Cross-posting here from the Apple Developer Forums.
I've built ("utils/build-toolchain toolchain.name") and activated a custom toolchain in Xcode. Build logs confirm that the custom swift compiler is being invoked when using default Xcode configuration. However, it doesn't work with the new (preview) build system. Build logs still show the correct "export TOOLCHAINS=..." line, but then it invokes "swift" binary from "XcodeDefault.xctoolchain".
Curiously, Xcode's new build system does pick up and use official release toolchain from swift.org and a custom one from the "test batch mode" thread. I wonder what makes it so different from mine. Is it code signature?