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

[vext01]

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)] {

[cuviper]

As a workaround, you can simplify this a little with a module:

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

[dtolnay]

I believe this sort of change would require an RFC.

[vext01]

OK, I'm closing this is favour of: rust-lang/rfcs#2312