72 lines
2.4 KiB
Markdown
72 lines
2.4 KiB
Markdown
# \#\[const_fn\]
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[](https://crates.io/crates/const_fn)
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[](https://docs.rs/const_fn)
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[](#license)
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[](https://www.rust-lang.org)
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[](https://github.com/taiki-e/const_fn/actions?query=workflow%3ACI+branch%3Amaster)
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An attribute for easy generation of const functions with conditional
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compilations.
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## Usage
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Add this to your `Cargo.toml`:
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```toml
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[dependencies]
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const_fn = "0.4"
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```
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*Compiler support: requires rustc 1.31+*
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## Examples
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```rust
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use const_fn::const_fn;
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// function is `const` on specified version and later compiler (including beta and nightly)
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#[const_fn("1.36")]
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pub const fn version() {
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/* ... */
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}
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// function is `const` on nightly compiler (including dev build)
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#[const_fn(nightly)]
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pub const fn nightly() {
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/* ... */
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}
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// function is `const` if `cfg(...)` is true
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#[const_fn(cfg(...))]
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pub const fn cfg() {
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/* ... */
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}
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// function is `const` if `cfg(feature = "...")` is true
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#[const_fn(feature = "...")]
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pub const fn feature() {
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/* ... */
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}
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```
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## Alternatives
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This crate is proc-macro, but is very lightweight, and has no dependencies.
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You can manually define declarative macros with similar functionality (see
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[`if_rust_version`](https://github.com/ogoffart/if_rust_version#examples)),
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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).
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(Note: the former approach requires more macros to be defined depending on the
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number of version requirements, the latter approach requires more functions to
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be maintained manually)
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## License
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Licensed under either of [Apache License, Version 2.0](LICENSE-APACHE) or
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[MIT license](LICENSE-MIT) at your option.
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Unless you explicitly state otherwise, any contribution intentionally submitted
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for inclusion in the work by you, as defined in the Apache-2.0 license, shall
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be dual licensed as above, without any additional terms or conditions.
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