Since there was interest to stabilize cfg_target_feature/target_feature to allow other RFCs to advance, I wrote a pre-RFC to fix their semantics on stable and nightly Rust before stabilization:
Since target feature is required for the SIMD RFCs, it would be great if the pre-RFC could get some eyes from the interested parties to make sure that the proposed semantics don't hinder any SIMD-related work.