Field aliases do not work in combination with flatten

[alex-berger]

Field aliases as introduced in #1458 do not work when deserializing "flattened" structures.

Expected behavior

Field aliases should work when deserializing "flattened" structures.

Observed behavior

Field aliases do not work when deserializing "flattened" structures.

How to reproduce

The following code can be used to reproduce the problem, also available as Rust Playground snippet here.

use serde::{Deserialize, Serialize};
use serde_json;

#[derive(Serialize, Deserialize)]
pub struct A {
    #[serde(rename = "y", alias = "z")]
    x: i64,
}

#[derive(Serialize, Deserialize)]
pub struct B {
    #[serde(flatten)]
    a: A,
}

fn main() -> Result<(),Box<std::error::Error>> {
     let i = r#"{ "z": 12 }"#; // using r#"{ "y": 12 }"# will work.
     let b: B = serde_json::from_str(i)?;
     assert_eq!(b.a.x,12);
     Ok(())
}

[emilio]

I just got bitten by this too.

[dtolnay]

This is a bug -- I would accept a PR to fix this.

[emilio]

I'll try to give it a shot, but first time poking at serde_derive, so no promises :)

[Jezza]

Yeah, I just got bit by this too.
I was using variants, so I thought it was related to that.
I can take a try at fixing this if there's been no progress.

use serde::Deserialize;
use serde_json::from_str;

#[derive(Debug, Deserialize)]
struct Value {
	#[serde(flatten)]
	data: Ser,
}

#[derive(Debug, Deserialize)]
enum Ser {
	#[serde(rename = "var", alias = "def")]
	Variant(u8)
}

fn main() {
	let input = r#"{ "def": 0 }"#;
	println!("{:?}", from_str::<Ser>(input).unwrap());
	println!("{:?}", from_str::<Value>(input).unwrap());
}

[Jezza]

After hunting around, I found the issue.

We pass an array of valid variants directly into the deserialize_enum.
The aliases aren't counted as variants, and so aren't added to the list.
We could add the aliases to the generated array, but I don't know if that might break other things.

I'm open to ideas.

EDIT: I forgot that structs have this issue as well.
We pass in an array of field names (non-aliased).

We could add the aliased names to the arrays, and then internally, when it actually goes to deserialise it, sort out which one to use, etc, like normal.