Improve compile times

[dignifiedquire]

I don't know when, but at some point in the last months our compile times have gone really bad. I don't have any real data, except for my impressions but things got pretty bad.

Where to start investigating

• different rust versions, could be a regression in nightlies or sth like that
• git bisect our code basis figuring out what changed when

From there we should have a clearer understanding on where the issues are and what we can do to improve things.

[dignifiedquire]

There is definitely some slow down coming from the compiler :(

commit: 62b43ca768bec267dc26015d70ae82cb7f663f53
command: cargo +stable build --all
rustc: rustc 1.32.0 (9fda7c223 2019-01-16)
time: 358.87 user - 28.36 system - 1:06.83 elapsed

commit: 62b43ca768bec267dc26015d70ae82cb7f663f53
command: cargo +nightly-2018-12-06 build --all
rustc: rustc 1.32.0-nightly (14997d56a 2018-12-05)
time: 365.17 user - 28.90 system - 1:10.30 elapsed

commit: 62b43ca768bec267dc26015d70ae82cb7f663f53
command: cargo +nightly-2019-02-24 build --all
rustc: rustc 1.34.0-nightly (aadbc459b 2019-02-23)
time: 401.57 user - 28.23 system - 1:13.43 - elapsed

commit: 62b43ca768bec267dc26015d70ae82cb7f663f53
command: cargo build --all
rustc: rustc 1.34.0-nightly (02c4c2892 2019-02-26)
time: 423.16 user - 30.80 system - 1:21.02 elapsed

[schomatis]

ouch, could we go back a few version? (or do we depend on 1.34?)

[dignifiedquire]

We don't (yet), but I reached out to the rust compiler team, and will follow up here with anything that I can find out. (Note master is still on the old 1.32 one, so everyone is fine, except my PR in #517)

[dignifiedquire]

Would be good to get some more details on which parts of the code are slow to compile and see if we can cfg somethings out by putting behind features or split into smaller crates.