generics - 为什么不能在Rc <dyn Fn()>内使用此闭包？

Question

我有这个结构:

pub struct ImmEval<Out> {
    closure: Rc<dyn Fn(&Self) -> Out>
}
impl<Out> From<Rc<dyn Fn(&Self) -> Out>> for ImmEval<Out> {
    /// Constructs the evaluator from the specified closure.
    fn from(cl: Rc<dyn Fn(&Self) -> Out>) -> Self {
        ImmEval {closure: cl}
    }
}
impl<Out> Evaluator for ImmEval<Out> {
    type Output = Out;
    /// Evaluates and returns the result.
    fn eval(&self) -> Self::Output {
        (*self.closure)(self)
    }
}

在这样的测试中使用:

let counter = Cell::new(0);
{
    let eval = ImmEval::<i32>::from(Rc::from(|_: &ImmEval<i32>| {
        counter.set(counter.get() + 1);
        counter.get()
    }));

    eval.eval();
    eval.eval();
}
assert_eq!(counter.get(), 2);

结构本身会编译，但测试不会编译。相反，它失败并显示以下rustc消息:

error[E0277]: the trait bound `ImmEval<i32>: std::convert::From<std::rc::Rc<[closure@src\tests.rs:8:51: 11:10 counter:_]>>` is not satisfied
  --> src\tests.rs:8:41
   |
8  |           let eval = ImmEval::<i32>::from(Rc::from(|_: &ImmEval<i32>| {
   |  _________________________________________^
9  | |             counter.set(counter.get() + 1);
10 | |             counter.get()
11 | |         }));
   | |__________^ the trait `std::convert::From<std::rc::Rc<[closure@src\tests.rs:8:51: 11:10 counter:_]>>` is not implemented for `ImmEval<i32>`
   |
   = help: the following implementations were found:
             <ImmEval<Out> as std::convert::From<std::rc::Rc<(dyn for<'r> std::ops::Fn(&'r ImmEval<Out>) -> Out + 'static)>>>
   = note: required by `std::convert::From::from`

当然，这意味着我的封包不是有效的Fn(&ImmEval<i32>) -> i32，但是为什么呢？

Here's the playground example。

Answer 1

在这种情况下无法进行类型推断。如果您编写所需的类型，则可以使用:

let f: Rc<dyn Fn(&ImmEval<i32>) -> i32> = Rc::new(|_: &ImmEval<i32>| {
    counter.set(counter.get() + 1);
    counter.get()
});
let eval = ImmEval::<i32>::from(f);

但这不能解决您的其他终生问题。