Cannot (de-)serialize rational64 with serde

[MalteSchledjewski]

Hello,

I am trying to (de-)serialize a uom::si::rational64::Frequency with serde but I cannot get it working.

Cargo.toml

[package]
name = "uom-serde"
version = "0.1.0"
edition = "2018"

[dependencies]
uom = { version = "0.31.1", default-features = false, features = [ "autoconvert", "rational64", "si", "std", "use_serde"]}
serde = { version = "1.0.130", features = ["derive"] }

lib.rs

use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize)]
pub struct Dummy {
    pub frequency: uom::si::rational64::Frequency,
}

Error:

cargo build
   Compiling uom v0.31.1
   Compiling uom-serde v0.1.0 
error[E0277]: the trait bound `Ratio<i64>: Serialize` is not satisfied
 --> src\lib.rs:5:5
  |
5 |     pub frequency: uom::si::rational64::Frequency,
  |     ^^^ the trait `Serialize` is not implemented for `Ratio<i64>`
  |
  = note: required because of the requirements on the impl of `Serialize` for `Quantity<dyn Dimension<M = Z0, J = Z0, N = Z0, I = Z0, T = NInt<UInt<UTerm, B1>>, L = Z0, Th = Z0, Kind = (dyn Kind + 'static)>, dyn uom::si::Units<Ratio<i64>, length = uom::si::length::meter, 
electric_current = uom::si::electric_current::ampere, luminous_intensity = uom::si::luminous_intensity::candela, thermodynamic_temperature = uom::si::thermodynamic_temperature::kelvin, mass = uom::si::mass::kilogram, time = uom::si::time::second, amount_of_substance = uom::si::amount_of_substance::mole>, Ratio<i64>>`
  = note: required by `_::_serde::ser::SerializeStruct::serialize_field`

error[E0277]: the trait bound `Ratio<i64>: Deserialize<'_>` is not satisfied
 --> src\lib.rs:5:5
  |
5 |     pub frequency: uom::si::rational64::Frequency,
  |     ^^^ the trait `Deserialize<'_>` is not implemented for `Ratio<i64>`
  |
  = note: required because of the requirements on the impl of `Deserialize<'_>` for `Quantity<(dyn Dimension<M = Z0, J = Z0, N = Z0, I = Z0, T = NInt<UInt<UTerm, B1>>, L = Z0, Th = Z0, Kind = (dyn Kind + 'static)> + 'static), (dyn uom::si::Units<Ratio<i64>, length = uom::si::length::meter, electric_current = uom::si::electric_current::ampere, luminous_intensity = uom::si::luminous_intensity::candela, thermodynamic_temperature = uom::si::thermodynamic_temperature::kelvin, mass = uom::si::mass::kilogram, time = uom::si::time::second, amount_of_substance = uom::si::amount_of_substance::mole> + 'static), Ratio<i64>>`
  = note: required by `next_element`

error[E0277]: the trait bound `Ratio<i64>: Deserialize<'_>` is not satisfied
 --> src\lib.rs:5:5
  |
5 |     pub frequency: uom::si::rational64::Frequency,
  |     ^^^ the trait `Deserialize<'_>` is not implemented for `Ratio<i64>`
  |
  = note: required because of the requirements on the impl of `Deserialize<'_>` for `Quantity<(dyn Dimension<M = Z0, J = Z0, N = Z0, I = Z0, T = NInt<UInt<UTerm, B1>>, L = Z0, Th = Z0, Kind = (dyn Kind + 'static)> + 'static), (dyn uom::si::Units<Ratio<i64>, length = uom::si::length::meter, electric_current = uom::si::electric_current::ampere, luminous_intensity = uom::si::luminous_intensity::candela, thermodynamic_temperature = uom::si::thermodynamic_temperature::kelvin, mass = uom::si::mass::kilogram, time = uom::si::time::second, amount_of_substance = uom::si::amount_of_substance::mole> + 'static), Ratio<i64>>`
  = note: required by `next_value`

error: aborting due to 3 previous errors

As far as I can see from the Cargo.lock file, the serde feature of num-rational is not activated.

[adamreichold]

As far as I can see from the Cargo.lock file, the serde feature of num-rational is not activated.

You should be able to work around this by manually enabling the feature by adding

num-rational = { version = "*", features = ["serde"] }

to your Cargo.toml.

I suspect "num-rational/serde" is missing from the feature list at https://github.com/iliekturtles/uom/blob/master/Cargo.toml#L80

[adamreichold]

While I opened #270 I see now that forwarding a feature to an optional dependency will also enable the dependency itself. (The so-called "Weak Dependency Feature" Cargo feature is still unstable.)

I therefore wonder whether this should be considered a usability issue and the documentation in the README be extended instead of the above?

[iliekturtles]

I'm leaning towards the second solution so that enabling serde doesn't pull in additional optional dependencies. Of course this makes it more complicated for users who need serde with storage types like rational64. Other thoughts?

[adamreichold]

@iliekturtles Friendly ping about considering the README paragraph from #272 for merging. Even if another solution like weak dependency features is chosen eventually, documenting this probably won't hurt until then.