred: E0658, E0706.
+For more information about an error, try `rustc --explain E0658`.