在函数体上有生命周期的泛型值上迭代时，借用值的生命周期不够长

Question

fn func<'a, T>(arg: Vec<Box<T>>)
where
    String: From<&'a T>,
    T: 'a,
{
    let s: Vec<String> = arg.iter().map(|s| String::from(s)).collect();
    do_something_else(arg);
}

fn do_something_else<T>(arg: Vec<Box<T>>) {}

编译器抱怨arg 的寿命不够长。为什么呢？

error[E0597]: `arg` does not live long enough
 --> src/lib.rs:6:26
  |
6 |     let s: Vec<String> = arg.iter().map(|s| String::from(s)).collect();
  |                          ^^^ borrowed value does not live long enough
7 |     do_something_else(arg);
8 | }
  | - borrowed value only lives until here
  |
note: borrowed value must be valid for the lifetime 'a as defined on the function body at 1:9...
 --> src/lib.rs:1:9
  |
1 | fn func<'a, T>(arg: Vec<Box<T>>)
  |         ^^

Answer 1

约束String: From<&'a T>，强调函数的生命周期参数'a，将允许您将对T 的引用转换为String。但是，对从迭代器获得的元素的引用比'a 更具限制性（因此，它们的寿命不够长）。

由于转换应该适用于任何生命周期的引用，因此您可以将约束替换为更高等级的 trait bound (HRTB)：

fn func<T>(arg: Vec<Box<T>>)
where
    for<'a> String: From<&'a T>,
{
    let s: Vec<String> = arg.iter().map(|s| String::from(s)).collect();
    do_something_else(arg);
}

在这里使用From 来获取拥有的字符串也不是我在野外见过的。也许你会对Display trait 感兴趣，这样你就可以调用to_string()：

fn func<T>(arg: Vec<Box<T>>)
where
    T: Display,
{
    let _: Vec<_> = arg.iter().map(|s| s.to_string()).collect();
    // ...
}

另见：

• How do I write the lifetimes for references in a type constraint when one of them is a local reference?
• How does for<> syntax differ from a regular lifetime bound?