We present Crust, a self-hosted compiler that accepts C, a subset of C++, a subset of Rust and a subset of rpython in a single translation unit, together with its own assembler, linker, freestanding runtime and operating system. After bootstrap the chain depends on no external toolchain: not GCC, not LLVM, not GNU as or ld, and not a host libc. The system is deliberately kept small enough that its entire call graph fits in the context window of a large language model, and we argue this constraint is what makes a system auditable in an era when the reader is increasingly a machine. As a worked example we take the removal of a layer usually treated as part of the machine — the return-address stack — and, with archived and reproducible measurements [20], show both a two-order-of-magnitude pitfall in the naive implementation and a regime in which abandoning the stack is faster than the hardware’s own return predictor. The point of the example is not the speed but the epistemics: the pitfall, the fix, and the regime were each found, made, and checked inside a toolchain small enough to read whole.