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

On Wed, Sep 09, 2026 at 02:59:13PM +0200, Miguel Ojeda wrote:

On Wed, Sep 9, 2026 at 12:22 PM Björn Baron [off-list ref] wrote:

quoted

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.

Yup, I am encouraged by 'until fairly recently' on that ;) so maybe just
bump version requirement to whatever this recently was?

quoted

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

--
Cheers, Lorenzo