rust - 如何获得对拥有对象的引用的生命周期？ (无法为生命周期参数推断出合适的生命周期)

Question

我如何编译它？

extern crate collections;
fn print_x_from_table(table: collections::BTreeMap<String, String>) {
    let x: &str = table
        .get(&String::from_str("x"))
        .map(|s: &String| &s[])  // ERROR!
        .unwrap_or("");
    println!("table contains x={}", x);
}

它给出了这个错误:

src/rusttest.rs:5:22: 5:25 error: cannot infer an appropriate lifetime for lifetime parameter 'a in function call due to conflicting requirements
src/rusttest.rs:5       .map(|s: &String| &s[])
                                           ^~~
src/rusttest.rs:7:34: 7:35 note: first, the lifetime cannot outlive the expression at 7:33...
src/rusttest.rs:7   println!("table contains x={}", x);
                                                    ^
note: in expansion of format_args!
<std macros>:2:43: 2:76 note: expansion site
<std macros>:1:1: 2:78 note: in expansion of println!
src/rusttest.rs:7:2: 7:37 note: expansion site
src/rusttest.rs:7:34: 7:35 note: ...so type `&str` of expression is valid during the expression
src/rusttest.rs:7   println!("table contains x={}", x);

如果 table 是一个引用参数，我知道我可以添加一个类型参数 'a 来指示 s 应该存在多长时间。但是当我拥有 table 时我该怎么做呢？

extern crate collections;
fn print_x_from_ref_table<'a>(table: &'a collections::BTreeMap<String, String>) {
    let x: &'a str = table
        .get(&String::from_str("x"))
        .map(|s: &'a String| &s[])
        .unwrap_or("");
    println!("table contains x={}", x);
}

Answer 1

我设法通过避免使用闭包和使用匹配来让它工作。我认为这是可行的，因为在 .map(|s| &s[]) 中，编译器会感到困惑，并认为引用的生命周期应该比实际生命周期短。

fn print_x_from_table(table: collections::BTreeMap<String, String>) {
    let x = match table.get("x") {
        Some(v) => &v[],
        None => ""
    };
    println!("table contains x={}", x);
}