Functions/modules using unstable *syntax* (e.g., `impl Trait`) can't be CFGed out on stable

[Stebalien]

This fails on stable because rust still parses the unstable module:

#[cfg(feature = "unstable")]
mod unstable {
    fn foo() -> impl Foo {}
}

It's possible to work around this by conditionally defining a macro and then having the macro define the module but that's a bit messy:

#[cfg(feature = "unstable")]
macro_rules! decl_unstable {
    () =>  {
        mod unstable {
            fn foo() -> impl Foo {}
        }
    }
}

#[cfg(not(feature = "unstable"))]
macro_rules! decl_unstable {
    () =>  {}
}

decl_unstable!();

[durka]

Or implement the module in another file. I think this is a fundamental limitation -- you can't skip forward over an item if you don't understand the syntax that the item is using. The only way we could solve it is with time travel, i.e., doing a point release of stable that knows the impl Trait syntax but not what it means.

[Stebalien]

Or implement the module in another file.

That's actually what I'm doing in my project (still the same problem). That is, I have the following in my lib.rs:

#[cfg(feature="unstable")]
mod unstable;

And a module, unstable.rs

fn foo() -> impl Foo {}

[retep998]

Modules are parsed before #[cfg]'s are parsed, which has been an issue for as long as I can remember. Can someone find whether there is an issue tracking this?

[durka]

Dupe of #27873. cc @jseyfried (cfg expert!)

[durka]

Though this'll be fixed by the above PR, the workaround for current stable is

#[cfg_attr(feature="nightly", path="nightly_stuff.rs")]
#[cfg_attr(not(feature="nightly"), path="stable_stuff.rs")]
mod stuff;