Fixed diagnostics for coroutines with () as input.

tests/ui/coroutine/arg-count-mismatch-on-unit-input.rs

@@ -0,0 +1,11 @@
+#![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
+
+use std::ops::Coroutine;
+
+fn foo() -> impl Coroutine<u8> {
+    //~^ ERROR type mismatch in coroutine arguments
+    #[coroutine]
+    |_: ()| {}
+}
+
+fn main() { }