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## 函数的入参是一个async function
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因为我们使用了trait bound，所以不能用`Fn() -> impl Future<Output=Result<Return, sql::Error>>`的方式.
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```rust
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use core::future::Future;
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pub async fn on_tran<F, Fut>(f: F) -> usize
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where F: Fn() -> Fut, Fut: Future<Output=usize> {
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f().await
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}
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#[tokio::main]
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async fn main() {
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let foo = || {
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async {
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8 as usize
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}
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};
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println!("{}", on_tran(foo).await);
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}
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```
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