There are some private attributes like
@_exported etc, that are considered unstable (i. e. they may go away any time), but nonetheless are quite important in some use cases (like the CryptoSwift library, which, according to its maintainer, is really slow without all those private attributes).
At the same time, it is unclear whether those attributes will ever make it through Swift Evolution, as their use cases are too narrow to be worth a proposal. Also, there's this "I want to write cool things in Swift right now, not wait 5 years until a language feature is maybe de-underscored" factor.
Those library authors that make use of those attributes are unintentionally putting their users in danger: any time their app may stop compiling after upgrading to a new Xcode version just because some dependency used those underscored attributes, and some of them went away in that new Xcode version.
Can the core team consider making unknown attributes a warning rather than an error? In this case, if, say,
@inline(__always) is removed, the libraries that make use of it won't break. In other words, can we adopt the same behavior as C++11 attributes?