rust - 实现闭包特征会导致绑定(bind)/具体生命周期不匹配

Question

我想为特定类型的闭包实现一个特征。这是一个最小的例子(playground):

trait Foo {
    fn foo(&self, x: &u32);
}

impl<F> Foo for F
    where F: Fn(&u32)
{
    fn foo(&self, x: &u32) {
        self(x)
    }
}

fn main() {
    let _: &FnOnce(&u32) = &|x| {};   // works
    let _: &Foo          = &|x| {};   // doesn't work
}

它导致了这个错误:

error: type mismatch resolving `for<'r> <[closure@<anon>:16:29: 16:35] as std::ops::FnOnce<(&'r u32,)>>::Output == ()`:
 expected bound lifetime parameter ,
    found concrete lifetime [--explain E0271]
  --> <anon>:16:28
   |>
16 |>     let _: &Foo          = &|x| {};
   |>                            ^^^^^^^
note: required because of the requirements on the impl of `Foo` for `[closure@<anon>:16:29: 16:35]`
note: required for the cast to the object type `Foo`

error: type mismatch: the type `[closure@<anon>:16:29: 16:35]` implements the trait `std::ops::Fn<(_,)>`, but the trait `for<'r> std::ops::Fn<(&'r u32,)>` is required (expected concrete lifetime, found bound lifetime parameter ) [--explain E0281]
  --> <anon>:16:28
   |>
16 |>     let _: &Foo          = &|x| {};
   |>                            ^^^^^^^
note: required because of the requirements on the impl of `Foo` for `[closure@<anon>:16:29: 16:35]`
note: required for the cast to the object type `Foo`

我已经尝试将 HRTB 显式添加到 where 子句中，如下所示:

where F: for<'a> Fn(&'a u32)

但这并没有帮助。我还在 impl block 上声明了生命周期，如下所示:

impl<'a, F> Foo for F
    where F: Fn(&'a u32) { ... }

但这会导致 impl block 中的生命周期错误。我认为那些错误是正确的，不能在 impl block 上声明生命周期参数。

我该如何修正这个例子？

Answer 1

检查这部分错误:

[...] implements the trait std::ops::Fn<(_,)>, but the trait for<'r> std::ops::Fn<(&'r u32,)> is required

我认为基本上没有足够的代码来正确推断类型。添加显式类型注释允许编译示例:

let _: &Foo          = &|x: &u32| {};