Rust 为什么我的trait 对象类型不匹配

我正在try 从函数返回特征对象的装箱结果.我的函数返回(似乎)满足该特征对象的正确类型.然而，编译器说类型不匹配.以下是我的代码:

use anyhow::Result;

#[derive(Debug)]
struct Bar {
    a: i32,
    b: i32,
}

trait Wrap {}

struct Wrapper1<'a> {
    bar: &'a Bar,
}

impl<'a> Wrapper1<'a> {
    fn new(bar: &'a Bar) -> Result<Self> {
        Ok(Self { bar })
    }
}

impl<'a> Wrap for Wrapper1<'a> {}

struct Wrapper2<'a> {
    bar: &'a Bar,
}

impl<'a> Wrapper2<'a> {
    fn new(bar: &'a Bar) -> Result<Self> {
        Ok(Self { bar })
    }
}

impl<'a> Wrap for Wrapper2<'a> {}

fn wrap<'a>(i: i32, bar: &'a Bar) -> Result<Box<dyn Wrap + 'a>> {
    if i % 2 == 0 {
        Wrapper1::new(bar).map(Box::new)
    } else {
        Wrapper2::new(bar).map(Box::new)
    }
}

这就是我得到的错误.

error[E0308]: mismatched types
  --> src/main.rs:37:9
   |
35 | fn wrap<'a>(i: i32, bar: &'a Bar) -> Result<Box<dyn Wrap + 'a>> {
   |                                      -------------------------- expected `Result<Box<(dyn Wrap + 'a)>, anyhow::Error>` because of return type
36 |     if i % 2 == 0 {
37 |         Wrapper1::new(bar).map(Box::new)
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Result<Box<dyn Wrap>, Error>`, found `Result<Box<Wrapper1<'_>>, Error>`
   |
   = note: expected enum `Result<Box<(dyn Wrap + 'a)>, _>`
              found enum `Result<Box<Wrapper1<'_>>, _>`
help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
   |
37 |         Ok(Wrapper1::new(bar).map(Box::new)?)
   |         +++                                ++

error[E0308]: mismatched types
  --> src/main.rs:39:9
   |
35 | fn wrap<'a>(i: i32, bar: &'a Bar) -> Result<Box<dyn Wrap + 'a>> {
   |                                      -------------------------- expected `Result<Box<(dyn Wrap + 'a)>, anyhow::Error>` because of return type
...
39 |         Wrapper2::new(bar).map(Box::new)
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Result<Box<dyn Wrap>, Error>`, found `Result<Box<Wrapper2<'_>>, Error>`
   |
   = note: expected enum `Result<Box<(dyn Wrap + 'a)>, _>`
              found enum `Result<Box<Wrapper2<'_>>, _>`
help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
   |
39 |         Ok(Wrapper2::new(bar).map(Box::new)?)
   |         +++                                ++

For more information about this error, try `rustc --explain E0308`.
error: could not compile `foo` due to 2 previous errors

编译器建议这样包装我的表达式，这确实有效.

Ok(Wrapper2::new(bar).map(Box::new)?)
+++                                ++

然而，我不明白为什么这是必要的，因为它似乎使类型完全相同.它也不太符合人体工程学.