Rfcs: Blocks for items

[clarfonthey]

It'd be really nice if we could do something like:

#[cfg(feature = "abc")]
{
    extern crate x;
    extern crate y;
    mod z;
    use z::{a, b, c};
}

Right now, we have to attach the attribute to every item, which is a bit redundant. We can put things in a module and then re-export them, but it'd be nice if we could do something like this instead.

[SimonSapin]

With https://github.com/alexcrichton/cfg-if you can make it look like:

#[macro_use] extern crate cfg_if;

cfg_if! {
    if #[cfg(feature = "abc")] {
        extern crate x;
        extern crate y;
        mod z;
        use z::{a, b, c};
    }
}

[scottmcm]

@withoutboats Would anonymous modules do this? As something like

#[cfg(feature = "abc")]
mod {
    extern crate x;
    extern crate y;
    mod z;
    use z::{a, b, c};
}

[clarfonthey]

I considered anonymous modules, but the main issue is that things like extern crate, etc. wouldn't semantically fit in a module. It could work, though.

[strega-nil]

@clarcharr it's actually a pattern, albeit an uncommon one, to put extern crates in submodules.

[clarfonthey]

@ubsan wait, is that actually allowed? huh.

[strega-nil]

@clarcharr https://github.com/ubsan/cafe/blob/master/src/parse/lexer.rs

[Centril]

I think this is #2377 so I'm closing in favor of that.