rust non-determinism

[Bernhard M. Wiedemann]

Hello fellow R-B-ings,

In previous months I had seen several rust-builds that were 
non-deterministic in similar ways.

Via https://github.com/pop-os/launcher/issues/230
I finally found one big clue that points to rust's default of
  codegen-units = 16 as the cause.

I'll advocate [1] to add to our (openSUSE) build envs:
export RUSTFLAGS="-C codegen-units=1"
that helps for many of these cases.

In my measurements, it only needs 5-10% longer to compile on a 4-core-VM 
and some builds are even faster on 1-core VMs.
Plus the docs say, it optimizes resulting binaries better, which makes 
it a worthy trade-off.


Ciao
Bernhard M.

[1] https://github.com/openSUSE/post-build-checks/pull/65
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