如何为对类型引用的操作指定通用特征？

Question

我正在尝试创建一个对通用数字类型进行抽象的特征。指定我希望特征需要T + T（即T: Add<T>）很容易。我什至可以在特征中指定T + &T，然后在实现该特征的某种类型的泛型函数中使用它。

但是，当我尝试为特征添加&T + T（即for<'a> &'a Self: Add<Self>）和&T + &T 的约束时，我遇到了问题。奇怪的是，在为数字特征编写毯子impl 时，以及为任何类型的特征编写通用函数时，我可以毫无问题地指定这些相同的约束，但我无法让编译器让我只指定这些而是在 trait 上约束一次。

简化示例

use std::ops::Add;

trait Numeric where
    Self: Sized,
    // T + T
    Self: Add<Output = Self>,
    // T + &T
    Self: for<'a> Add<&'a Self, Output = Self>,
    // &T + T
    // should specify &T + T on this trait but instead causes compile
    // errors everywhere I try to use Numeric claiming no implementation
    // for &T + T
//  for<'a> &'a Self: Add<Self, Output = Self>,
    // &T + &T
    // should specify &T + &T on this trait but instead causes compile
    // errors everywhere I try to use Numeric claiming no implementation
    // for &T + &T
//  for<'a, 'b> &'a Self: Add<&'b Self, Output = Self>,
{}

impl <T> Numeric for T where
    // T + T
    T: Add<Output = Self>
    + Sized,
    // T + &T
    for<'a> T: Add<&'a T, Output = T>,
    // &T + T
    for<'a> &'a T: Add<T, Output = T>,
    // &T + &T
    for<'a, 'b> &'a T: Add<&'b T, Output = T>,
{}

// works
fn generic_add_0<T: Numeric>(a: T, b: T) -> T {
    a + b
}

// works
fn generic_add_1<T: Numeric>(a: T, b: &T) -> T {
    a + b
}

// doesn't infer &T + T from the trait?
fn generic_add_2<T: Numeric>(a: &T, b: T) -> T {
    a + b
}

// doesn't infer &T + &T from the trait?
fn generic_add_3<T: Numeric>(a: &T, b: &T) -> T {
    a + b
}

// works
fn generic_add_4<T: Numeric>(a: &T, b: &T) -> T
// I want to not have to specify this every time I use Numeric,
// I want Numeric to imply this constraint for me
where for<'a, 'b> &'a T: Add<&'b T, Output = T> {
    a + b
}

fn main() {
    generic_add_0(1.0, 2.0);
    generic_add_1(1.0, &2.0);
    generic_add_2(&1.0, 2.0);
    generic_add_3(&1.0, &2.0);
    generic_add_4(&1.0, &2.0);
}

Answer 1

我想我在盯着num-traits 做了很长一段时间并不断试错之后，已经设法回答了我自己的问题。

我最初的示例的这个修改版本创建了两个额外的特征，使您能够非常接近最初的目标。尽管您仍然需要 where 子句，但它不会随着您为其他操作添加更多约束而增加大​​小。

use std::ops::Add;

/**
 * Trait defining what a numeric type is, in this case just something
 * that can be added to a right hand side and yield itself as output
 */
trait NumericByValue<Rhs = Self, Output = Self>: Sized + Add<Rhs, Output = Output> {}

/**
 * All types implemeting the by value operations are NumericByValue
 */
impl <T, Rhs, Output> NumericByValue<Rhs, Output> for T where
    T: Add<Rhs, Output = Output> {}

/**
 * The trait to define &T op T and &T op &T versions for NumericByValue
 * based off the MIT/Apache 2.0 licensed code from num-traits 0.2.10
 * http://opensource.org/licenses/MIT
 * https://docs.rs/num-traits/0.2.10/src/num_traits/lib.rs.html#112
 *
 * The trick is that all types implementing this trait will be references,
 * so the first constraint expresses some &T which can be operated on with
 * some right hand side type T to yield a value of type T.
 *
 * In a similar way the second constraint expresses &T op &T -> T operations
 */
trait NumericRef<T>:
    // &T op T -> T
    NumericByValue<T, T>
    // &T op &T -> T
    + for<'a> NumericByValue<&'a T, T> {}

/**
 * All types implementing the operations from NumericByValue by reference,
 * are NumericRef<T>, ie a type like &u8 is NumericRef<u8>.
 */
impl <RefT, T> NumericRef<T> for RefT where
    RefT: NumericByValue<T, T>
    + for<'a> NumericByValue<&'a T, T> {}

/**
 * A trait extending the constraints in NumericByValue to
 * types which also support the operations with a right hand side type
 * by reference.
 *
 * When used together with NumericRef this can express all 4 by value
 * and by reference combinations for the operations using the
 * following
 *
 * ```ignore
 *  fn function_name<T: Numeric>()
 *  where for<'a> &'a T: NumericRef<T> {
 * ```
 */
trait Numeric: NumericByValue + for<'a> NumericByValue<&'a Self> {}

/**
 * All types implemeting the operations in NumericByValue with a right hand
 * side type by reference are Numeric.
 */
impl <T> Numeric for T where T: NumericByValue + for<'a> NumericByValue<&'a T> {}

fn generic_add_0<T: Numeric>(a: T, b: T) -> T
where for<'a> &'a T: NumericRef<T> {
    a + b
}

fn generic_add_1<T: Numeric>(a: T, b: &T) -> T
where for<'a> &'a T: NumericRef<T> {
    a + b
}

fn generic_add_2<T: Numeric>(a: &T, b: T) -> T
where for<'a> &'a T: NumericRef<T> {
    a + b
}

fn generic_add_3<T: Numeric>(a: &T, b: &T) -> T
where for<'a> &'a T: NumericRef<T> {
    a + b
}

fn main() {
    generic_add_0(1.0, 2.0);
    generic_add_1(1.0, &2.0);
    generic_add_2(&1.0, 2.0);
    generic_add_3(&1.0, &2.0);
}