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This crate should be superseded by keyword genericity in the future.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

Const type

The const type of type T is stipulatively defined as a type whose parameterizations represent various constants of type T.

The const type for a constant value const VAL: T is stipulatively defined as the parametrization of the constant type of T that represents this constant.

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

Using a derive macro

use optionally_const::{OptionallyConst, FieldlessEnumConstType};

#[derive(FieldlessEnumConstType, Debug)]
#[const_type(ConstTypeName)]
enum FieldlessEnum {
    A,
    B,
    C,
}

fn print_fieldless_enum<T>(value: T)
where
    T: OptionallyConst<FieldlessEnum>,
{
    if let Some(value) = T::MAYBE_CONST {
        println!("Const value: {:?}", value);
    } else {
        let value: FieldlessEnum = T::into_value(value);
        println!("Non-const value: {:?}", value);
    }
}

fn main() {
    print_fieldless_enum(FieldlessEnum::A);
    print_fieldless_enum(FieldlessEnum::B);
    print_fieldless_enum(FieldlessEnum::C);
    print_fieldless_enum(ConstTypeName::<{ FieldlessEnum::A as usize }>);
    print_fieldless_enum(ConstTypeName::<{ FieldlessEnum::B as usize }>);
    print_fieldless_enum(ConstTypeName::<{ FieldlessEnum::C as usize }>);

    let Some(_const_type_instance) = FieldlessEnum::A.try_into_const_type_instance::<{FieldlessEnum::A as usize}>() else {
        panic!("The conversion from a variant to a corresponding const type instance failed");
    };
}

Using a manual implementation

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);
}

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.
Higher control over constant propagation in generic contexts: you can make it more likely to generate function instantiations that will accept fewer parameters at the ABI level, at the cost of higher complexity of the code and higher likelyhood of generic (aka template) bloat.

optionally_const 0.3.2