Rust serde untagged enum fails - rust

Question

I'm tying to deserialize one of two structs using serde. The input comes from a csv file.
use serde::Deserialize;
use std::io;
#[derive(Deserialize)]
struct A {
value: i8,
}
#[derive(Deserialize)]
struct B {
value: String,
}
#[derive(Deserialize)]
#[serde(untagged)]
enum C {
One(A),
Two(B),
}
fn main() {
let mut rdr = csv::Reader::from_reader(io::stdin());
for result in rdr.deserialize() {
let record: Result<C, csv::Error> = result;
match record {
Ok(value) => {
println!("ok");
}
Err(error) => {
println!("Error parsing line: {}", error);
}
}
}
}
If I understand untagged enums correctly, this should try parsing it as an A struct, so just an i8, if that fails, try parsing it as a B struct, so just a String.
I've verified that my structs deserialize correctly on their own.
Here's the command I run:
echo "value\nTest\n1" | cargo r
Here's the output:
Error parsing line: CSV deserialize error: record 1 (line: 2, byte: 6): data did not match any variant of untagged enum C
Error parsing line: CSV deserialize error: record 2 (line: 3, byte: 11): data did not match any variant of untagged enum C

Answer 1

If you switch your model from "enum of structs" to "struct containing enum", the csv input works as expected:
use serde::{Deserialize, Deserializer, Serialize};
#[derive(Deserialize)]
enum Value {
One(i8),
Two(String),
}
#[derive(Deserialize)]
struct Data {
value: Value,
}
fn main() {
let mut rdr = csv::Reader::from_reader(io::stdin());
for result in rdr.deserialize() {
let record: Result<Data, csv::Error> = result;
match record {
Ok(value) => {
println!("ok");
}
Err(error) => {
println!("Error parsing line: {}", error);
}
}
}
}
It seems that the csv crate isn't able to understand that the value field in your struct A and the value field in your struct B should belong to the same column. Since they have the same name, defining as a struct containing an untagged enum makes more sense.