Deprecate `#![plugin]` & `#[plugin_registrar]`
This PR deprecates `#![plugin]` and `#[plugin_registrar]`.
~A removal deadline is set: 1.44.0. This will be in 9 months from now and should give everyone who is still relying on the feature ample time to rid themselves of this dependency.~
cc https://github.com/rust-lang/rust/issues/29597
r? @Mark-Simulacrum
The syntax crate contains those things concerned purely with syntax
– that is, the AST ("abstract syntax tree"), parser, pretty-printer,
lexer, macro expander, and utilities for traversing ASTs.
For more information about how these things work in rustc, see the
rustc guide: