Once for each stage, yes. Otherwise mix complains about missing dependencies.
idi527
wolf4earth
I can confirm that caching deps in addition to _build after running mix compile does result in no more recompilations. Lead to nice speed boost in our CircleCI builds.
Is this something which should be brought up with the rebar team? I think it’s counter intuitive that rebar places in deps on recompilation.
0
tristan
Rebar3 Core Team
We were made aware of this recently and got it fixed. Didn’t realize all compilation wasn’t done to deps by mix 
The rebar3 version installed by mix should now be 3.13.1 which supports an additional argument of where to send output.
I’m not sure what version of mix uses the new output option… Will go look in a bit.
2
tristan
Rebar3 Core Team
I’m not seeing a patch in elixir repo actually, so this may not be supported yet, but it is supported in rebar3.
0
tristan
Rebar3 Core Team
Verified this is not yet supported in mix and likely won’t get attention until the end of May.
So if someone has time and wants to patch mix before then it’d be a great help. The change is to add -o <outputdir> where outputdir is the path to the dep under _build/ to the rebar3 bare compile command mix runs, if rebar3 version is >= 3.13.1 (or error if it fails with “invalid option” and tell the user to upgrade rebar3 I guess).
2