Add special handling of Extension and Clone for debug_handler

[davidpdrsn]

To help with cases like #2293

Basically generate T: Clone for Extension<T> arguments.

We already do something similar for argument ordering so should be possible to extend that.

[loganbnielsen]

Macros operate on the syntax tree and therefore don't generally have knowledge as to whether a trait is implemented for a type. I'm not sure if it's possible to make a check for this using similar techniques from debug_handler.rs. AFAIK, we could only identify whether it's Extension and the T but not whether the T implements Clone (without doing something infeasible like registering all types that implement clone haha)

[jplatte]

@loganbnielsen If you look at the existing checks in debug_handler.rs, we already have checks like "inner type of Json in return type position implements Serialize". It works by generating a function like fn assert_serialize<T: Serialize>() {} and calling it as assert_serialize::<InnerType>();.

[loganbnielsen]

I can publish a PR for this today

[jplatte]

Go ahead! This issue is help wanted so PRs are welcome :)

I'll also assign the issue to you so it's easy to see that it has somebody working on it.

[loganbnielsen]

PR request here.