Interaction between serde and strum that results in a compiler error

[agodnic]

Hi!

I've noticed that using strum's derive EnumDiscriminants with some serde variant attributes, results in a compiler error.

I'm not sure of this is an issue in strum, in serde, or both.

Here's a program to reproduce the issue:

Cargo.toml

[dependencies]
serde = { version = "1.0", features = ["derive"] }
strum = "0.16.0"
strum_macros = "0.16.0"

src/main.rs

extern crate strum;
#[macro_use] extern crate strum_macros;
extern crate serde;

use std::str::FromStr;

use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize, Debug, EnumDiscriminants)]
#[serde(tag = "type")]
enum Message {
    #[serde(rename = "request" )]
    Request { id: String, method: String, params: u8 },
    #[serde(rename = "response" )]
    Response { id: String, result: u8 },
}

fn main() {
    println!("Hello, world!");
}

Compiler error

error[E0658]: the attribute `serde` is currently unknown to the compiler and may have meaning added to it in the future
  --> src/main.rs:12:5
   |
12 |     #[serde(rename = "request" )]
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/29642

error[E0658]: the attribute `serde` is currently unknown to the compiler and may have meaning added to it in the future
  --> src/main.rs:14:5
   |
14 |     #[serde(rename = "response" )]
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/29642

warning: unused import: `std::str::FromStr`
 --> src/main.rs:5:5
  |
5 | use std::str::FromStr;
  |     ^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0658`.

Additional info

The program compiles successfully if the lines #[serde(rename = "response" )] are removed.

[Peternator7]

Hi!

Thanks for reporting this issue, and sorry for the delay. The problem is that the newly created Discriminants enum doesn't implement Serialize so the compiler doesn't know what serde means in the context of the new type. EnumDiscriminants copies the attributes on each of the variants over to the new enum type, but it doesn't copy the derives over from the original type.

The current solution is to add a strum_discriminants attribute to the original enum and include
additional traits to derive. Here's and example that implements strum::EnumIter on the discriminant enum. Github Link

In this particular case, it would look something like:

#[derive(Serialize, Deserialize, Debug, EnumDiscriminants)]
#[serde(tag = "type")]
#[strum_discriminants(derive(Serialize, Deserialize))]
enum Message {
    #[serde(rename = "request" )]
    Request { id: String, method: String, params: u8 },
    #[serde(rename = "response" )]
    Response { id: String, result: u8 },
}

[azriel91]

Heya, I've opened #82 which is one way to resolve this. Hopefully the whitelist is sensible -- I've been using it for a number of projects and haven't run into any weird problems.