Hi @zetanumbers,
We've been hard at work on the new compiler written in zig (see src folder) so we have not upgraded the old compiler written in rust. I suspect it will take much more changes before you can get all of CI to pass. If you want to make those changes I will review them but we do not see substantial value in a rust upgrade of the old compiler. Sorry to disappoint!

After some testing I can say a large portion (16.05s -> 8.13s) of build time reduction was caused by rust switching to lld by default on x86_64 linux (rust-lang/rust#140525).

Oh yeah, we were using lld in some places but not everywhere I think. I would still prefer to avoid making the switch now, the old compiler had several flaky issues and changing things up here could make them worse

You should delete the old compiler code if it's not maintained.

The new compiler is still missing a lot of features, so we keep the old one around. We also still fix docs issues and other issues that are really bothering people. I just don't want to risk making the compiler or CI more flaky for a speedup that would mainly benefit compiler developers.

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I am gonna close this for now. Although I am still interested in old rust compiler as a case study of suboptimal incremental compilation performance on the rustc side.