Rust 生命周期，数据流入其他引用

Question

我编写了以下代码来过滤数据流，在我从解析简单数字更改为还具有绑定(bind)到生命周期的类型(如 &str)之前，这些数据流工作正常。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，编译器会失败:

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 在 fn filter_matches<'s>(&self, filter: &Filter<'s>) -> bool 中应该有相同的生命周期。但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

我不明白原因，Filter<'s>绑定(bind)到 SCHEME这是延迟生成并绑定(bind)到 'static不允许 filter.execute 引用 &self.name.as_str() 是有道理的因为它会过时但不是filter.execute(&ctx)签名是pub fn execute(&self, ctx: &ExecutionContext<'s>) -> Result<bool, SchemeMismatchError>应该在它完成后立即删除引用，因为它没有其他生命周期？

为了尝试编译上面的代码，您可以使用 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 inside filter_matches 来解决。方法，但这有点糟糕，因为用户只会在尝试过滤时收到解析错误，而且速度可能会变慢。

Answer 1

我看到有两种方法可以解决这个问题:
1) 延长 self.name 的生命周期.这可以通过收集 expensive_data_iterator 来实现进入，比如说，Vec。

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

2) 缩短 filter 的生命周期.

--- 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()))

我省略了一些其他的小改动。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_struct超过生命周期filter .好吧，如果这两种方法都有些不好，那么您可以将它们结合起来!流程data按 block ，将每个 block 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少 N 倍