Separate compilation is one solution to the problem of slow compilation.

Binary patching is another one. It feels a bit messy and I am sceptical that it can be maintained assuming it works at all.

I think a much better approach would be too make the compilers faster. Why does compiling 1M LOC take more than 1s in unoptimized mode for any language? My guess is part of blame lies with bloated backends and meta programming (including compile time evaluation, templates, etc.)