We haven't modified the compiler but instead plugged into it an external instrumentation tool using the compiler option `-toolexec`. With this option, the comp

We haven't modified the compiler but instead plugged into it an external instrumentation tool using the compiler option `-toolexec`. With this option, the compiler invokes every toolchain binary (compile, asm, link, etc.) through the provided program. So you can basically write a proxy program intercepting calls to `compile` to do source-code instrumentation, exactly like you would with go generate, but now automatically done during compilation on every package.