In Rayon's parallel iterator implementation, with_producer is the mechanism used to convert a parallel iterator into a Producer. This is essential when reaching the start of an iterator chain or when coordinating multiple inputs (e.g., in zip()).
Because the Producer type often contains lifetimes that are local to the with_producer call, Rayon cannot use standard Rust closures (FnOnce) to implement this. Instead, it uses a dedicated callback trait called ProducerCallback.
Key Concepts:
with_producer: A method on IndexedParallelIterator that initiates the conversion to a producer via a callback.ProducerCallback: A trait used instead of closures to allow the with_producer signature to remain generic over the producer type without needing to name it explicitly in the trait definition. This bypasses lifetime issues associated with associated types.- The Pattern: To implement a combinator (like
map), you must create a wrapper Producer (e.g., MapProducer) and a manual Callback struct that implements ProducerCallback to wrap the base producer with your new logic.
// The conceptual pattern for implementing a parallel iterator combinator
impl<I, F> IndexedParallelIterator for Map<I, F>
where I: IndexedParallelIterator,
F: MapOp<I::Item>,
{
fn with_producer<CB>(self, callback: CB) -> CB::Output
where CB: ProducerCallback<Self::Item>
{
// 1. Wrap the provided callback and necessary data into a manual Callback struct
self.base.with_producer(Callback { callback: callback, map_op: self.map_op })
struct Callback<CB, F> {
callback: CB,
map_op: F,
}
// 2. Implement ProducerCallback for the manual Callback struct
impl<T, F, CB> ProducerCallback<T> for Callback<CB, F>
where F: MapOp<T>,
CB: ProducerCallback<F::Output>
{
type Output = CB::Output;
fn callback<P>(self, base: P) -> CB::Output
where P: Producer<Item=T>
{
// 3. Wrap the base producer with the new logic (e.g., MapProducer)
let producer = MapProducer { base: base, map_op: &self.map_op };
// 4. Pass the wrapped producer to the original callback
self.callback.callback(producer)
}
}
}
}
