Generate error for async trait fucntions

Fixes Rust-GCC#2767

gcc/rust/ChangeLog:

	* checks/errors/rust-ast-validation.cc (ASTValidation::visit):
	Added check for `async` function inside trait.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2767.rs: New test.

Signed-off-by: Kushal Pal <kushalpal109@gmail.com>

gcc/rust/checks/errors/rust-ast-validation.cc

@@ -107,6 +107,11 @@ ASTValidation::visit (AST::Function &function)
     rust_error_at (function.get_locus (), ErrorCode::E0379,
 		   "functions in traits cannot be declared const");
 
+  // may change soon
+  if (qualifiers.is_async () && context.back () == Context::TRAIT_IMPL)
+    rust_error_at (function.get_locus (), ErrorCode::E0706,
+		   "functions in traits cannot be declared %<async%>");
+
   if (valid_context.find (context.back ()) == valid_context.end ()
       && function.has_self_param ())
     rust_error_at (

gcc/testsuite/rust/compile/issue-2767.rs

@@ -0,0 +1,13 @@
+// { dg-additional-options "-frust-edition=2018" }
+trait Foo {
+    fn f() -> u32;
+}
+
+impl Foo for u32 {
+    async fn f() -> u32 {
+        // { dg-error "functions in traits cannot be declared .async." "" { target *-*-* } .-1 }
+        22
+    }
+}
+
+fn main() {}