Schemars not understanding #[serde(serialize_with = "serialize_function")] macro

[xylix]

Trying to serialize a struct like this

pub enum Example {   
 #[serde(serialize_with = "serialize_function")]
    Function(String, functions::QueryFn),
}

I get

error[E0277]: the trait bound `for<'r, 's, 't0> fn(std::vec::Vec<datatype::DataType>, &'r std::collections::HashMap<&'s str, datatype::DataType>, &'t0 aw_datastore::worker::Datastore) -> std::result::Result<datatype::DataType, QueryError>: schemars::JsonSchema` is not satisfied
  --> aw-query/src/datatype.rs:18:27
   |
18 | #[derive(Clone,Serialize, JsonSchema)]
   |                           ^^^^^^^^^^ the trait `schemars::JsonSchema` is not implemented for `for<'r, 's, 't0> fn(std::vec::Vec<datatype::DataType>, &'r std::collections::HashMap<&'s str, datatype::DataType>, &'t0 aw_datastore::worker::Datastore) -> std::result::Result<datatype::DataType, QueryError>`
   |
   = note: required because of the requirements on the impl of `schemars::JsonSchema` for `(std::string::String, for<'r, 's, 't0> fn(std::vec::Vec<datatype::DataType>, &'r std::collections::HashMap<&'s str, datatype::DataType>, &'t0 aw_datastore::worker::Datastore) -> std::result::Result<datatype::DataType, QueryError>)`
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

[GREsau]

I can't think of any robust way for schemars to work out the schema itself here, as it can't really "look inside" the serialize_function.

What I could certainly do though is allow you to specify another function which returns the schema you want, which would look something like:

pub enum Example {
  #[schemars(schema_with = "schema_function")]
  #[serde(serialize_with = "serialize_function")]
  Function(String, functions::QueryFn),
}

fn schema_function() -> Schema {
 // ...
}

Would that solve your problem?

[GREsau]

It should already be possible to do something similar using with, as long as you have another type that has the same schema as the desired schema for Function:

pub enum Example {
  #[schemars(with = "T")]
  #[serde(serialize_with = "serialize_function")]
  Function(String, functions::QueryFn),
}

... where T is some type that implements JsonSchema

However, when I just tried that out I realised that schemars currently doesn't respect attributes set on enum variants! I'll try to get that fixed soon

[xylix]

Thanks for the information. I think being able to define custom schema_function's would exactly solve the problem.

[chipsenkbeil]

@GREsau, I know there is a #[schemars(with = "T")] now, but I think having the ability to provide a custom schema function would still be very handy.

I've written custom serializers and deserializers for std::io::Error and std::io::ErrorKind such that I can ship these across the wire. I obviously cannot add JsonSchema as a derive to these types and my serialization of ErrorKind is a giant mapping of serialize_unit_variant while the io::Error just encapsulates the description, os code, and error kind fields.

If I could provide a custom schema function, I'd be able to list out the error kind variants myself.

The alternative would be to create an equivalent enum that maps to/from io::ErrorKind that derives JsonSchema and plug that enum into my other structures.

[GREsau]

The #[schemars(schema_with = "...")] attribute is now implemented in v0.7.3, and usable as:

pub enum Example {
  #[schemars(schema_with = "schema_function")]
  #[serde(serialize_with = "serialize_function")]
  Function(String, functions::QueryFn),
}

fn schema_function(gen: &mut SchemaGenerator) -> Schema {
 // ...
}

Could you give it a try and let me know if it works as you expect?