cfg: composing/nested and+not does not work

[pnkfelix]

It is hard (maybe impossible) to compile certain combinations of cfg flags.
E.g.: I believe the following should always compile, for any choice of a and b cfg switches:

#[cfg(and(not(a),not(b)))]
fn f() { println!("Got it"); }

#[cfg(a)]
fn f() { println!("in a"); }

#[cfg(b)]
fn f() { println!("in b"); }

fn main() {
    f();
}

But currently you get:

% rustc d.rs
d.rs:11:4: 11:5 error: unresolved name `f`.
d.rs:11     f();
            ^
error: aborting due to previous error
task 'rustc' failed at 'explicit failure', /Users/fklock/Dev/Mozilla/rust.git/src/libsyntax/diagnostic.rs:101
task '<main>' failed at 'explicit failure', /Users/fklock/Dev/Mozilla/rust.git/src/librustc/lib.rs:396

I believe this is because our current cfg implementation is largely just a hack, rather than a proper expression evaluator. See also #2119.

[huonw]

FWIW, not is the only predicate handled by cfg, and it cannot even be nested, i.e. #[cfg(not(not(a)))] is testing that the flag not(a) doesn't exist, not that a does exist.

[pnkfelix]

Oh wait, I misunderstood the current syntax. There is no cfg(and(..)) operator; and-ing is implicit by feeding multiple arguments into cfg

[pnkfelix]

(Closing as not-a-real bug, other than the potential desire to implement #2119 properly.)

((Though this does illustrate the serious need to document the cfg semantics properly somewhere.))