closures - 闭包作为函数参数 "cannot infer an appropriate lifetime due to conflicting requirements"

Question

我正在尝试使用闭包作为函数参数:

fn foo(f: Box<Fn() -> bool>) -> bool {
    f()
}

fn main() {
    let bar = 42;
    foo(Box::new(|| bar != 42));
}

但我得到了这个生命周期错误:

src/main.rs:7:24: 7:36 error: cannot infer an appropriate lifetime due to conflicting requirements
src/main.rs:7   let n = foo(Box::new(|| bar != 42));
                                     ^~~~~~~~~~~~
src/main.rs:7:15: 7:23 note: first, the lifetime cannot outlive the     expression at 7:14...
src/main.rs:7   let n = foo(Box::new(|| bar != 42));
                            ^~~~~~~~
src/main.rs:7:15: 7:23 note: ...so that the type `[closure src/main.rs:7:24: 7:36]` will meet its required lifetime bounds
src/main.rs:7   let n = foo(Box::new(|| bar != 42));
                            ^~~~~~~~
src/main.rs:7:15: 7:37 note: but, the lifetime must be valid for the call at 7:14...
src/main.rs:7   let n = foo(Box::new(|| bar != 42));
                            ^~~~~~~~~~~~~~~~~~~~~~
src/main.rs:7:24: 7:36 note: ...so that argument is valid for the call
src/main.rs:7   let n = foo(Box::new(|| bar != 42));
                                     ^~~~~~~~~~~~
error: aborting due to previous error

我不明白为什么没有正确推断生命周期。我该怎么做才能解决这个问题？

$ rustc --version
rustc 1.0.0-nightly (6c065fc8c 2015-02-17) (built 2015-02-18)

Answer 1

如果你想使用盒装闭包，你需要使用move || {}。

fn foo(f: Box<Fn() -> bool>)
       -> bool {
    f()
}

fn main() {
    let bar = 42;
    let blub = foo(Box::new(move || bar != 42));
}

另一方面，您不能直接使用未装箱的闭包，因为它可能包含任意数量的捕获元素，因此没有大小限制。通过使用泛型，您可以轻松规避此限制:

fn foo<T>(f: T)
          -> bool
          where T : Fn() -> bool {
    f()
}

fn main() {
    let bar = 42;
    let blub = foo(|| bar != 42);
}