Returning structure in Substrate RPC

Question

We're trying to return structure in RPC, but as far as I understand it should be serializable:

    error[E0277]: the trait bound `pallet_spaces::Space<T>: serde::de::Deserialize<'_>` is not satisfied
  --> pallets/spaces/rpc/src/lib.rs:15:1
   |
15 | #[rpc]
   | ^^^^^^ the trait `serde::de::Deserialize<'_>` is not implemented for `pallet_spaces::Space<T>`
   |
   = note: required because of the requirements on the impl of `for<'de> serde::de::Deserialize<'de>` for `std::vec::Vec<pallet_spaces::Space<T>>`
   = note: required because of the requirements on the impl of `serde::de::DeserializeOwned` for `std::vec::Vec<pallet_spaces::Space<T>>`
   = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

The problem is that we use T::Moment from pallet_timestamp and it's not serializable, so we stuck at this point:

    error[E0277]: the trait bound `<T as pallet_timestamp::Trait>::Moment: _::_serde::Serialize` is not satisfied
  --> pallets/spaces/src/lib.rs:25:5
   |
25 |     pub created: WhoAndWhen<T>,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `_::_serde::Serialize` is not implemented for `<T as pallet_timestamp::Trait>::Moment`
   |
   = note: required because of the requirements on the impl of `_::_serde::Serialize` for `pallet_utils::WhoAndWhen<T>`
   = note: required by `_::_serde::ser::SerializeStruct::serialize_field`

What can you suggest to easily return a structure like this?

#[derive(Encode, Decode, Clone, Eq, PartialEq, RuntimeDebug, Serialize, Deserialize)]
pub struct Space<T: Trait> {
    pub id: SpaceId,
    pub created: WhoAndWhen<T>,
    pub updated: Option<WhoAndWhen<T>>,

    pub owner: T::AccountId,

    // Can be updated by the owner:
    pub parent_id: Option<SpaceId>,
    pub handle: Option<Vec<u8>>,
    pub content: Content,
    pub hidden: bool,

    pub posts_count: u32,
    pub hidden_posts_count: u32,
    pub followers_count: u32,

    pub score: i32,

    /// Allows to override the default permissions for this space.
    pub permissions: Option<SpacePermissions>,
}

#[derive(Encode, Decode, Clone, Eq, PartialEq, RuntimeDebug, Serialize, Deserialize)]
pub struct WhoAndWhen<T: Trait> {
    pub account: T::AccountId,
    pub block: T::BlockNumber,
    pub time: T::Moment,
}

Answer 1

Your main problem is that you are mixing std and no-std here. Substrate only depends on serde in std mode, as you can learn about in literally any Cargo.toml file in the project.

Start by fixing this: You only derive serde::* when you are in std mode.

#[cfg_attr(feature = "std", derive(Serialize, Deserialize))]
#[derive(Encode, Decode, Clone, Eq, PartialEq, RuntimeDebug)]
pub struct Space<T: Trait> {
  // snip.. 
}