Add AcceptContext::expect_no_args

[scrabsha]

This PR adds AcceptContext::expect_no_args that follows the same style as what was added in #155696 and #155831 (albeit much smaller this time).

• There is still one (1) usage of AttributeDiagnosticContext::expected_no_args in proc_macro_attrs:136 and I have no idea how to remove it. Any suggestion on what to replace this call with is welcome :)
• Some usages of ArgParser::no_args still remain in places where we emit custom errors. They show up in the diff due to the no_args -> as_no_args renaming.

I did a little experiment in which I made AcceptContext::expect_no_args unconditionally return Some(()) so parsing always continues, but it led to lots of gibberish errors being emitted. Full diff for this is available here: expect_no_args_experiment.patch. This means there's no simple way to get rid of the let _ = cx.expect_no_args(/* ... */); and I am not super happy with that.

[JonathanBrouwer]

The problem with expect_no_args is that it is too easy to forget to check it. This problem is ofcourse not introduced by this PR, and I still think this PR is an improvement over the current situation so happy to continue with it, but I hope to see the meta_item_no_args function from #155193 be used in more places after that PR finally merges

[scrabsha]

Absolutely. I would LOVE opening a PR that essentially removes every call to AcceptContext::expect_no_args 🥰

r? JonathanBrouwer

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

[JonathanBrouwer]

@bors r+ rollup

View changes since this review

[rust-bors]

📌 Commit f77fc05 has been approved by JonathanBrouwer

It is now in the queue for this repository.