我想将基于async_trait的代码迁移到新的async traits feature（将在Rust 1.75中发布）。我现有的遗留代码：

#[async_trait::async_trait]
pub trait SourceOld: Send {
    async fn get_async(&self) -> Result<String, ()>;
    fn clone(&self) -> Box<dyn SourceOld>;
}

pub fn process_old_sources(_sources: Vec<Box<dyn SourceOld>>) {
    todo!()
}

把它改写成...

pub trait SourceNew: Send {
    async fn get_async(&self) -> Result<String, ()>;
    fn clone(&self) -> Box<dyn SourceNew>;
}

pub fn process_new_sources(_sources: Vec<Box<dyn SourceNew>>) {
    todo!()
}

编译它会对两种Box<dyn SourceNew>用法产生此错误。

error[E0038]: the trait `SourceNew` cannot be made into an object
  --> src/lib.rs:10:28
   |
10 |     fn clone(&self) -> Box<dyn SourceNew>;
   |                            ^^^^^^^^^^^^^ `SourceNew` cannot be made into an object
   |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
  --> src/lib.rs:9:14
   |
7  | pub trait SourceNew: Send {
   |           --------- this trait cannot be made into an object...
8  |     // fn get_async(&self) -> impl std::future::Future<Output = Result<String, ()>> + Send;
9  |     async fn get_async(&self) -> Result<String, ()>;
   |              ^^^^^^^^^ ...because method `get_async` is `async`
   = help: consider moving `get_async` to another trait

nightly编译器也建议用这个替换get_async，但这似乎没问题。

fn get_async(&self) -> impl std::future::Future<Output = Result<String, ()>> + Send;