This is a direct replacement for now removed `#![feature(custom_attribute)]` (https://github.com/rust-lang/rust/issues/29642), except it doesn't rely on implicit fallback from unresolved attributes to custom attributes (which was always hacky and is the primary reason for the removal of `custom_attribute`) and requires registering the attribute explicitly. ```rust #![register_attr(my_attr)] #[my_attr] // OK fn main() {} ``` It's not yet clear whether this should go through stabilization or not. It's quite possible that all the uses should migrate to `#![register_tool]` (https://github.com/rust-lang/rust/issues/66079) instead.