Added support to Parse ASYNC function

Fixes issue Rust-GCC#2560
The parser now parses ASYNC functions.
gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_item):Added ASYNC case.
	(Parser::parse_vis_item):Added a switch case to handel ASYNC.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2650.rs: New test.

Signed-off-by: M V V S Manoj Kumar <mvvsmanojkumar@gmail.com>

gcc/rust/parse/rust-parse-impl.h

@@ -1109,6 +1109,8 @@ Parser<ManagedTokenSource>::parse_item (bool called_from_statement)
 	  add_error (std::move (error));
 	}
       return nullptr;
+
+    case ASYNC:
     case PUB:
     case MOD:
     case EXTERN_TOK:
@@ -1385,6 +1387,25 @@ Parser<ManagedTokenSource>::parse_vis_item (AST::AttrVec outer_attrs)
 	  lexer.skip_token (1); // TODO: is this right thing to do?
 	  return nullptr;
 	}
+    case ASYNC:
+      t = lexer.peek_token (1);
+
+      switch (t->get_id ())
+	{
+	case UNSAFE:
+	case FN_TOK:
+	  return parse_function (std::move (vis), std::move (outer_attrs));
+
+	default:
+	  add_error (
+	    Error (t->get_locus (),
+		   "unexpected token %qs in some sort of async production",
+		   t->get_token_description ()));
+
+	  lexer.skip_token (1);
+	  return nullptr;
+	}
+
     case STATIC_TOK:
       return parse_static_item (std::move (vis), std::move (outer_attrs));
     case AUTO:

gcc/testsuite/rust/compile/issue-2650.rs

@@ -0,0 +1,3 @@
+pub async fn a() -> u32 {
+    1
+}