[5.0.0-rc] [generics]: add T: ToSchema bounds on impl instead of requiring it on definition?

[oxalica]

I have generic types like this:

#[derive(utoipa::ToSchema)]
struct Signed<T> {
    payload: T,
    sig: String,
}

#[utoipa::path(post, path = "/api", request_body = Signed<String>)]
async fn post_message() {}

It fails to compile with:

error[E0277]: the trait bound `T: PartialSchema` is not satisfied
 --> src/main.rs:3:14
  |
3 |     payload: T,
  |              ^ the trait `ComposeSchema` is not implemented for `T`, which is required by `T: PartialSchema`
  |
  = note: required for `T` to implement `PartialSchema`
help: consider restricting type parameter `T`
  |
2 | struct Signed<T: utoipa::__dev::ComposeSchema> {
  |                ++++++++++++++++++++++++++++++

error[E0277]: the trait bound `T: ToSchema` is not satisfied
 --> src/main.rs:3:14
  |
3 |     payload: T,
  |              ^ the trait `ToSchema` is not implemented for `T`
  |
help: consider restricting type parameter `T`
  |
2 | struct Signed<T: utoipa::ToSchema> {
  |                ++++++++++++++++++

But the suggestion to define struct Signed<T: utoipa::ToSchema> would change the public API to require ToSchema for any Signed<T>. It would break other non-API-related code using Signed<T> on non-ToSchema types. My use case is to make utoipa dependency optional, so it's not possible to introduce breaking change on whether user enables utoipa feature.

I think a better solution would be generating the ToSchema bound on impl block, instead of requiring them on the struct definition. That is:

#[derive(ToSchema)]
struct Signed<T> { .. }
/// generates:
impl<T: ToSchema /*<- added by derive-macro*/> ToSchema for Signed<T> { .. }

[juhaku]

Good idea, thanks for taking this into action and submitting a patch 🙂