Emit an error if a variant of an untagged enum is annoted with #[serde(rename)] or #[serde(alias)]

[devnetsec]

Fixes #2787.

[Mingun]

Ideally, error should point to the each problematic attribute instead of a variant name. Also, compile tests should be added to test_suite/tests/ui

[devnetsec]

I fixed those two things. Are there any other attributes that need to have error messages like this?

[Mingun]

Thanks, much better, but it is better to point to the alias = "..." and rename = "..." parts only, so when you have multiple attributes on a single line, errors pointed to the corresponding parts of that line.

Don't known about other attributes. You may check other if they need improvements.

[devnetsec]

I made it point towards the alias and rename parts to be even more clear. I think #[serde(rename_all)] may be another attribute that has this issue, but I couldn't get that error to work in 6d72564.

[oli-obk]

Can someone run crater-at-home on this to gauge the impact? I worry someone accidentally is using it in a crate that will impact many other crates.

Or switch it to emitting a warning instead (can we do that from proc macros?)

[devnetsec]

Unfortunately, I don't think this can be switched to a warning. Compile errors are handled by serde::internals::ctxt, which holds Syn errors. Syn uses Rust's compile_error! macro to print them. It looks like we're waiting on the nightly-only Diagnostic API to stabilize. See this Stack Overflow thread. Also, the test suite only compares expected and actual output when compilation fails.