This is a hack, but I don't think we can do much better as long as `derive` is
running at the syntax expansion phase.
If the custom_derive feature gate is enabled, this works with user-defined
traits and syntax extensions. Without the gate, you can't use e.g. #[derive_Clone]
directly, so this does not change the stable language.
This commit also cleans up the deriving code somewhat, and forbids some
previously-meaningless attribute syntax. For this reason it's technically a
[breaking-change]
This leaves the `Share` trait at `std::kinds` via a `#[deprecated]` `pub use`
statement, but the `NoShare` struct is no longer part of `std::kinds::marker`
due to #12660 (the build cannot bootstrap otherwise).
All code referencing the `Share` trait should now reference the `Sync` trait,
and all code referencing the `NoShare` type should now reference the `NoSync`
type. The functionality and meaning of this trait have not changed, only the
naming.
Closes#16281
[breaking-change]
