How does tonic library dynamically set up custom middleware

```
pub struct GrpcStruct
where
F: tonic::service::interceptor::Interceptor + Clone + Send + Sync + 'static,
{
middleware: Vec,
}

Setting the layer manually is fine

let ss = self.middleware[0].clone();
let ss1 = self.middleware[1].clone();
let ss2 = self.middleware[2].clone();

tonic::transport::Server::builder()
.layer(interceptor(ss))
.layer(interceptor(ss1))
.layer(interceptor(ss2))
.add_service(srv)
.serve(address)
.await
.unwrap();

// But change to dynamic add, will report an error, the error at the bottom

let mut app = tonic::transport::Server::builder();
for mid in self.middleware.iter() {
app = app.layer(interceptor(mid.clone()));
}
app.add_service(srv).serve(address).await.unwrap();

```

Report an error：

Reproduce the shown tonic::transport::Server builder example and inspect the compiler error for the looped .layer(interceptor(mid.clone())) calls. Compare it with the manually chained version and consult tonic's Server builder and layer type constraints; done means the middleware can be assembled dynamically without the reported error.

Written by the indexing model from the issue text.