rust - 可转换为 isize 的枚举的特征

Question

我有几个枚举。它们每个都有数字标签，可以转换为整数 (isize)。我有一个他们都定义的特征。但该特征的实例不再可转换为整数，因为编译器无法证明所有实现实际上都是无参数枚举。

是否有任何特征表明类型是无参数枚举或者它可以转换为整数(以任何方式)，我可以在需要获取数值的泛型中用作约束？

---

更具体地说，这是一个代码示例(also on play):

enum Foo {
    a = 1,
    b = 2,
    c = 3,
}

enum Bar {
    d = 4,
    e = 5,
    f = 6,
}

trait Process : std::marker::MarkerTrait {
    fn process(v: isize) -> String;
}

impl Process for Foo {
    fn process(v: isize) -> String { format!("foo{}", v) }
}

impl Process for Bar {
    fn process(v: isize) -> String { format!("bar{}", v) }
}

// just for sake of argument; something more complex in reality, of course
fn extern_call(v: isize) -> isize { 2 * v + 211 }

fn process<T: Process>(v: T) -> String {
    T::process(extern_call(v as isize))
}

fn main() {
    println!("a: {}", process(Foo::a));
}

很明显问题出在process，v as isize编译失败

error: non-scalar cast: T as isize

所以我想做类似的事情

process<T: Process + scalar>

或

process<T: Process + ToPrimitive>

或者告诉编译器只允许可以转换为 isize 的类型，但我不知道那可能是什么。

Answer 1

向您的特征添加一个方法，将其强制为 isize。然后编译器可以验证对象是否满足要求的标准:

enum Foo {
    A = 1,
    B = 2,
    C = 3,
}

enum Bar {
    D = 4,
    E = 5,
    F = 6,
}

trait Process: {
    fn as_isize(self) -> isize;
    fn process(v: isize) -> String;
}

impl Process for Foo {
    fn as_isize(self) -> isize { self as isize }
    fn process(v: isize) -> String { format!("foo{}", v) }
}

impl Process for Bar {
    fn as_isize(self) -> isize { self as isize }
    fn process(v: isize) -> String { format!("bar{}", v) }
}

// just for sake of argument; something more complex in reality, of course
fn extern_call(v: isize) -> isize { 2 * v + 211 }

fn process<T: Process>(v: T) -> String {
    T::process(extern_call(v.as_isize()))
}

fn main() {
    println!("a: {}", process(Foo::A));
}

仅供引用，此处不需要MarkerTrait(并且不合惯用):

MarkerTrait is intended to be used as the supertrait for traits that don't have any methods