Rust 如何在 struct 上创建一个大小合适的闭包或实现 FnFnMutFnOnce

基本上，我想写一个返回闭包的函数.我怎么能不用返回Box<FnOnce(u32)>就完成这项任务呢？

从closures chapter of the rust book中，我了解到闭包只是 struct 和impl FnOnce的语法糖.以下是我的try :

#[derive(Debug)]
struct MyError {
    code: u32,
    location: &'static str,
}
// Here is my closure:
struct MyErrorPartial {
    location: &'static str,
}
impl FnOnce(u32) for MyErrorPartial {
    type Output = MyError;

    fn call_once(self, args: u32) -> MyError {
        MyError {
            code: args,
            location: self.location,
        }
    }
}
fn error_at(location: &'static str) -> MyErrorPartial {
    MyErrorPartial {location: location}
}

fn function_returning_code() -> Result<(), u32> {
    Err(123)
}
fn function_with_error() -> Result<(), MyError> {
    try!(function_returning_code().map_err(error_at("line1")));
    try!(function_returning_code().map_err(error_at("line2")));
    Ok(())
}
fn main() {
    function_with_error().unwrap();
}

它目前给出了一个错误:

<anon>:11:12: 11:17 error: associated type bindings are not allowed here [E0229]
<anon>:11 impl FnOnce(u32) for MyErrorPartial {
                     ^~~~~