Why do I get an UnsupportedType error when serializing to TOML with a manually implemented Serialize for an enum with struct variants? - serialization

Question

I'm trying to implement Serialize for an enum that includes struct variants. The serde.rs documentation indicates the following:
enum E {
// Use three-step process:
// 1. serialize_struct_variant
// 2. serialize_field
// 3. end
Color { r: u8, g: u8, b: u8 },
// Use three-step process:
// 1. serialize_tuple_variant
// 2. serialize_field
// 3. end
Point2D(f64, f64),
// Use serialize_newtype_variant.
Inches(u64),
// Use serialize_unit_variant.
Instance,
}
With that in mind, I proceeded to implemention:
use serde::ser::{Serialize, SerializeStructVariant, Serializer};
use serde_derive::Deserialize;
#[derive(Deserialize)]
enum Variants {
VariantA,
VariantB { k: u32, p: f64 },
}
impl Serialize for Variants {
fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
where
S: Serializer,
{
match *self {
Variants::VariantA => serializer.serialize_unit_variant("Variants", 0, "VariantA"),
Variants::VariantB { ref k, ref p } => {
let mut state =
serializer.serialize_struct_variant("Variants", 1, "VariantB", 2)?;
state.serialize_field("k", k)?;
state.serialize_field("p", p)?;
state.end()
}
}
}
}
fn main() {
let x = Variants::VariantB { k: 5, p: 5.0 };
let toml_str = toml::to_string(&x).unwrap();
println!("{}", toml_str);
}
The code compiles, but when I run it it fails:
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: UnsupportedType', src/libcore/result.rs:999:5
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
I figured the issue must be in my use of the API, so I consulted the API documentation for StructVariant and it looks practically the same as my code. I'm sure I'm missing something, but I don't see it based on the docs and output.

Answer 1

Enabling external tagging for the enum enables Serde to serialize/deserialize it to TOML:
#[derive(Deserialize)]
#[serde(tag = "type")]
enum Variants {
VariantA,
VariantB { k: u32, p: f64 },
}
toml::to_string(&Variants::VariantB { k: 42, p: 13.37 })
serializes to
type = VariantB
k = 42
p = 13.37
This works well in Vecs and HashMaps, too.

Answer 2

The TOML format does not support enums with values:
use serde::Serialize; // 1.0.99
use toml; // 0.5.3
#[derive(Serialize)]
enum A {
B(i32),
}
fn main() {
match toml::to_string(&A::B(42)) {
Ok(s) => println!("{}", s),
Err(e) => eprintln!("Error: {}", e),
}
}
Error: unsupported Rust type
It's unclear what you'd like your data structure to map to as TOML. Using JSON works just fine:
use serde::Serialize; // 1.0.99
use serde_json; // 1.0.40
#[derive(Serialize)]
enum Variants {
VariantA,
VariantB { k: u32, p: f64 },
}
fn main() {
match serde_json::to_string(&Variants::VariantB { k: 42, p: 42.42 }) {
Ok(s) => println!("{}", s),
Err(e) => eprintln!("Error: {}", e),
}
}
{"VariantB":{"k":42,"p":42.42}}