Rust 实现 Arc to Arc<Bar> to Arc<dyn Foo 的性状>

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质对象。问题是 trait 对象需要包装在一个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()
}

此操作失败，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)`

我可能错过了一些明显的东西，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。