Forward on parse Error in Json::from_data instead of failing

[LorenzSchueler]

Hello,
I'm using rocket v0.5 and tried to use multiple routes to the same URL. When using different parameters this works perfectly. However I also tried to use this with POST request where the payload can be a Json of type a or type b. The problem is that the FromData implementation for Json returns an Outcome::Failure when it encounters a Error::Parse instead of forwarding it.
I thought I could just write my own FromData implementation which internally uses the FromData implementation of Json but converts an Outcome::Failure with Status::UnprocessableEntity to an Outcome::Forward. the problem here is that from_data consumes data but data is not Clone so that I could put it into Outcome::Forward.
I'm wondering if I'm missing something or if this is intended because cloning data could be expensive.

[jebrosen]

When using different parameters this works perfectly. However I also tried to use this with POST request where the payload can be a Json of type a or type b.

Since Rocket's JSON support is based on serde_json, one way to accomplish a single route with multiple kinds of payload would be to use serde's support for several different enum representations to have serde determine which type to deserialize based on the data.

I'm wondering if I'm missing something or if this is intended because cloning data could be expensive.

Yes, I think this is more or less the same as issue #775. I explained the reason for this limitation in #775 (comment), along with a few potential solutions for the future. But, none of them will be ready for 0.5 - and they would likely require parsing the same data multiple times, making it a slower option than serde anyway.