Rollup merge of #60437 - davidtwco:issue-60236, r=nikomatsakis

Ensure that drop order of `async fn` matches `fn` and that users cannot refer to generated arguments.

Fixes #60236 and fixes #60438.

This PR 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.

This PR also stops users from referring to the generated `__argN`
identifiers.

r? @nikomatsakis

0 files changed, 0 insertions, 0 deletions