dcontext 0.3.0

Distributed context propagation for Rust — scoped, type-safe, serializable
Documentation 
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use std::any::Any;

use crate::error::ContextError;

/// Type-erased context value. Stored as trait objects in the scope stack.
pub(crate) trait ContextValue: Any + Send + Sync {
    /// Clone into a new boxed trait object.
    fn clone_boxed(&self) -> Box<dyn ContextValue>;
    /// Downcast to &dyn Any.
    fn as_any(&self) -> &dyn Any;
    /// Serialize this value to bytes (bincode).
    /// Returns `LocalOnlyKey` for local-only (non-serializable) values.
    fn serialize_value(&self) -> Result<Vec<u8>, ContextError>;
    /// Whether this value is local-only (excluded from serialization).
    fn is_local(&self) -> bool;
}

impl<T> ContextValue for T
where
    T: Clone + Send + Sync + serde::Serialize + serde::de::DeserializeOwned + 'static,
{
    fn clone_boxed(&self) -> Box<dyn ContextValue> {
        Box::new(self.clone())
    }

    fn as_any(&self) -> &dyn Any {
        self
    }

    fn serialize_value(&self) -> Result<Vec<u8>, ContextError> {
        bincode::serialize(self).map_err(|e| ContextError::SerializationFailed(e.to_string()))
    }

    fn is_local(&self) -> bool {
        false
    }
}

/// Wrapper for local-only values that don't implement Serialize/DeserializeOwned.
/// These are excluded from serialization but propagate via snapshot/attach.
pub(crate) struct LocalValue<T>(pub T);

impl<T> ContextValue for LocalValue<T>
where
    T: Clone + Send + Sync + 'static,
{
    fn clone_boxed(&self) -> Box<dyn ContextValue> {
        Box::new(LocalValue(self.0.clone()))
    }

    fn as_any(&self) -> &dyn Any {
        &self.0
    }

    fn serialize_value(&self) -> Result<Vec<u8>, ContextError> {
        Err(ContextError::LocalOnlyKey(
            std::any::type_name::<T>().to_string(),
        ))
    }

    fn is_local(&self) -> bool {
        true
    }
}