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Exposing a raw pointer in the trait API means drivers that want to "reuse input buffer" need to write unsafe code right? unsafe { *out_len = ... }; I believe the unsafe code should be confined to the abstraction layer instead, not in the driver. How about using using an enum return type instead to properly wrap inplace versus driver allocated outputs? pub enum RpcOutput { Allocated(KVec), InPlace(usize), } fn fw_rpc( uctx: &mut UserCtx, scope: u32, rpc_in: &mut [u8], ) -> Result; Then the abstraction handles the pointer write: match T::fw_rpc(ctx, scope, rpc_in_slice) { Ok(RpcOutput::Allocated(kvec)) => { ... } Ok(RpcOutput::InPlace(len)) => { unsafe { *out_len = len }; ... } } fw_rpc() as a bonus also gets 1 less function parameter and cleaner return signature. -- Joel Fernandes