loops - 是否可以明确指定循环迭代的生命周期？

Question

以下代码利用生命周期省略进行编译:

fn g(a: &mut str, closure: impl Fn(&mut str, &str) -> ()) {
    let data = vec![];
    for d in data.iter() {
        closure(a, d);
    }
}

但是，假设我需要显式指定闭包中变量的生命周期(在我的例子中，调用站点是不明确的，除非我这样做)。如果我显式添加生命周期，编译器会提示 a 在闭包的先前迭代中可变地借用。

fn g<'b, 'd>(a: &'b mut str, closure: impl Fn(&'d mut str, &str) -> ()) {
    let data: Vec<&str> = vec![];
    'd: for d in data.iter() {
        closure(a, d);
    }
}

warning: label name `'d` shadows a lifetime name that is already in scope
 --> src/lib.rs:3:5
  |
1 | fn g<'b, 'd>(a: &'b mut str, closure: impl Fn(&'d mut str, &str) -> ()) {
  |          -- first declared here
2 |     let data: Vec<&str> = vec![];
3 |     'd: for d in data.iter() {
  |     ^^ lifetime 'd already in scope

error[E0312]: lifetime of reference outlives lifetime of borrowed content...
 --> src/lib.rs:4:17
  |
4 |         closure(a, d);
  |                 ^
  |
note: ...the reference is valid for the lifetime 'd as defined on the function body at 1:10...
 --> src/lib.rs:1:10
  |
1 | fn g<'b, 'd>(a: &'b mut str, closure: impl Fn(&'d mut str, &str) -> ()) {
  |          ^^
note: ...but the borrowed content is only valid for the lifetime 'b as defined on the function body at 1:6
 --> src/lib.rs:1:6
  |
1 | fn g<'b, 'd>(a: &'b mut str, closure: impl Fn(&'d mut str, &str) -> ()) {
  |      ^^

我认为这是因为编译器不会猜测我只想 'd 持续循环的每次迭代。我寻找一种引用生命周期的方法，但我能找到的只是我能够在 Rust 引用书中指定“生命周期或标签”，这似乎只是一个用于所有意图和目的的标签。

是否有任何方法可以显式指定每个循环迭代的生命周期，或者显式写出编译器在第一个示例中省略的内容？

Answer 1

Is it possible to specify the lifetime of an iteration of a loop explicitly?

没有。这样的事情根本不需要。

---

您没有提供与原始问题相对应的 MCVE，因此不可能提供解决该问题的解决方案。我的直觉和你的评论“写出编译器省略的内容”告诉我你可能应该看看:

• How do I bound a generic type with a trait that requires a lifetime parameter if I create the reference inside the function?
• How does for<> syntax differ from a regular lifetime bound?

TL；博士:

fn g(a: &mut str, closure: impl for<'a, 'd> Fn(&'a mut str, &'d str) -> ())