Problem with serde impl for k256::ecdsa::VerifyingKey in 0.14.0-pre.2

[tmpfs]

Seems like the serde derive implementation is not being compiled for k256::ecdsa::VerifyingKey when the serde feature flag is active.

Minimal reproduction:

[package]
name = "k256-serde"
version = "0.1.0"
edition = "2021"

[dependencies]
k256 = { version = "0.14.0-pre.2", default-features = false, features = ["serde", "ecdsa", "std"] }
serde_json = "1"
rand = "0.8"

use k256::ecdsa::SigningKey;
use rand::rngs::OsRng;

fn main() {
    let key = SigningKey::random(&mut OsRng);
    let verifying_key = key.verifying_key();
    serde_json::to_string(verifying_key).unwrap();
}

Yields the compiler error:

error[E0277]: the trait bound `ecdsa::verifying::VerifyingKey<Secp256k1>: serde::ser::Serialize` is not satisfied
    --> src/main.rs:7:27
     |
7    |     serde_json::to_string(verifying_key).unwrap();
     |     --------------------- ^^^^^^^^^^^^^ the trait `serde::ser::Serialize` is not implemented for `ecdsa::verifying::VerifyingKey<Secp256k1>`
     |     |
     |     required by a bound introduced by this call
     |
     = note: for local types consider adding `#[derive(serde::Serialize)]` to your `ecdsa::verifying::VerifyingKey<Secp256k1>` type
     = note: for types from other crates check whether the crate offers a `serde` feature flag
     = help: the following other types implement trait `serde::ser::Serialize`:
               &'a T
               &'a mut T
               ()
               (T,)
               (T0, T1)
               (T0, T1, T2)
               (T0, T1, T2, T3)
               (T0, T1, T2, T3, T4)
             and 142 others
note: required by a bound in `serde_json::to_string`
    --> /Users/muji/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.128/src/ser.rs:2209:17
     |
2207 | pub fn to_string<T>(value: &T) -> Result<String>
     |        --------- required by a bound in this function
2208 | where
2209 |     T: ?Sized + Serialize,
     |                 ^^^^^^^^^ required by this bound in `to_string`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `k256-serde` (bin "k256-serde") due to 1 previous error

Is this an error my end or a problem with propagating the feature flag perhaps?

[tmpfs]

Oh my bad, I noticed I had to enable the pem flag too, is it worth automatically enabling pem when serde is present perhaps?

[tarcieri]

It's really pkcs8 that's needed, and it could be auto-enabled along with serde, yes

[tarcieri]

This changes it to work like the elliptic-curve crate, where only the serde feature needs to be enabled: RustCrypto/signatures#874