Optional const

Optional constness on stable Rust.

This crate should be superseded by keyword genericity in the future.

Usage

use optionally_const::{const_type_instance, OptionallyConst};

fn print_flag<T: OptionallyConst<bool>>(flag: T) {
    if let Some(flag) = T::MAYBE_CONST {
        println!("flag is const: {flag}");
    } else {
        let flag: bool = flag.into_value();
        println!("flag is not const: {flag}");
    };
}

fn main() {
    print_flag(true);
    print_flag(false);
    print_flag(const_type_instance!(true));
    print_flag(const_type_instance!(false));
}

Limitations

Rust currently doesn't allow defining a type like struct ConstType<T, const VAL: T>; because the type of const parameters must not depend on other generic parameters [E770]. Consequently, one can't provide a canonical "const type" for any const value.
The const_type_instance! macro currently supports only bool type. However, it can be extended to support other types in the future.
Due to lack of support for negative trait bounds and [E770], it's impossible to implement OptionallyConst<T> for all types that implement Const<T>. The current implementation only supports bool type. However, you can implement both OptionallyConst<T> and Const<T> for your own types.

Optional constness for user-defined types

Enums

use optionally_const::Const;

enum MyEnum {
    A,
    B,
    C,
}

impl std::fmt::Display for MyEnum {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            MyEnum::A => write!(f, "A"),
            MyEnum::B => write!(f, "B"),
            MyEnum::C => write!(f, "C"),
        }
    }
}

struct MyEnumAConstType;
struct MyEnumBConstType;
struct MyEnumCConstType;

// Ideally, OptionallyConst<T> should be implemented for
// all types that implement Const<T>.
//
// However, impl of OptionallyConst<T> for all `T` conflicts with the
// impl of OptionallyConst<T> for all `U: Const<T>` in the absence of
// negative trait bounds.

impl Const<MyEnum> for MyEnumAConstType {
    const VALUE: MyEnum = MyEnum::A;
}

impl OptionallyConst<MyEnum> for MyEnumAConstType {
    const MAYBE_CONST: Option<MyEnum> = Some(MyEnum::A);

    fn into_value(self) -> MyEnum {
        MyEnum::A
    }
}

impl Const<MyEnum> for MyEnumBConstType {
    const VALUE: MyEnum = MyEnum::B;
}

impl OptionallyConst<MyEnum> for MyEnumBConstType {
    const MAYBE_CONST: Option<MyEnum> = Some(MyEnum::B);

    fn into_value(self) -> MyEnum {
        MyEnum::B
    }
}

impl Const<MyEnum> for MyEnumCConstType {
    const VALUE: MyEnum = MyEnum::C;
}

impl OptionallyConst<MyEnum> for MyEnumCConstType {
    const MAYBE_CONST: Option<MyEnum> = Some(MyEnum::C);

    fn into_value(self) -> MyEnum {
        MyEnum::C
    }
}

fn print_my_enum<T: OptionallyConst<MyEnum>>(value: T) {
    if let Some(value) = T::MAYBE_CONST {
        println!("value is const: {value}");
    } else {
        let value: MyEnum = value.into_value();
        println!("value is not const: {value}");
    };
}

fn main() {
    print_my_enum(MyEnum::A);
    print_my_enum(MyEnum::B);
    print_my_enum(MyEnum::C);
    print_my_enum(MyEnumAConstType);
    print_my_enum(MyEnumBConstType);
    print_my_enum(MyEnumCConstType);
}

Note: this can be done even easier with the FieldlessEnumConstType derive macro, which is exposed when the derive feature is enabled.

Use cases

More generic const parameters: at the moment of writing, only a handful of types can be used as generic const parameters. However, with this crate, you can accept T: Const<U> where U is an enum type.