ac7beab527
mbe: Implement `unsafe` attribute rules This implements `unsafe attr` rules for declarative `macro_rules!` attributes, as specified in [RFC 3697](https://github.com/rust-lang/rfcs/pull/3697). An invocation of an attribute that uses an `unsafe attr` rule requires the `unsafe(attr(...))` syntax. An invocation of an attribute that uses an ordinary `attr` rule must *not* use the `unsafe(attr(...))` syntax. `unsafe` is only supported on an `attr` rule, not any other kind of `macro_rules!` rule. Tracking issue for `macro_rules!` attributes: https://github.com/rust-lang/rust/issues/143547