Question

为什么要这么做：

extern crate crossbeam;
fn main(){
    let v: Vec<i32> = vec![1, 2, 3];

    crossbeam::scope(|s|{
        s.spawn(|_|{
            for e in v{
                println!("{}", e);
            }
        });
    }).unwrap();
}

如果不这样做：

extern crate crossbeam;
fn main(){
    let v: Vec<i32> = vec![1, 2, 3];
    let closure = |_|{
        for e in v{
            println!("{}", e);
        }
    };

    crossbeam::scope(|s|{
        s.spawn(closure);
    }).unwrap();
}

编译器给我错误：

error[E0308]: mismatched types
  --> src/main.rs:11:11
   |
11 |         s.spawn(closure);
   |           ^^^^^ one type is more general than the other
   |
   = note: expected type `std::ops::FnOnce<(&crossbeam::thread::Scope<'_>,)>`
              found type `std::ops::FnOnce<(&crossbeam::thread::Scope<'_>,)>`

是因为更高的特质界限吗？

如果是的话，如何在保持闭包定义在范围之外的同时进行修复？

我是rust的新手，我发现该错误令人困惑，因为据我所知，它找到的类型是预期的类型。

Answer 1

问题在于rust无法正确地推断出封闭的类型（不做详细介绍，假设编译器在封闭之间的类型推断不太好）。

例如，您只需要在所有闭包参数上添加类型注释

extern crate crossbeam;
fn main(){
    let v: Vec<i32> = vec![1, 2, 3];
    let closure =  |sr:& crossbeam::thread::Scope<'_> /* anotate the closure arguments types */|{
        for e in v{
            println!("{}", e);
        }
    };

    crossbeam::scope(|s|{
        s.spawn(closure);
    }).unwrap();
}

请注意，您的issus与https://users.rust-lang.org/t/solved-one-type-is-more-general-than-the-other-but-they-are-exactly-the-same/25194/4
类似

为什么我不能在横梁范围内生成预定义的闭包

1 个答案: