Change 531288 had a related patch set uploaded (by Daniel Kinzler; owner: Daniel Kinzler):
[mediawiki/core@master] WIP: use classes as wiring.

https://gerrit.wikimedia.org/r/531288

From a perf perspective, I've not perceived the status quo as needing better run-time performance. I don't recall seeing the middleware between MediaWikiServices and ServiceWiring.php take up much if any time in flame graphs or debug traces.

Typically the frame for MediaWikiServices::getSomething, through frames ServiceContainer::get, ServiceContainer::createService and {closure:ServiceWiring:#Something} are reported as taking the same amount of time, as if they go directly from one to the other. Eg. in case of getResourceLoader() from yesterday's flame graph I see a factor of ~0.002 difference (e.g. 678 vs 676 samples).

If changing this is preferred for other reasons, we have no objections so long as performance is the same or better. If it is motivated by performance, then I'd recommend measuring the creation time for a few services and determine the impact on latency with a representative production workload.

@Krinkle: IIRC the performance concern was the time taken by ServiceWiring.php itself, creating all the closures and putting them into an array. The time for that would be accounted under MediaWikiServices::newInstance() → ServiceContainer::loadWiringFiles(), I believe, not under the getter for an individual service.

At https://gerrit.wikimedia.org/r/c/mediawiki/core/+/531288#message-f1da7e094986f91f0b3189f49e43b9aeb974f629 @tstarling measured a 200us improvement in wall clock time for a simple request to the REST API.

There's also mentioned that this change might improve the readability of the wiring file.

Moving to roadmapping process. It's not much work, maybe a couple of weeks.