It builds Swift without any standard library, runtime or linkage to other components. This isn't normally possible with the Swift compiler.

The solution doesn't involve changing the compiler or building everything into the executable. Instead, this is a makefile coordinated pipeline (front-end swiftc, back-end clang) plus a start.s and a system.c file that omit any need for typical standard library components in the first place.

i.e. a "bare metal" compilation