ios/third_party/swift_cxxstdlib/README.md
The src folder contains a partial copy of the source repository since we only need the sources for standard library components that have a dependency on libc++.
The iOS build may use the swift compiler's C++ interop feature. Some swift standard library modules that provide support for C++ interop have dependencies on libc++. The module binaries that are packaged with stable versions of the swift compiler were built using a revision of libc++ that is different from the revision in the Chromium toolchain and are therefore incompatible.
The solution is to use custom builds of these modules that were built using Chromium's revion of libc++.
The use of swift/C++ interop is experimental at this time. It is not used in official builds or on CQ bots.
At this interim stage, we do not yet have a proper mirror set up on chromium.googlesource.com. We're just using a manual copy of files from the upstream repo.