Proposal: Add the ability to do `#[cfg(...)] { ... }` at the top level of a file.

[vext01]

Hi,

I've never used this RFC repo, so apologies if I'm doing this wrong :)

In rust-lang/rust#47712 someone suggested an RFC would be required.

Occasionally I find I need to conditionally use several libraries. For example, to import data structures used in a conditionally compiled function.

#[cfg(debug_assertions)]    
use std::slice;    
#[cfg(debug_assertions)]    
use std::fs::File;    
#[cfg(debug_assertions)]    
use std::io::prelude::*;    
#[cfg(debug_assertions)]    
use std::convert::AsRef;    
#[cfg(debug_assertions)]    
use std::path::Path;    

Whereas, I'd like to write:

#[cfg(debug_assertions)] {
    use std::slice;
    use std::fs::File;    
    use std::io::prelude::*;    
    use std::convert::AsRef;    
    use std::path::Path;
}

This currently gives:

error: expected item after attributes
  --> src/lib.rs:53:24
   |
53 | #[cfg(debug_assertions)] {

Thanks

[Centril]

Note that an issue does not constitute an RFC, only pull requests following the format do. .

On the issue itself, I agree that this is a problem in need of solving.
Syntactically, I think it is sufficient to allow special scopes at the item level which may only contain items.
This might, and I stress the might here, lead people to believe that expressions are allowed inside those scopes, but that is unlikely.

I am preliminarily positively inclined towards the additions proposed here.

[vext01]

Thanks. I will raise a proper RFC once I find some time.

[burdges]

There was an RFC for use std::{slice, fs::{File, FileType}, io::prelude::*, path::Path}; which solves this particular problem nicely, especially if one added use { std::{ .. }, rand::{ .. } }. I forget if it passed or encountered issues. Also, there is the submodule trick, but it's less pretty and the pubs can get tricky:

#[cfg(debug_assertions)]
use self::imports::*
#[cfg(debug_assertions)]
mod imports {
    pub(super) use std::slice;
    pub(super) use std::fs::File;    
    pub(super) use std::io::prelude::*;    
    pub(super) use std::convert::AsRef;    
    pub(super) use std::path::Path;
}

[Ixrec]

The RFC in question was #2128, and it was accepted (very quickly and with basically no controversy).

[clarfonthey]

The cfg-if crate solves this problem quite nicely and without any extra language features.

[Centril]

Closing this issue since the associated RFC #2377 has been filed.