Rust 如何在具体的生命周期中实现FromStr

我想为具有生存期参数的 struct 实现FromStr:

use std::str::FromStr;

struct Foo<'a> {
    bar: &'a str,
}

impl<'a> FromStr for Foo<'a> {
    type Err = ();
    fn from_str(s: &str) -> Result<Foo<'a>, ()> {

        Ok(Foo { bar: s })
    }
}

pub fn main() {
    let foo: Foo = "foobar".parse().unwrap();
}

然而，编译器抱怨:

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
  --> src/main.rs:11:12
   |
11 |         Ok(Foo { bar: s })
   |            ^^^
   |
help: consider using an explicit lifetime parameter as shown: fn from_str(s: &'a str) -> Result<Foo<'a>, ()>
  --> src/main.rs:9:5
   |
9  |     fn from_str(s: &str) -> Result<Foo<'a>, ()> {
   |     ^

将impl更改为

impl<'a> FromStr for Foo<'a> {
    type Err = ();
    fn from_str(s: &'a str) -> Result<Foo<'a>, ()> {
        Ok(Foo { bar: s })
    }
}

给出了这个错误

error[E0308]: method not compatible with trait
  --> src/main.rs:9:5
   |
9  |     fn from_str(s: &'a str) -> Result<Foo<'a>, ()> {
   |     ^ lifetime mismatch
   |
   = note: expected type `fn(&str) -> std::result::Result<Foo<'a>, ()>`
   = note:    found type `fn(&'a str) -> std::result::Result<Foo<'a>, ()>`
note: the anonymous lifetime #1 defined on the block at 9:51...
  --> src/main.rs:9:52
   |
9  |     fn from_str(s: &'a str) -> Result<Foo<'a>, ()> {
   |                                                    ^
note: ...does not necessarily outlive the lifetime 'a as defined on the block at 9:51
  --> src/main.rs:9:52
   |
9  |     fn from_str(s: &'a str) -> Result<Foo<'a>, ()> {
   |                                                    ^
help: consider using an explicit lifetime parameter as shown: fn from_str(s: &'a str) -> Result<Foo<'a>, ()>
  --> src/main.rs:9:5
   |
9  |     fn from_str(s: &'a str) -> Result<Foo<'a>, ()> {
   |     ^

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