Derive Macro ExplicitDiscriminant 

#[derive(ExplicitDiscriminant)]
{
    // Attributes available to this derive:
    #[pattern]
}

Require that all variants of an enum have an explicit discriminant defined.

Example:

// This works
#[derive(ExplicitDiscriminant)]
pub enum MyEnum {
    One = 1,
    Two = 2,
    Three = 3,
}

// But this won't compile
#[derive(ExplicitDiscriminant)]
pub enum MyOtherEnum {
    One = 1,
    Two,
    Three = 3,
}