rkyv 0.8.16

Zero-copy deserialization framework for Rust
Documentation 
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use std::{collections::HashMap, fmt};

use rkyv::{access, rancor::Error, Archive, Deserialize, Serialize};

#[derive(Archive, Debug, Deserialize, Serialize)]
// We have a recursive type, which requires some special handling
//
// First the compiler will return an error:
//
// > error[E0275]: overflow evaluating the requirement `HashMap<String,
// > JsonValue>: Archive`
//
// This is because the implementation of Archive for Json value requires that
// JsonValue: Archive, which is recursive!
// We can fix this by adding #[omit_bounds] on the recursive fields. This will
// prevent the derive from automatically adding a `HashMap<String, JsonValue>:
// Archive` bound on the generated impl.
//
// Next, the compiler will return these errors:
//
// > error[E0277]: the trait bound `__S: ScratchSpace` is not satisfied
// > error[E0277]: the trait bound `__S: Serializer` is not satisfied
//
// This is because those bounds are required by HashMap and Vec, but we removed
// the default generated bounds to prevent a recursive impl.
// We can fix this by manually specifying the bounds required by HashMap and Vec
// in an attribute, and then everything will compile:
#[rkyv(serialize_bounds(
    __S: rkyv::ser::Writer + rkyv::ser::Allocator,
    __S::Error: rkyv::rancor::Source,
))]
#[rkyv(deserialize_bounds(__D::Error: rkyv::rancor::Source))]
// We need to manually add the appropriate non-recursive bounds to our
// `CheckBytes` derive. In our case, we need to bound
// `__C: rkyv::validation::ArchiveContext`. This will make sure that our `Vec`
// and `HashMap` have the `ArchiveContext` trait implemented on the validator.
// This is a necessary requirement for containers to check their bytes.
//
// With those two changes, our recursive type can be validated with `access`!
#[rkyv(bytecheck(
    bounds(
        __C: rkyv::validation::ArchiveContext,
    )
))]
pub enum JsonValue {
    Null,
    Bool(bool),
    Number(JsonNumber),
    String(String),
    Array(#[rkyv(omit_bounds)] Vec<JsonValue>),
    Object(#[rkyv(omit_bounds)] HashMap<String, JsonValue>),
}

impl fmt::Display for ArchivedJsonValue {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Null => write!(f, "null")?,
            Self::Bool(b) => write!(f, "{}", b)?,
            Self::Number(n) => write!(f, "{}", n)?,
            Self::String(s) => write!(f, "{}", s)?,
            Self::Array(a) => {
                write!(f, "[")?;
                for (i, value) in a.iter().enumerate() {
                    write!(f, "{}", value)?;
                    if i < a.len() - 1 {
                        write!(f, ", ")?;
                    }
                }
                write!(f, "]")?;
            }
            Self::Object(h) => {
                write!(f, "{{")?;
                for (i, (key, value)) in h.iter().enumerate() {
                    write!(f, "\"{}\": {}", key, value)?;
                    if i < h.len() - 1 {
                        write!(f, ", ")?;
                    }
                }
                write!(f, "}}")?;
            }
        }
        Ok(())
    }
}

#[derive(Archive, Debug, Deserialize, Serialize)]
pub enum JsonNumber {
    PosInt(u64),
    NegInt(i64),
    Float(f64),
}

impl fmt::Display for ArchivedJsonNumber {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::PosInt(n) => write!(f, "{}", n),
            Self::NegInt(n) => write!(f, "{}", n),
            Self::Float(n) => write!(f, "{}", n),
        }
    }
}

fn main() {
    let mut hash_map = HashMap::new();
    hash_map.insert("name".into(), JsonValue::String("ferris".into()));
    hash_map.insert("age".into(), JsonValue::Number(JsonNumber::PosInt(10)));
    hash_map.insert("is_crab".into(), JsonValue::Bool(true));
    hash_map.insert("project".into(), JsonValue::Null);
    let value = JsonValue::Object(hash_map);

    let buf = rkyv::to_bytes::<Error>(&value).unwrap();
    let archived_value = access::<ArchivedJsonValue, Error>(&buf).unwrap();

    println!("{}", archived_value);
}