I tried to do this recently and it failed. Some things to remember:

• Search for and replace references to run-make-fulldeps
• Update path to ../../run-make-fulldeps/tools.mk from just ../tools.mk
• bootstrap has an explicit reference to fulldeps in test.rs to build RustDemangler and set --rust-demangler-path, which should be changed to support the move to run-make without fulldeps

I forgot about that last one and its possible that was the only issue.

If not, check bootstrap for other fulldeps dependencies, and note that the error I thought I saw had to do with llvm-profdata or llvm-cov. If so, those may require additional special handling in bootstrap or elsewhere.