FromForm: Shared parameters?

[darksylinc]

Hi!

I'm getting more confident with Rocket and Rust and porting of our backend is going well. But I noticed I am going into "too much copy-paste" (which leads to human error) and wish to consult if there are better ways:

I have the following:

#[derive(Debug, FromForm, Serialize)]
#[serde(crate = "rocket::serde")]
struct BaseParams<'a>
{
       #[field(name = "userId")]
       pub user_id: Option<u64>,
       pub game: Option<&'a str>,
}

Many (but not all) of our APIs take user_id & game. And many take more parameters:

#[derive(Debug, FromForm, Serialize)]
#[serde(crate = "rocket::serde")]
struct GetObjs<'a> {
       #[field(name = "userId")]
       pub user_id: Option<u64>,
       pub game: Option<&'a str>,
       pub region_id: u16,
}

Normally in Rust to avoid so much repetition this would be solved through composition, something like:

#[derive(Debug, FromForm, Serialize)]
#[serde(crate = "rocket::serde")]
struct GetObjs<'a> {
       pub base: BaseParams<'a>,
       pub region_id: u16,
}

However as far as I can tell, Rocket's FromForm does not support seeing into "base". And even if it does, I would need Rocket to look for "game" and not "base_game" or similar.
Unfortunately, Rust does not support inheritance out of the box either.

I tried with a macro that emulates inheritance:

macro_rules! AddAuthenticationFields {
    ($pub:vis struct $name:ident<$lifetime:tt> { $( $field:ident: $ty:ty ),* $(,)* }) => {
        #[derive(Debug, FromForm, Serialize)]
        #[serde(crate = "rocket::serde")]
        $pub struct $name<$lifetime> {
            #[field(name = "userId")]
            pub user_id: Option<u64>,
            pub game: Option<&$lifetime str>,
            $( $field: $ty ),*
        }
    };
}

AddAuthenticationFields! {
 pub struct EmptyParams<'a> {}
}

AddAuthenticationFields! {
 pub struct GetObjs<'a> {
       pub region_id: u16,
} }

The first one EmptyParams works perfectly.

However the second one, GetObjs gets errors. Rust complains about many things and it seems to boil down to the order in which FromForm macro is executed (I don't know how to make it execute after my macro).

Is there a way to support this? Maybe I need to implement a trait?

If the answer is no then I think it's probably less error prone for me to parse it by hand using p: Form<Contextual<'_, &str>>.

Cheers and Thanks!

[blackghost1987]

I think what you are looking for is called "flatten" in Serde world. Actually you can do this already with responses (e.g. for JSON) see: https://serde.rs/attr-flatten.html

Unfortunately this logic is not (yet) implemented for FromForm, at least as far as I know. There was an attempt to implement "flatten" for Forms a couple years back, but it got stuck in Draft, see: #1916

I can't really help you with the macro implementation though, I'm just trying provide some (hopefully helpful) context.

[darksylinc]

Ahh! Thank you! So I wasn't crazy!

Thank you.

So it's a known issue with a PR in draft.

So I guess I'll roll it by hand when necessary. Thanks!