add `if_cfg!` macro

[BennoLossin]

The following code pattern is becoming more common:

impl ops::Div for Delta {
    type Output = Self;

    #[inline]
    fn div(self, rhs: Self) -> Self::Output {
        #[cfg(CONFIG_64BIT)]
        {
            Self {
                nanos: self.nanos / rhs.nanos,
            }
        }

        #[cfg(not(CONFIG_64BIT))]
        {
            Self {
                // SAFETY: This function is always safe to call regardless of the input values
                nanos: unsafe { bindings::div64_s64(self.nanos, rhs.nanos) },
            }
        }
    }
}

It would be great if we had a if_cfg! macro:

impl ops::Div for Delta {
    type Output = Self;

    #[inline]
    fn div(self, rhs: Self) -> Self::Output {
        Self {
            nanos: if_cfg!(if CONFIG_64BIT {
                self.nanos / rhs.nanos
            } else {
                // SAFETY: This function is always safe to call regardless of the input values
                nanos: unsafe { bindings::div64_s64(self.nanos, rhs.nanos) },
            })
        }
    }
}

Therefore implement a if_cfg! macro that expands into #[cfg(cond)] { ... } and #[cfg(not(cond))] { ... }. It should:

• only contain valid rust syntax to be formattable by rustfmt & easily understood
• be possible to use in both expression & statement position

---

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[tgross35]

Could this just vendor https://github.com/rust-lang/cfg-if/blob/dbfd66354537a7d47d84c95ea28b9a6f169ba9d1/src/lib.rs? Possibly tweaking the syntax since #[cfg(...)] is noisy

[BennoLossin]

Hmm that's also an idea. But why isn't it just part of the stdlib?

[tgross35]

Soon :) rust-lang/rust#115585

[BennoLossin]

yeah then vendoring might make more sense, no need to reinvent the wheel :)