Feature Request: A way to deprecate features

[llogiq]

I have a crate that has a feature that is no longer needed. For now, I keep the feature, but it does nothing. I will at some point remove the feature, but I'd like to be able to issue a warning before removing it.

An easy an approachable way would be to nominate a special "deprecated" feature that is always available, so we could write my_feature = "deprecated"in our Cargo.toml. However, this would need a survey of crates to see if any already define such a feature.

Another option is to stabilize the proc_macro_diagnostics feature in rustc, that would allow us to issue a compile-time warning if the feature is selected, albeit with more ceremony.

[ehuss]

I'm not sure if proc_macro_diagnostics would work here since registry dependencies have diagnostics capped at Allow, so it would probably be hidden in most circumstances.

[tgross35]

This would be covered as part of rust-lang/rfcs#3416 if accepted

[schneems]

@tgross35 rust-lang/rfcs#3416 was merged in. What are the next steps to allow feature deprecation?