Speed up compilation by 23%

[Enselic]

Before this commit, serde_derive is built before serde. But serde does not depend on serde_derive, so that is not needed. Instead, build serde and serde_derive in parallel. That speeds up compilation by 23%.

Before:

$ hyperfine --prepare 'cargo clean' 'cargo build'
Benchmark 1: cargo build
Time (mean ± σ): 4.979 s ± 0.044 s [User: 4.887 s, System: 0.714 s]
Range (min … max): 4.915 s … 5.068 s 10 runs

After:

$ hyperfine --prepare 'cargo clean' 'cargo build'
Benchmark 1: cargo build
Time (mean ± σ): 3.821 s ± 0.065 s [User: 5.083 s, System: 0.740 s]
Range (min … max): 3.749 s … 3.955 s 10 runs

[GuillaumeGomez]

Thanks!

[aDotInTheVoid]

This will get blown away the next time update.sh is run. To avoid this, I'm going to submit a corresponding change to rust-lang/rust. EDIT: rust-lang/rust#141989

Also, we should use triagebot to add a comment for all PR's that touch src/ as that's almost always wrong.

[Enselic]

Thanks. For the record, there used to be a version mismatch problem to do this split up, but that was fixed over a year ago.

[aDotInTheVoid]

Published as d7f859a (0.46.1), which also specifies that we need serde to be at least 1.0.186