<commit_before>Rust 2015 does not permit the use of `async fn`.

Example of erroneous code:

```compile_fail,E0670
async fn foo() {}<commit_msg>Rollup merge of #73436 - GuillaumeGomez:cleanup-e0670, r=Dylan-DPC<commit_after>Rust 2015 does not permit the use of `async fn`.

Erroneous code example:

```compile_fail,E0670
async fn foo() {}
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{'dir': 'git-commits-cleaned', 'id': '4144102', 'n_tokens_mistral': 138, 'n_tokens_neox': 116, 'n_words': 40}