如何在包含 &'static str 的结构上实现 serde::Deserialize？

Question

我正在尝试在SourceConfig 结构上实现serde::Deserialize，它包装了一个包含&'static str 的结构以及它自己的一些数据(playground)：

use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize)]
pub struct Config {
    pub name: &'static str,
}

#[derive(Serialize, Deserialize)]
struct SourceConfig {
    config: Config,
    id: u32,
}

但这给了我一生的错误：

   Compiling playground v0.0.1 (/playground)
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'de` due to conflicting requirements
  --> src/lib.rs:10:5
   |
10 |     config: Config,
   |     ^^^^^^
   |
note: first, the lifetime cannot outlive the lifetime `'de` as defined on the impl at 8:21...
  --> src/lib.rs:8:21
   |
8  | #[derive(Serialize, Deserialize)]
   |                     ^^^^^^^^^^^
note: ...so that the types are compatible
  --> src/lib.rs:10:5
   |
10 |     config: Config,
   |     ^^^^^^
   = note: expected `SeqAccess<'_>`
              found `SeqAccess<'de>`
   = note: but, the lifetime must be valid for the static lifetime...
note: ...so that the types are compatible
  --> src/lib.rs:10:5
   |
10 |     config: Config,
   |     ^^^^^^
   = note: expected `Deserialize<'_>`
              found `Deserialize<'static>`
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'de` due to conflicting requirements
  --> src/lib.rs:10:5
   |
10 |     config: Config,
   |     ^^^^^^
   |
note: first, the lifetime cannot outlive the lifetime `'de` as defined on the impl at 8:21...
  --> src/lib.rs:8:21
   |
8  | #[derive(Serialize, Deserialize)]
   |                     ^^^^^^^^^^^
note: ...so that the types are compatible
  --> src/lib.rs:10:5
   |
10 |     config: Config,
   |     ^^^^^^
   = note: expected `MapAccess<'_>`
              found `MapAccess<'de>`
   = note: but, the lifetime must be valid for the static lifetime...
note: ...so that the types are compatible
  --> src/lib.rs:10:5
   |
10 |     config: Config,
   |     ^^^^^^
   = note: expected `Deserialize<'_>`
              found `Deserialize<'static>`
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors

我尝试将#[serde(borrow)] 添加到SourceConfig 中的config，但这不起作用，因为Config 没有被借用。将其添加到Config 中的name 也不起作用。如何在SourceConfig 上正确实现Deserialize？

Answer 1

您可以在SourceConfig 结构上添加bound，这样'de 的生命周期就等于'static 的生命周期。

#[derive(Debug, serde::Deserialize)]
pub struct Config {
    pub name: &'static str,
}

#[derive(Debug, serde::Deserialize)]
#[serde(bound(deserialize = "'de: 'static"))]
struct SourceConfig {
    config: Config,
    id: u32,
}

fn main() {
    let j = r#"
{
    "id": 123,
    "config": {
        "name": "John Smith"
    }
}
    "#;
    
    let sc: SourceConfig = serde_json::from_str(&j).unwrap();
    dbg!(sc);
}

但是，您可能不想反序列化 &'static str，因为这需要您有一个 &'static str 可以借用。这将您限制为字符串文字，或者您需要泄漏内存来获取&'static str。 您可能需要String，或者，如果您想支持字符串文字，则需要Cow<'a, str>。

&'a str 的另一个问题是，如果使用转义序列对字符串进行序列化，它将无法工作。例如，包含换行符的 JSON 字符串将具有 \n，在反序列化期间需要替换它，因此如果您使用 &'a str，则会失败。