Re: [PATCH 20/23] kbuild: rust: parallelise rustc front end

[Miguel Ojeda]

On Wed, Sep 9, 2026 at 12:22 PM Björn Baron <bjorn3_gh@xxxxxxxxxxxxxx> wrote:
>
> There are still several known reproducibility issues for the parallel
> frontend[1] and at least until fairly recently there were also deadlock
> issues and incorrect errors[2], so I would not recommend enabling it without
> an explicit request from the user. Perhaps even only allow it on nightly
> rustc for now, then you don't need to check which of the two spellings of
> the flag is supported.

Thanks a lot for taking a look Björn.

If there are still major issues even in nightly, then I would say
let's not add it yet -- some users/CIs are testing with nightly
routinely, so it is probably best to avoid surprises for them. (If
someone wants to test the flag, they can still pass it manually.)

On the other hand, if there are no major issues left and it is just
reproducibility issues, then I guess we could add it to help figure
those issues out and avoid surprises later on.

> By the way, currently backend parallelism is entirely disabled due to the
> usage of --emit obj (which forces -Ccodegen-units=1). For -O2/3 builds

Yeah, we even set `-Ccodegen-units=1` explicitly at the moment, so we
would need to change that too (and likely make it an option, because
normally end users will want the best results, while developers may
want speed of iteration).

Cheers,
Miguel