这段代码无法编译（playground）：

struct Mapper<T, U, F>
where
    F: Fn(T) -> U,
{
    mapper: F,
    // _t: PhantomData<T>,
    // _u: PhantomData<U>,
}

impl<T, U, F> Mapper<T, U, F>
where
    F: Fn(T) -> U,
{
    fn new(mapper: F) -> Self {
        Self {
            mapper,
            // _t: PhantomData,
            // _u: PhantomData,
        }
    }

    fn call(&self, t: T) -> U {
        (self.mapper)(t)
    }
}

字符串
编译器说

error[E0392]: parameter `T` is never used
 --> src/main.rs:3:15
  |
3 | struct Mapper<T, U, F>
  |               ^ unused parameter
  |
  = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
  = help: if you intended `T` to be a const parameter, use `const T: usize` instead

error[E0392]: parameter `U` is never used
 --> src/main.rs:3:18
  |
3 | struct Mapper<T, U, F>
  |                  ^ unused parameter
  |
  = help: consider removing `U`, referring to it in a field, or using a marker such as `PhantomData`
  = help: if you intended `U` to be a const parameter, use `const U: usize` instead

型
然而，这段代码编译，虽然我只为T添加了PhantomData：

use std::marker::PhantomData;

struct Mapper<T, U, F>
where
    F: Fn(T) -> U,
{
    mapper: F,
    _t: PhantomData<T>,
    // _u: PhantomData<U>,
}

impl<T, U, F> Mapper<T, U, F>
where
    F: Fn(T) -> U,
{
    fn new(mapper: F) -> Self {
        Self {
            mapper,
            _t: PhantomData,
            // _u: PhantomData,
        }
    }

    fn call(&self, t: T) -> U {
        (self.mapper)(t)
    }
}

型
另一方面，如果我只为U添加PhantomData，代码将无法编译：

use std::marker::PhantomData;

struct Mapper<T, U, F>
where
    F: Fn(T) -> U,
{
    mapper: F,
    // _t: PhantomData<T>,
    _u: PhantomData<U>,
}

impl<T, U, F> Mapper<T, U, F>
where
    F: Fn(T) -> U,
{
    fn new(mapper: F) -> Self {
        Self {
            mapper,
            // _t: PhantomData,
            _u: PhantomData,
        }
    }

    fn call(&self, t: T) -> U {
        (self.mapper)(t)
    }
}

fn main() {
    let mapper = Mapper::new(|a: usize| -> isize { -(a as isize) });
    println!("{}", mapper.call(3));
}

型
为什么？为什么？