allow module-level feature flags

[thestinger]

You may want to use an experimental feature in an isolate module without allowing it to infect the entire crate. This is also currently needed for rust-core to use macros, because until we have both static linking and whole program optimization it's going to be a module.

[alexcrichton]

We had originally explicitly decided to not have module-level control over the feature set, but that decision didn't really have a reason other than "it feels right".

One thing to consider with module-level control is whether modules should the have the ability to deny features? Do we want something like #[feature(+feature1, -feature2)]? I don't think we'd want to do that, but I can imagine that someone will at some point want to do that.

[huonw]

We still don't have the ability for this. It's possibly useful in terms of restricting which parts of a the crate are experimental (e.g. I could imagine most of a crate being stable, with certain parts pushing the boundaries of the type system etc. with feature flags). On the note of infection, rust-lang/rfcs#475 means these will be forced to infect the entire crate with only-usable-with-nightly-itis.

[alexcrichton]

Quite a bit of time has passed since this was opened, and with the current design of feature gates it's intentional that it's only crate-level, so closing.