Failing build: https://hydra.nixos.org/build/134175791
ChangeLog: https://github.com/racer-rust/racer/blob/v2.1.40/CHANGELOG.md
A few more things are worth noting:
* It's not possible to update to latest version (2.1.42) at the time of
committing since this requires a newer `rustc` (1.51 to be precise) to
compile.
* For proper completion, `rustLibSrc` rather than `rustcSrc` must be
used now. The two were separated here previously[1].
* Dropped the `checkPhase` and replaced it with a list of `checkFlags`.
This has the benefit that the default `checkPhase` from
`buildRustPackage` can be used which properly configures parallelism
and which target to use (i.e. `release` in this case which reduces
build time).
[1] 68060f6f6f
61b8a2c720