Make macro emit a warning instead of an error?

[Xaeroxe]

I believe it should be possible to emit a warning, using proc_macro::Diagnostic. With that in mind, I believe it'd be preferable to have the default behavior be to warn, and then opt in to it being an error.

This also helps with #1, because when a dependency emits a warning Cargo silently ignores it.

[Xaeroxe]

Though I suppose this only works with a nightly compiler

[parker-codes]

I'm investigating a solution in #5. Ideally we could show an error/warning in the editor and only log out warnings for the current project. Since diagnostics are not on stable Rust, in #1 someone showed how to use a deprecated mark to almost do the same thing. It works really well, except it creates a strikethrough effect in the editor which makes it hard to read.

[parker-codes]

Just merged the PR to not fail builds. It still fails tests though, so I'll be looking for a solution to that.