error[E0425]: cannot find value `__arg0` in this scope
  --> tests/ui/arg-implementation-detail.rs:18:17
   |
18 |         let _ = __arg0;
   |                 ^^^^^^ not found in this scope
   |
help: an identifier with the same name is defined here, but is not accessible due to macro hygiene
  --> tests/ui/arg-implementation-detail.rs:7:16
   |
 5 | #[async_trait]
   | -------------- in this attribute macro expansion
 6 | pub trait Trait {
 7 |     async fn f((_a, _b): (Struct, Struct)) {
   |                ^^^^^^^^
   = note: this error originates in the attribute macro `async_trait` (in Nightly builds, run with -Z macro-backtrace for more info)