rls no longer builds after rust-lang/rust#62507

[rust-highfive]

Hello, this is your friendly neighborhood mergebot.
After merging PR #62507, I observed that the tool rls no longer builds.
A follow-up PR to the repository https://github.com/rust-lang/rls is needed to fix the fallout.

cc @petrochenkov, do you think you would have time to do the follow-up work?
If so, that would be great!

cc @alexcrichton, the PR reviewer, and @rust-lang/compiler -- nominating for prioritization.

[phil-opp]

This affects rustfmt too: https://rust-lang.github.io/rustup-components-history/

[Xanewok]

Happened because of

2019-08-01T13:23:36.8982444Z �[0m�[0m�[1m�[32m   Compiling�[0m rustc-ap-rustc_data_structures v542.0.0
2019-08-01T13:23:37.0454949Z �[0m�[1m�[38;5;9merror[E0658]�[0m�[0m�[1m: use of unstable library feature 'test': `bench` is a part of custom test frameworks which are unstable�[0m
2019-08-01T13:23:37.0456824Z �[0m    �[0m�[0m�[1m�[38;5;12m--> �[0m�[0m/cargo/registry/src/github.com-1ecc6299db9ec823/rustc-ap-rustc_data_structures-542.0.0/bit_set.rs:1269:3�[0m

in https://dev.azure.com/rust-lang/rust/_build/results?buildId=4891.

IIUC this will need a coordinated upgrade to rustc-ap-* once again
@topecongiro @kngwyu

blame