Ensure that drop order of `async fn` matches `fn`.

This commit modifies the lowering of `async fn` arguments so that the
drop order matches the equivalent `fn`.

Previously, async function arguments were lowered as shown below:

    async fn foo(<pattern>: <ty>) {
      async move {
      }
    } // <-- dropped as you "exit" the fn

    // ...becomes...
    fn foo(__arg0: <ty>) {
      async move {
        let <pattern> = __arg0;
      } // <-- dropped as you "exit" the async block
    }

After this PR, async function arguments will be lowered as:

    async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
      async move {
      }
    } // <-- dropped as you "exit" the fn

    // ...becomes...
    fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
      async move {
        let __arg2 = __arg2;
        let <pattern> = __arg2;
        let __arg1 = __arg1;
        let <pattern> = __arg1;
        let __arg0 = __arg0;
        let <pattern> = __arg0;
      } // <-- dropped as you "exit" the async block
    }

If `<pattern>` is a simple ident, then it is lowered to a single
`let <pattern> = <pattern>;` statement as an optimization.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/libsyntax/ext/placeholders.rs b/src/libsyntax/ext/placeholders.rs
index 68cd3c28676..f5e18e98436 100644
--- a/src/libsyntax/ext/placeholders.rs
+++ b/src/libsyntax/ext/placeholders.rs
@@ -199,7 +199,10 @@ impl<'a, 'b> MutVisitor for PlaceholderExpander<'a, 'b> {
 
         if let ast::IsAsync::Async { ref mut arguments, .. } = a {
             for argument in arguments.iter_mut() {
-                self.next_id(&mut argument.stmt.id);
+                self.next_id(&mut argument.move_stmt.id);
+                if let Some(ref mut pat_stmt) = &mut argument.pat_stmt {
+                    self.next_id(&mut pat_stmt.id);
+                }
             }
         }
     }