Flatten + deserialize_with generates incorrect Deserialize code

[dtolnay]

The following flattened field needs to invoke our deserialize_with function, not Point::deserialize.

#[macro_use]
extern crate serde_derive;

extern crate serde;

use serde::Deserializer;

#[derive(Deserialize)]
struct Location {
    #[serde(flatten, deserialize_with = "de")]
    point: Point,
}

struct Point;

fn de<'de, D>(_deserializer: D) -> Result<Point, D::Error>
where
    D: Deserializer<'de>,
{
    unimplemented!()
}

error[E0277]: the trait bound `Point: serde::Deserialize<'_>` is not satisfied
 --> src/main.rs:8:10
  |
8 | #[derive(Deserialize)]
  |          ^^^^^^^^^^^ the trait `serde::Deserialize<'_>` is not implemented for `Point`
  |
  = note: required by `serde::Deserialize::deserialize`

Sub-issue of #1239.

[dtolnay]

Fixed in cb2b92f.

[Stargateur]

Seem related, playground.

use std::collections::HashMap;
use std::fmt;
use std::str::FromStr;

use serde; // 1.0.85
use serde::de::{self, Deserialize, MapAccess, Visitor}; // 1.0.85
use serde::Deserializer;
use serde_derive::Deserialize; // 1.0.85
use toml; // 0.4.10
use void::Void; // 1.0.2

#[derive(Debug)]
struct Obj {
    x: isize,
    y: String,
}

struct ObjVisitor;

#[derive(Debug, Deserialize)]
struct ObjAux {
    x: isize,
    y: String,
}

impl<'de> Visitor<'de> for ObjVisitor {
    type Value = Obj;

    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        formatter.write_str("string or map")
    }

    fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
    where
        E: de::Error,
    {
        Ok(FromStr::from_str(value).unwrap())
    }

    fn visit_map<M>(self, visitor: M) -> Result<Self::Value, M::Error>
    where
        M: MapAccess<'de>,
    {
        let aux: ObjAux = Deserialize::deserialize(de::value::MapAccessDeserializer::new(visitor))?;
        Ok(Obj { x: aux.x, y: aux.y })
    }
}

impl<'de> Deserialize<'de> for Obj {
    fn deserialize<D>(deserializer: D) -> Result<Obj, D::Error>
    where
        D: Deserializer<'de>,
    {
        deserializer.deserialize_any(ObjVisitor)
    }
}

impl FromStr for Obj {
    type Err = Void;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        Ok(Obj {
            x: 0,
            y: s.to_owned(),
        })
    }
}

#[derive(Debug, Deserialize)]
struct Simple {
    obj: Obj,
}

#[derive(Debug, Deserialize)]
struct InsideHashMap {
    objs: HashMap<String, Obj>,
}

fn main() {
    // Basic deserialization of Obj
    let toml = r#"
        x = 5
        y = "hello"
    "#;
    let obj: Obj = toml::from_str(toml).unwrap();
    println!("{:?}", obj);

    // Basic deserialization of Obj as a field in a struct
    let toml = r#"
        [obj]
        x = 5
        y = "hello"
    "#;
    let simple: Simple = toml::from_str(toml).unwrap();
    println!("{:?}", simple);

    // Basic deserialization of Obj as a field in a struct as a string or struct
    let toml = r#"
        obj = "hello"
    "#;
    let simple: Simple = toml::from_str(toml).unwrap();
    println!("{:?}", simple);

    // Deserialization of an Obj inside a HashMap
    let toml = r#"
        [objs]
        a = { x = 5, y = "hello" }
    "#;
    let working: InsideHashMap = toml::from_str(toml).unwrap();
    println!("{:?}", working);

    // Deserialization of Obj inside a HashMap field as a string or struct
    let toml = r#"
        [objs]
        a = "hello"
    "#;
    let not_working: InsideHashMap = toml::from_str(toml).unwrap();
    println!("{:?}", not_working);
}

Look like deserialise_with is not used in this case too.

Stackoverflow question