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rust - 为什么不可能取消引用装箱的迭代器特征对象？

转载 作者：行者123 更新时间：2023-11-29 08:26:19 25 4
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<分区>

我有以下三个例子:

fn iterate_with_iterator<T: std::fmt::Display, I: Iterator<Item = T>>(iter: I) {
    for x in iter {
         println!("{}", x);
    }
}

fn iterate_with_boxed_iterator<'a, T: std::fmt::Display>(iter: Box<dyn Iterator<Item = T> + 'a>) {
    for x in iter {
         println!("{}", x);
    }
}

fn iterate_with_deref_boxed_iterator<'a, T: std::fmt::Display>(iter: Box<dyn Iterator<Item = T> + 'a>) {
    let diter = *iter;
    for x in diter {
         println!("{}", x);
    }
}

Playground

编译iterate_with_deref_boxed_iterator时出现以下错误:

error[E0277]: the size for values of type `dyn std::iter::Iterator<Item = T>` cannot be known at compilation time
  --> src/main.rs:14:9
   |
14 |     let diter = *iter;
   |         ^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `dyn std::iter::Iterator<Item = T>`
   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
   = note: all local variables must have a statically known size
   = help: unsized locals are gated as an unstable feature

error[E0277]: the size for values of type `dyn std::iter::Iterator<Item = T>` cannot be known at compilation time
  --> src/main.rs:15:14
   |
15 |     for x in diter {
   |              ^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `dyn std::iter::Iterator<Item = T>`
   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
   = note: required by `std::iter::IntoIterator::into_iter`

为什么取消引用会中断编译？

25 4 0
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