Handwritten generic type bounds

When deriving Serialize and Deserialize implementations for structs with generic type parameters, most of the time Serde is able to infer the correct trait bounds without help from the programmer. It uses several heuristics to guess the right bound, but most importantly it puts a bound of T: Serialize on every type parameter T that is part of a serialized field and a bound of T: Deserialize on every type parameter T that is part of a deserialized field. As with most heuristics, this is not always right and Serde provides an escape hatch to replace the automatically generated bound by one written by the programmer.

use serde::{de, Deserialize, Deserializer};

use std::fmt::Display;
use std::str::FromStr;

#[derive(Deserialize, Debug)]
struct Outer<'a, S, T: 'a + ?Sized> {
    // When deriving the Deserialize impl, Serde would want to generate a bound
    // `S: Deserialize` on the type of this field. But we are going to use the
    // type's `FromStr` impl instead of its `Deserialize` impl by going through
    // `deserialize_from_str`, so we override the automatically generated bound
    // by the one required for `deserialize_from_str`.
    #[serde(deserialize_with = "deserialize_from_str")]
    #[serde(bound(deserialize = "S: FromStr, S::Err: Display"))]
    s: S,

    // Here Serde would want to generate a bound `T: Deserialize`. That is a
    // stricter condition than is necessary. In fact, the `main` function below
    // uses T=str which does not implement Deserialize. We override the
    // automatically generated bound by a looser one.
    #[serde(bound(deserialize = "Ptr<'a, T>: Deserialize<'de>"))]
    ptr: Ptr<'a, T>,

/// Deserialize a type `S` by deserializing a string, then using the `FromStr`
/// impl of `S` to create the result. The generic type `S` is not required to
/// implement `Deserialize`.
fn deserialize_from_str<'de, S, D>(deserializer: D) -> Result<S, D::Error>
    S: FromStr,
    S::Err: Display,
    D: Deserializer<'de>,
    let s: String = Deserialize::deserialize(deserializer)?;

/// A pointer to `T` which may or may not own the data. When deserializing we
/// always want to produce owned data.
enum Ptr<'a, T: 'a + ?Sized> {
    Ref(&'a T),

impl<'de, 'a, T: 'a + ?Sized> Deserialize<'de> for Ptr<'a, T>
    Box<T>: Deserialize<'de>,
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        D: Deserializer<'de>,

fn main() {
    let j = r#"
            "s": "1234567890",
            "ptr": "owned"

    let result: Outer<u64, str> = serde_json::from_str(j).unwrap();

    // result = Outer { s: 1234567890, ptr: Owned("owned") }
    println!("result = {:?}", result);