Polymorphic Constant

A macro to generate numerical constants in multiple types at once.

You can have the number Pi be available in f64 or f32

It was designed with three goals in mind:

Catch all overflow errors on compile-time
Minimize the code footprint
Be readable and easy to use

This is meant as a temporary fix to the year-long debate over Rust Pre-RFC #1337

Syntax

```rust use polymorphicconstant::polymorphicconstant;

polymorphic_constant! {
    static PI: f32 | f64 = 3.141592653589793;
}

// Which can then be used as

fn pi_squared () -> f64 {
    PI.f64 * PI.f64
}

```

A few features are supported:

```rust use polymorphicconstant::polymorphicconstant;

polymorphic_constant! {

    /// Doc comment attributes
    static PI: f32 | f64 = 3.141592653589793;

    // Visibility modifiers (for both constant and type)
    pub (crate) static E: f32 | f64 = 2.7182818284590452;

    // Nonzero numeric types (NonZeroI32, NonZeroU8, etc)
    static ASCII_LINE_RETURN: u8 | nz_u8 = 10;
}

// You can handle constants like any static struct
static PI_COPY: PI = PI;
static PI_F32: f32 = PI.f32;

// Into is implemented for every variant of the constant
fn times_pi<T: std::ops::Mul<T>> (value: T) -> <T as std::ops::Mul>::Output
where
    PI: Into<T>,
{
    value * PI.into()
}

assert_eq!(times_pi(2.0), 6.283185307179586f64);

```

Safety

This system ensures that you keep all the safeties and warnings given by rust, but no more

Any incompatible type will prevent compilation:

Float literals cannot be stored if it would convert them to infinity ```compile_fail

use polymorphicconstant::polymorphicconstant;

polymorphic_constant! {
```
static FAILS: f32 | f64 =  3141592653589793238462643383279502884197.0;
```
}

```
Literals cannot be stored in a type too small to hold them ```compile_fail

use polymorphicconstant::polymorphicconstant;

polymorphic_constant! {
```
static FAILS: u64 | nz_i8 = 128;
```
}

```
Negative numbers cannot be stored in unsigned types ```compile_fail

use polymorphicconstant::polymorphicconstant;

polymorphic_constant! {
```
static FAILS: i64 | u8 = -1;
```
}

```
0 cannot be stored in non-zero types ```compile_fail

use polymorphicconstant::polymorphicconstant;

polymorphic_constant! {
```
static FAILS: nz_u8 | nz_u16 | nz_u32 = 0;
```
}

```
However, floats may lose precision, and a lot of it ```rust

use polymorphicconstant::polymorphicconstant;

polymorphic_constant! {
```
static SUCCEEDS: f32 = 3.141592653589793238462643383279;
```
}

```

Warnings

Currently, the same constant cannot hold both int and float variants ```compilefail # use polymorphicconstant::polymorphic_constant;

# polymorphic_constant! {
    static FAIL: i32 = 0.1;
# }

compilefail # use polymorphicconstant::polymorphic_constant;

# polymorphic_constant! {
    static FAIL: f32 = 0;
# }

```

The constant also has to be initialized with an untyped literal ```compilefail # use polymorphicconstant::polymorphic_constant;

# polymorphic_constant! {
    static FAIL: i32 = 0u32;
# }

```

It is still unclear if accepting the examples above could be dangerous, thus the conservative choice.

Example

``` use polymorphicconstant::polymorphicconstant;

polymorphic_constant! { static HEIGHT: i8 | u8 | i16 | u16 | i32 | u32 = 16; static WIDTH: i8 | u8 | i16 | u16 | i32 | u32 = 32; }

fn main() { let size = HEIGHT.i16 * WIDTH.i16;

assert_eq!(size, 16 * 32);

let height_copy:i32 = HEIGHT.into();

assert_eq!(HEIGHT.i32, height_copy);

} ```

Support

I would love any feedback on usage, for future ameliorations and features.