Getting access to the Request in a FromForm instance OR requiring a guard in an uri! macro

[42triangles]

I am curious how to best handle what conceptually feels in between a request guard and a form, where the form data has to be validated against the database (⇒ FromRequest), while it still should be necessary for uri! macros (⇒ FromForm).

With a POST request that wouldn't be an issue because based on what I've seen, FromData does have access to the Request.
However, for GET requests the same doesn't work, since doing the ?<form..> thing requires FromForm which does not allow using the Request to get in this case access to the database connection.
And while a request guard could handle the validation (even though Request::query_value does note that it is much more expensive than the normal way of retrieving them, and is to be avoided if possible), it is not something that then becomes necessary in the uri! macro which I would like to use for safe URI generation.

A possible way of getting close that I've considered would be using a FromForm instance that doesn't do anything with the data it is given, and a request guard that does the validation; though the effective double lookup does feel rather... unidiomatic.

Another way would just be to do validation in the body of the function, which is probably the best course of action unless I have overlooked anything.