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我编写了以下代码来过滤数据流，这些数据流运行良好，直到我从解析简单数字更改为也具有绑定到生命周期的类型，例如&str和&[u8]。

use wirefilter::{ExecutionContext, Filter, Scheme};

lazy_static::lazy_static! {
    static ref SCHEME: Scheme = Scheme! {
        port: Int,
        name: Bytes,
    };
}

#[derive(Debug)]
struct MyStruct {
    port: i32,
    name: String,
}

impl MyStruct {
    fn scheme() -> &'static Scheme {
        &SCHEME
    }

    fn filter_matches<'s>(&self, filter: &Filter<'s>) -> bool {
        let mut ctx = ExecutionContext::new(Self::scheme());
        ctx.set_field_value("port", self.port).unwrap();
        ctx.set_field_value("name", self.name.as_str()).unwrap();

        filter.execute(&ctx).unwrap()
    }
}

fn main() -> Result<(), failure::Error> {
    let data = expensive_data_iterator();
    let scheme = MyStruct::scheme();
    let filter = scheme
        .parse("port in {2 5} && name matches \"http.*\"")?
        .compile();

    for my_struct in data
        .filter(|my_struct| my_struct.filter_matches(&filter))
        .take(2)
    {
        println!("{:?}", my_struct);
    }

    Ok(())
}

fn expensive_data_iterator() -> impl Iterator<Item = MyStruct> {
    (0..).map(|port| MyStruct {
        port,
        name: format!("http {}", port % 2),
    })
}

如果我尝试编译它，编译器将失败：

error[E0623]: lifetime mismatch
  --> src/main.rs:26:16
   |
21 |     fn filter_matches<'s>(&self, filter: &Filter<'s>) -> bool {
   |                           -----           ----------
   |                           |
   |                           these two types are declared with different lifetimes...
...
26 |         filter.execute(&ctx).unwrap()
   |                ^^^^^^^ ...but data from `self` flows into `filter` here

error: aborting due to previous error

error: Could not compile `wirefilter_playground`.

To learn more, run the command again with --verbose.

Process finished with exit code 101

我的第一个想法是 self 和 filter 应该具有相同的生命周期，fn filter_matches<'s>(&self, filter: &Filter<'s>) -> bool但是如果我将签名更改为，fn filter_matches<'s>(&'s self, filter: &Filter<'s>) -> bool我将开始收到此错误：

error: borrowed data cannot be stored outside of its closure
  --> src/main.rs:38:29
   |
33 |     let filter = scheme
   |         ------ ...so that variable is valid at time of its declaration
...
38 |         .filter(|my_struct| my_struct.filter_matches(&filter))
   |                 ----------- ^^^^^^^^^ -------------- cannot infer an appropriate lifetime...
   |                 |           |
   |                 |           cannot be stored outside of its closure
   |                 borrowed data cannot outlive this closure

error: aborting due to previous error

error: Could not compile `wirefilter_playground`.

To learn more, run the command again with --verbose.

Process finished with exit code 101

我无法理解原因，Filter<'s>绑定到SCHEME哪个是延迟生成的，并且绑定到'static哪个有意义，不允许 filter.execute 引用，&self.name.as_str()因为它会过期，但是，不是filter.execute(&ctx)哪个签名pub fn execute(&self, ctx: &ExecutionContext<'s>) -> Result<bool, SchemeMismatchError>应该删除引用一旦它结束，因为它没有其他生命周期？

为了尝试编译上面的代码，你可以使用这个Cargo.toml：

[package]
name = "wirefilter_playground"
version = "0.1.0"
edition = "2018"

[dependencies]
wirefilter-engine = "0.6.1"
failure = "0.1.5"
lazy_static = "1.3.0"

PS：这可以通过编译 as insidefilter_matches方法来解决，但这有点糟糕，因为用户在尝试过滤时只会收到解析错误，并且可能会更慢。

4

1 回答 1

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我看到 2 种方法来解决这个问题：
1）延长self.name. 这可以通过收集expensive_data_iterator到 Vec 来实现。 

--- let data = expensive_data_iterator();
+++ let data: Vec<_> = expensive_data_iterator().collect();

2) 减少filter.

--- let filter = scheme.parse("...")?.compile();
+++ let filter = scheme.parse("...")?;

--- .filter(|my_struct| my_struct.filter_matches(&filter))
+++ .filter(|my_struct| my_struct.filter_matches(&filter.clone().compile()))

我省略了其他一些小改动。是的，filter_matches<'s>(&'s self, ...)在任何一种情况下都是强制性的。

PS 是的，第二个选项有效，因为my_structoutlifes filter。好吧，如果这两种方法都有些糟糕，那么您可以将它们结合起来！按块处理data，将每一个收集到向量中。 

const N: usize = 10; // or any other size
loop {
    let cur_chunk: Vec<_> = data.by_ref().take(N).collect();
    if cur_chunk.is_empty() {
        break;
    }
    let cur_filter = filter.clone().compile();
    // etc
}

它只使用 O(N) 内存并且编译过滤器 N 次

于 2019-05-03T11:48:15.407 回答