feature serialization: #[serde(flatten)] does not work for different types

[jfrimmel]

Hello,
I encountered an unexpected behavior when using #[serde(flatten)] with this crate. See the example below.

Reproduction

Cargo.toml:

[package]
name = "valgrind-xml"
version = "0.0.0"
edition = "2018"

[dependencies]
quick-xml = { version="0.22", features=["serialize"] }
serde = { version="1", features=["derive"] }
serde_json = "1"

src/main.rs:

use serde::Deserialize;

#[derive(Debug, Deserialize)]
struct S {
    name: String,
    #[serde(flatten)]
    resource: Resource,
}

#[derive(Debug, Deserialize)]
struct Resource {
    a: i32,
    b: u64,
}

// should have the same behavior as `S`, but everything is inlined
#[derive(Debug, Deserialize)]
struct S2 {
    name: String,
    a: i32,
    b: u64,    
}

fn main() {
    let r: Result<S, _> = quick_xml::de::from_str(r#"<root name="omg" a="-1" b="42"/>"#); // doesn't work
    dbg!(r);
    let s: S2 = quick_xml::de::from_str(r#"<root name="omg" a="-1" b="42"/>"#).unwrap(); // works
    dbg!(s);

    // serde_json as reference:
    let s: S = serde_json::de::from_str(r#"{ "name": "omg", "a": -1, "b": 42 }"#).unwrap(); // works
    dbg!(s);
    let s: S2 = serde_json::de::from_str(r#"{ "name": "omg", "a": -1, "b": 42 }"#).unwrap(); // works
    dbg!(s);
}

Output of cargo run:

[src/main.rs:25] r = Err(
    Custom(
        "invalid type: map, expected i32",
    ),
)
[src/main.rs:27] s = S2 {
    name: "omg",
    a: -1,
    b: 42,
}
[src/main.rs:29] s = S {
    name: "omg",
    resource: Resource {
        a: -1,
        b: 42,
    },
}
[src/main.rs:31] s = S2 {
    name: "omg",
    a: -1,
    b: 42,
}

[Mingun]

#[serde(flatten)] is very dependent on the ability of the deserializer to report correct types, but XML deserializer has only two types under the hood: Map and String.

This is impossible to fix from the deserializer side, the fix of serde-rs/serde#1183 is required.

For now avoid using #[serde(flatten)] for types that you want to deserialize from XML.