condtype

Choose Rust types at compile-time via boolean constants, brought to you by Nikolai Vazquez.

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Conditional Typing

The CondType type and condval! macro choose types at compile-time using bool constants, just like std::conditional_t in C++. Unlike Either, the chosen type is directly used, rather than wrapping it with an enum type. This may be considered a form of dependent typing, but it is limited in ability and is restricted to compile-time constants rather than runtime values.

Examples

In the following example, CondType aliases &str or i32:

use condtype::CondType;

let str: CondType<true,  &str, i32> = "hello";
let int: CondType<false, &str, i32> = 42;

// Unsized types are also supported:
let str: &CondType<true, str, [u8]> = "world";

condval! enables choosing differently-typed values without specifying types. In the following example, val is inferred to be either &str or i32, depending on COND.

use condtype::condval;

const COND: bool = true;

let val = condval!(if COND {
    "hello"
} else {
    42
});

This crate also enables making code for some platforms more efficient by using smaller-sized types depending on platform-specific constants. In the following example, the RlimOption type can either be Option<rlim_t> or rlim_t itself, where rlim_t::MAX can be treated as a sentinel value for Option::None.

use condtype::{condval, CondType};
use libc::{rlim_t, RLIM_INFINITY};

const RLIM_INFINITY_IS_MAX: bool = RLIM_INFINITY == rlim_t::MAX;

type RlimOption = CondType<RLIM_INFINITY_IS_MAX, Option<rlim_t>, rlim_t>;

const RLIM_NONE: RlimOption = condval!(if RLIM_INFINITY_IS_MAX {
    None::<rlim_t>
} else {
    rlim_t::MAX
});

// Convert from either `RlimOption` type to `Option` via the `Into` trait:
let rlim_none: Option<rlim_t> = RLIM_NONE.into();

Without this crate, one could otherwise use cfg_if! to achieve the same goal. However, using #[cfg] requires maintaining a list of platforms and being more fine-grained if RLIM_INFINITY is dependent on CPU architecture.

use cfg_if::cfg_if;
use libc::rlim_t;

cfg_if! {
    // Platforms where `RLIM_INFINITY != rlim_t::MAX`:
    if #[cfg(any(
        target_os = "macos",
        target_os = "freebsd",
        target_os = "solaris",
        // ad nauseam...
    ))] {
        type RlimOption = rlim_t;
        const RLIM_NONE: RlimOption = rlim_t::MAX;
    } else {
        type RlimOption = Option<rlim_t>;
        const RLIM_NONE: RlimOption = None;
    }
}

Limitations

It is currently not possible to use CondType or condval! with a generic constant because Rust does not yet consider trait implementations based on booleans to be exhaustive. Once that issue is resolved, all versions of this crate should just work with generic constants.

fn generic<const B: bool>() {
    let val: CondType<B, &str, i32> = condval!(if B {
        "hello"
    } else {
        42
    });
}

Install

This crate is available on crates.io and can be used by running the following cargo command in your project directory:

cargo add condtype

or by manually adding the following to your project's Cargo.toml:

[dependencies]
condtype = "1.1.0"

License

Like the Rust project, this library may be used under either the MIT License or Apache License (Version 2.0).