Question

为什么会这样

fn main() {
    let test = "5% of foo".to_string();
    let result: i32 = test.split('%').collect()[0].parse().unwrap_or(0);
}

导致错误

error[E0282]: type annotations needed
 --> src/main.rs:4:23
  |
4 |     let result: i32 = test.split('%').collect()[0].parse().unwrap_or(0);
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for `B`

这也无济于事：

let result: i32 = test.to_string().split('%').collect()[0].parse().unwrap_or(0i32);

Answer 1

  # Message [/messages/{id}]
  This resource represents one particular message identified by its *id*.

  ## Retrieve Message [GET]
  + Response 200 (application/json)

          {"message":"Hello World!"}

  + Response 200 (text/plain)

          Hello world!

  + Response 401 (text/plain)

          There is no such a message for you, dear guest.

  + Response 410

          The message you are searching for does not exist anymore.

fn main() { let test = "5% of foo".to_string(); let result: i32 = test.split('%').collect::<Vec<_>>()[0].parse().unwrap_or(0); }可以成为实现collect()的任何类型，因此需要提供类型提示。

或者，您可以通过使用延迟迭代器来提高效率。

FromIterator

调用Iterator :: collect时，“需要类型注释”/“无法推断类型”

1 个答案: