Also, is there a list of attributes used in the standard library, much like the one in The Swift Programming Language book?
@_transparent is an underscored (stdlib-private) attribute. You shouldn’t use it in your own libraries or applications (given the sub-forum, you probably know this - just making it clear for anyone else).
But FWIW, it is documented: https://github.com/apple/swift/blob/master/docs/TransparentAttr.md
Other attributes are documented in the standard library programmer’s manual (but not all of them - for instance, it doesn’t appear that
@_alwaysEmitInToClient is documented there): https://github.com/apple/swift/blob/master/docs/StandardLibraryProgrammersManual.md#annotations
@_transparent is an extremely strong form of
@inlinable that ensures the call’s stack frame isn’t even preserved in the debug info. For example, if you try to set a breakpoint in
Int.+(_:_:), you’ll probably never see it hit in your program; this is because the operator is marked
Like all underscored attributes,
@_transparent is not intended for ordinary language users and its meaning could change if we happen to need something different from it.