54998d1b3f
rustc_plugin: Remove `Registry::register_attribute` Legacy plugins cannot register inert attributes anymore. The preferred replacement is to use `register_tool` ([tracking issue](https://github.com/rust-lang/rust/issues/66079)). ```rust #![register_tool(servo)] #[servo::must_root] struct S; ``` The more direct replacement is `register_attribute` ([tracking issue](https://github.com/rust-lang/rust/issues/66080)) ```rust #![register_attr(must_root)] #[must_root] struct S; ``` , but it requires registering each attribute individually rather than registering the tool once, and is more likely to be removed rather than stabilized.