Rust 实现 Arc<Bar> 到 Arc<dyn Foo> 的 Into 特征

Question

我正在尝试将实现特征的结构转换为具有相同特征的特征对象。问题是特征对象需要包装在Arc 中，而我无法为Arc 实现Into 特征。由于以下代码未编译：

use std::sync::Arc;

trait Foo { }

struct Bar {}

impl Foo for Bar { }

impl Into<Arc<dyn Foo>> for Arc<Bar> {
    fn into(self) -> Arc<dyn Foo> { self }
}

fn bar_to_foo(bar: Arc<Bar>) -> Arc<dyn Foo> {
    bar.into()
}

这将失败并显示以下编译器消息：

error[E0119]: conflicting implementations of trait `std::convert::Into<std::sync::Arc<(dyn Foo + 'static)>>` for type `std::sync::Arc<Bar>`:
 --> src/lib.rs:9:1
  |
9 | impl Into<Arc<dyn Foo>> for Arc<Bar> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: conflicting implementation in crate `core`:
          - impl<T, U> std::convert::Into<U> for T
            where U: std::convert::From<T>;
  = note: upstream crates may add a new impl of trait `std::convert::From<std::sync::Arc<Bar>>` for type `std::sync::Arc<(dyn Foo + 'static)>` in future versions

error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
 --> src/lib.rs:9:1
  |
9 | impl Into<Arc<dyn Foo>> for Arc<Bar> {
  | ^^^^^------------------^^^^^--------
  | |    |                      |
  | |    |                      `std::sync::Arc` is not defined in the current crate
  | |    `std::sync::Arc` is not defined in the current crate
  | impl doesn't use only types from inside the current crate
  |
  = note: define and implement a trait or new type instead

error: aborting due to 2 previous errors

但是我也不能执行以下操作：

use std::sync::Arc;

trait Foo { }

struct Bar {}

impl Foo for Bar { }

impl Into<dyn Foo> for Bar {
    fn into(self) -> dyn Foo { self }
}

fn bar_to_foo(bar: Arc<Bar>) -> Arc<dyn Foo> {
    bar.into()
}

因为这会导致：

error[E0277]: the size for values of type `(dyn Foo + 'static)` cannot be known at compilation time
 --> src/lib.rs:9:6
  |
9 | impl Into<dyn Foo> for Bar {
  |      ^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `(dyn Foo + 'static)`

我可能遗漏了一些明显的东西，但任何帮助将不胜感激！

Answer 1

我是个白痴：

use std::sync::Arc;

trait Foo { }

struct Bar {}

impl Foo for Bar { }

fn bar_to_foo(bar: Arc<Bar>) -> Arc<dyn Foo> {
    bar
}

我会展示自己的。