Crate crossfig 

Provides helpful configuration macros, allowing features to be introspected across crates at compile time.

The two main features that power this crate are switch and alias.

switch

switch provides a match-like syntax to make complex compile-time configuration more ergonomic.

// Without `crossfig`:

#[cfg(feature = "fastest")]
// Use the fastest algorithm!

#[cfg(all(not(feature = "fastest"), feature = "fast"))]
// Use the fast algorithm

#[cfg(all(not(feature = "fastest"), not(feature = "fast")))]
// Use the slow algorithm

// With `crossfig`:

crossfig::switch! {
    #[cfg(feature = "fastest")] => {
        // Use the fastest algorithm!
    }
    #[cfg(feature = "fast")] => {
        // Use the fast algorithm
    }
    _ => {
        // Use the slow algorithm
    }
}

alias

alias instead allows for creating a short-hand for common configurations.

// Define an alias
alias! {
    std: { #[cfg(feature = "std")] }
}

// Use it for conditional compilation
std! {
    // This code is only compiled with feature = "std"
}

Combined

Individually, both are useful tools for reducing repetition in conditional compilation. But where they really shine is when combined:

alias! {
    std: { #[cfg(feature = "std")] },
    log: { #[cfg(feature = "log")] },
}

switch! {
    log => {
        // Use the log crate
    }
    std => {
        // Use println! for logging
    }
    _ => {
        // Don't bother logging
    }
}

Cross-Crate Configuration Introspection

One last trick; aliases created with alias can be exported in your public API.

// In the `foo` crate:
crossfig::alias! {
    pub multi_threading: { #[cfg(feature = "multi_threading")] }
}

// In a consumer's crate:
foo::multi_threading! {
    // This code will be compiled if and only if foo/multi_threading
    // is enabled.
}

This can be extremely powerful, as it allows consuming crates to introspect on the enabled features of their dependencies.

macro_rules_attribute

As a quality of life improvement, you can also use macro_rules_attribute. This provides an attribute proc-macro apply, which can be used to apply aliases to items as an attribute.

#![no_std]

use macro_rules_attribute::apply;

alias! {
    std: { #[cfg(feature = "std")] }
}

#[apply(std)]
extern crate std;

Macros

alias

Defines an alias for a particular configuration. This has two advantages over directly using #[cfg(...)]:

disabled

Examples

enabled

Examples

switch

Provides a match-like expression similar to cfg_if and based on the experimental cfg_match. The name switch is used to avoid conflict with the match keyword. Arms are evaluated top to bottom, and an optional wildcard arm can be provided if no match can be made.