Lower async fn in traits.

compiler/rustc_ast_lowering/src/item.rs

@@ -836,9 +836,17 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)))
             }
             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, Some(ref body))) => {
-                let body_id = self.lower_fn_body_block(i.span, &sig.decl, Some(body));
-                let (generics, sig) =
-                    self.lower_method_sig(generics, sig, trait_item_def_id, false, None, i.id);
+                let asyncness = sig.header.asyncness;
+                let body_id =
+                    self.lower_maybe_async_body(i.span, &sig.decl, asyncness, Some(&body));
+                let (generics, sig) = self.lower_method_sig(
+                    generics,
+                    sig,
+                    trait_item_def_id,
+                    false,
+                    asyncness.opt_return_id(),
+                    i.id,
+                );
                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)))
             }
             AssocItemKind::TyAlias(box TyAliasKind(_, ref generics, ref bounds, ref default)) => {

src/test/ui/async-await/async-trait-fn.rs

@@ -2,6 +2,10 @@
 trait T {
     async fn foo() {} //~ ERROR functions in traits cannot be declared `async`
     async fn bar(&self) {} //~ ERROR functions in traits cannot be declared `async`
+    async fn baz() { //~ ERROR functions in traits cannot be declared `async`
+        // Nested item must not ICE.
+        fn a() {}
+    }
 }
 
 fn main() {}

src/test/ui/async-await/async-trait-fn.stderr

@@ -20,6 +20,22 @@ LL |     async fn bar(&self) {}
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
-error: aborting due to 2 previous errors
+error[E0706]: functions in traits cannot be declared `async`
+  --> $DIR/async-trait-fn.rs:5:5
+   |
+LL |       async fn baz() {
+   |       ^----
+   |       |
+   |  _____`async` because of this
+   | |
+LL | |         // Nested item must not ICE.
+LL | |         fn a() {}
+LL | |     }
+   | |_____^
+   |
+   = note: `async` trait functions are not currently supported
+   = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0706`.