Question

我正在尝试实现greps项目，但我仍陷入搜索功能。

fn search<'a, T>(query: &T, contents: &'a str) -> Vec<&'a str> {
    let mut results = Vec::new();

    for line in contents.lines() {
        if line.contains(query) {
            results.push(line);
        }
    }
    results
}

我收到此错误：

rustc 1.18.0 (03fc9d622 2017-06-06)
error[E0277]: the trait bound `T: std::ops::Fn<(char,)>` is not satisfied
  --> <anon>:39:17
   |
39 |         if line.contains(query) {
   |                 ^^^^^^^^ the trait `std::ops::Fn<(char,)>` is not implemented for `T`
   |
   = help: consider adding a `where T: std::ops::Fn<(char,)>` bound
   = note: required because of the requirements on the impl of `std::ops::FnOnce<(char,)>` for `&T`
   = note: required because of the requirements on the impl of `std::str::pattern::Pattern<'_>` for `&T`

error[E0277]: the trait bound `T: std::ops::FnOnce<(char,)>` is not satisfied
  --> <anon>:39:17
   |
39 |         if line.contains(query) {
   |                 ^^^^^^^^ the trait `std::ops::FnOnce<(char,)>` is not implemented for `T`
   |
   = help: consider adding a `where T: std::ops::FnOnce<(char,)>` bound
   = note: required because of the requirements on the impl of `std::str::pattern::Pattern<'_>` for `&T`

error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
  --> <anon>:57:40
   |
57 |         assert_eq!(vec!["KAMEHAMEHA"], search(query, contents));
   |                                        ^^^^^^ the trait `std::marker::Sized` is not implemented for `str`
   |
   = note: `str` does not have a constant size known at compile-time
   = note: required by `search`

为什么我需要Fn特质？添加该特性并不能解决我的问题。我正在使用泛型，我知道我真的不需要泛型，但我想了解这个主题。

以下是Rust playground的完整代码。

Answer 1

问题（或其中之一）在此功能中：

fn search<'a, T>(query: &T, contents: &'a str) -> Vec<&'a str> {
    let mut results = Vec::new();

    for line in contents.lines() {
        if line.contains(query) {
            results.push(line);
        }
    }
    results
}

函数line.contains()需要一个实现特征Pattern的参数。但是您的T参数不受约束。该错误有点令人困惑，因为您可能不关心Fn和FnOnce实现，它们恰好是Pattern的超类型。

您将面临的下一个问题是Pattern本身尚未稳定，因此除非您切换到编译器的每晚构建，否则无法显式使用它。否则，您可以约束T，以便至少可以将其转换为已经实现Pattern的其他内容，例如&str：

fn search<'a, T: 'a>(query: &'a T, contents: &'a str) -> Vec<&'a str> 
    where &'a T: Into<&'a str>
{
    let mut results = Vec::new();

    for line in contents.lines() {
        if line.contains(query.into()) {
            results.push(line);
        }
    }
    results
}

此后你还会有更多的错误，但希望这会让你经历最令人困惑的错误。

特征`std :: ops :: Fn＆lt;（char，）＆gt;`没有实现`T`

1 个答案: