rubycc builds Ruby C extensions on a machine that has no gcc, no binutils, no make and no shell. It is a C11-subset compiler, an assembler-free ELF writer, a linker, an ar, a make, a pkg-config shim and a preprocessor, written in Ruby, plus the libc headers a build needs -- so a distroless image with no libc development package still compiles ruby.h. Targets x86-64 and AArch64 Linux (ELF64) against glibc and musl. Generated code is unoptimized: the point is that the extension builds and its own test suite passes, which is the level at which every gem recorded in data/verified_gems.json was verified. See the README for the verified gems, the measured limits, and what is out of scope.