The following code:
use serde::{Deserialize, Serialize};
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
#[serde(tag = "type")]
enum Message {
Request {
id: u32,
resource: String,
operation: String,
},
Response {
id: u32,
value: String,
},
}
fn main() {
let value = Message::Response {
id: 60069,
value: "Foobar".into(),
};
let serialized = ron::to_string(&value).unwrap();
println!("Serialized:");
println!("{serialized}");
println!();
let deserialized: Message = ron::from_str(&serialized).unwrap();
println!("Deserialized:");
println!("{deserialized:#?}");
}with the following dependencies:
[dependencies]
ron = "0.10.1"
serde = { version = "1.0.219", features = ["derive"] }succeeds when run on Rust 1.89, but when rustc 1.91.0-nightly (425a9c0a0 2025-08-17) — the first version on which I have observed this problem — is used instead, the program fails with:
Serialized:
(type:"Response",id:60069,value:"Foobar")
thread 'main' (5637368) panicked at src/main.rs:35:60:
called `Result::unwrap()` on an `Err` value: SpannedError { code: ExpectedIdentifier, position: Position { line: 1, col: 7 } }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
The following code:
with the following dependencies:
succeeds when run on Rust 1.89, but when
rustc 1.91.0-nightly (425a9c0a0 2025-08-17)— the first version on which I have observed this problem — is used instead, the program fails with: