Emit warning on empty proxy type list

[LukasKalbertodt]

The attribute #[auto_impl()] should lead to a warning on nightly. Sadly, we cannot emit warnings on stable (since the proc_macro_diagnostic API is still unstable).

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I can mentor anyone interested in tackling this issue :)
Just ping me (via email, this issue, or in any other way)

Instructions: Two things need to be done:

• First, we need to extend a few things in diag.rs. There, we have a few traits that abstract over nightly/stable diagnostic. Since, as said above, proc_macro_diagnostic is still unstable, we report errors on stable via a hack. Thus you'll find a few #[cfg(feature = "nightly")] attributes there. We need to add a warn() method to SpanExt first. With the nightly feature, we should simply return a proc_macro::Diagnostic. On stable... good question. We probably want to modify our own Diagnostic type so that it can also "be a warning". But if it's a warning, the emit() will just don't do anything.

• With that out of the way, we can finally tackle the core of this issue: emit the warning. I think the easiest way to do that is to add some code to parse_types() in proxy.rs.

If anything is unclear, just go ahead and ask!

[felipehfs]

Hello, I'm a junior developer in Brazil and I want to contribute but I don't know how to get started.

[felipehfs]

My email is felipehfsouza@gmail.com if you want contact me.

[LukasKalbertodt]

@felipehfs Great! I wrote you a mail :)