if_test!() macro

[bblum]

Often I find myself writing code such as:

#[cfg(test)]
fn assert_something_expensive() { ... }
#[cfg(not(test))]
fn assert_something_expensive() { return; }

Sometimes this is logic that is not just an assert, in the case of rt::comm::block_on(). I wish for a macro so this could be done more concisely without needing to define clunky new function items.

[emberian]

#7869

[huonw]

It seems like this might be useful as a general if_cfg!():

if_cfg!(test {
   ...
} else {
  ....
})

if_cfg!(target_word_size="64" { } else { })
if_cfg!(not(test) { ... })

[jbclements]

Processing the cfgs before doing macro expansion makes this a bit harder, but how about something like this?:

#[cfg(test)]
#[macro_escape]
mod anonymous20130731 {
    macro_rules! if_test(($t:expr) => ($t))
}

#[cfg(not(test))]
#[macro_escape]
mod anonymous20130731 {
    macro_rules! if_test(($t:expr) => (()))
}


fn f() {
    if_test!(std::io::println("I am in testing mode."));
    std::io::println("The function f is all done.");
}

fn main () {
    f();
}

#[test] fn t1 () {
    f();
}

[jbclements]

@huonw points out that
a) it should be possible to write a macro that expands into something that uses #[cfg...] flags, and that
b) it's probably not actually simpler.

[jbclements]

... and also that the exsting "debug!" macro should cover this issue's request. @bblum , do you think this issue should not be closed?

[SimonSapin]

@jbclements , debug! only does logging, it’s unrelated to this.

Until something more pretty is added, I worked around this with:

#[cfg(not(test))]
static CFG_TEST: bool = false;

#[cfg(test)]
static CFG_TEST: bool = true;

… and using if CFG_TEST { assert!(…) }, counting on the compiler to optimize the test away.

[huonw]

I just opened #8188 which implements cfg!, which can be used like:

if cfg!(test) {
    assert!(something_expensive());
}

Is this an appropriate solution?

[SimonSapin]

@huonw looks good, thanks!

[bblum]

Yeah that's actually better than i was hoping for. Brilliant.