Iterating over a `CxxSequence` that is not a `CxxRandomAccessCollection` triggers a copy of the C++ collection. Let's disable the automatic conformances until we find a more efficient solution.
This means that for now developers won't be able to iterate over a `std::set` or `std::list` with a Swift for-in loop. I will submit a separate patch with an alternative solution for such types.
C++ random access collections, such as `std::vector` or `std::string`, are not affected.
This lifts the requirement for the user to explicitly add `import Cxx` in Swift code that relies on protocols from the `Cxx` module, such as `CxxSequence`.
This makes ClangImporter automatically conform C++ sequence types to `Cxx.CxxSequence` protocol.
We consider a C++ type to be a sequence type if it defines `begin()` & `end()` methods that return iterators of the same type which conforms to `UnsafeCxxInputIterator`.
This teaches ClangImporter to synthesize conformances of C++ iterator types to `UnsafeCxxInputIterator` protocol from the `Cxx` module.
We consider a C++ type to be an iterator if it defines a subtype (usually a typedef or a using decl) called `iterator_category` that inherits from `std::input_iterator_tag`.
rdar://96235368
This allows projects that don't want to pull in the entire C++ standard library to use stdlib-independent C++ interop utilities like `CxxSequence`.
This also makes the utilities available on platforms where we don't currently have the `std` overlay available, e.g. Windows.
This change adds basic helper protocols and structs that are going to be used for making C++ sequences and collection safe and Swifty by adding conformances to `Swift.Sequence`, `Swift.Collection`, etc.
This is not meant to be a final design.
