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## Motivation I'm trying to implement a `Layer` that has something very similar to tracing-error's [`WithContext`](66cd79f72a/tracing-error/src/layer.rs (L32)) to support erasing types and getting access to the current span's state. To do that, I implement `Layer::downcast_raw` in [the same way that tracing-error does](66cd79f72a/tracing-error/src/layer.rs (L55-L63)). This works great when the layer is not filtered. However, once I filter the layer ```rust let filter = tracing_subscriber::filter::Targets::new().with_default(tracing::Level::INFO); let layer = MyLayer::new(); tracing_subscriber::registry().with(layer.with_filter(filter)).init(); ``` I'm not able to get a `WithContext` instance anymore, because `Filtered` [handles `downcast_raw`, and doesn't forward it](66cd79f72a/tracing-subscriber/src/filter/layer_filters.rs (L379-L391)) to `MyLayer::downcast_raw`. ## Solution If `Filtered::downcast_raw` does not know how to handle the given type, forward it to the wrapped layer's `Layer::downcast_raw` implementation. Fixes #1618