# \#\[const_fn\]

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

## Usage

Add this to your `Cargo.toml`:

```toml
[dependencies]
const_fn = "0.4"
```

## Examples

```rust
use const_fn::const_fn;

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

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

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

// function is `const` if `cfg(feature = "...")` is true
#[const_fn(feature = "...")]
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`.

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

<!--
TODO: document the behavior on the version on the nightly channel.
      https://github.com/taiki-e/const_fn/issues/27
      https://github.com/rust-lang/rust/pull/81468
-->

## 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`](https://github.com/ogoffart/if_rust_version#examples)),
or [you can define the same function twice with different cfg](https://github.com/crossbeam-rs/crossbeam/blob/0b6ea5f69fde8768c1cfac0d3601e0b4325d7997/crossbeam-epoch/src/atomic.rs#L340-L372).
(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)

## License

Licensed under either of [Apache License, Version 2.0](LICENSE-APACHE) or
[MIT license](LICENSE-MIT) at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.
