Crate const_fn

A lightweight attribute for easy generation of const functions with conditional compilations.

Examples

use const_fn::const_fn;

// function is `const` on specified version and later compiler (including beta, nightly, and dev build)
#[const_fn("1.36")]
pub const fn version() {
    /* ... */
}

// function is `const` on nightly compiler (including dev build)
#[const_fn(nightly)]
pub const fn nightly() {
    /* ... */
}

// function is `const` if `cfg(...)` is true
#[const_fn(cfg(...))]
pub const fn cfg() {
    /* ... */
}

// function is `const` if `cfg(feature = "...")` is true
#[const_fn(feature = "...")]
pub const fn feature() {
    /* ... */
}

Use this crate as an optional dependency

If no arguments are passed, const_fn will always make the function const.

Therefore, you can use const_fn as an optional dependency by combination with cfg_attr.

// function is `const` if `cfg(feature = "...")` is true
#[cfg_attr(feature = "...", const_fn::const_fn)]
pub fn optional() {
    /* ... */
}

Alternatives

This crate is proc-macro, but is very lightweight, and has no dependencies.

You can manually define declarative macros with similar functionality (see if_rust_version), or you can define the same function twice with different cfg. (Note: the former approach requires more macros to be defined depending on the number of version requirements, the latter approach requires more functions to be maintained manually)

Attribute Macros

• const_fn
  A lightweight attribute for easy generation of const functions with conditional compilations.