No telemetry in the Rust compiler: metrics without betraying user privacy

"No telemetry in the Rust compiler: metrics without betraying user privacy"

I have some thoughts about something I believe we need: local-only stable-compiler metrics. For a while I've felt that we don't really have enough visibility into the way rustc
really works on users' machines. Long gone are the times when most of the community relied on nightly, causing some features to have a trial by fire where their only real usage happens after their stabilization. There are issues that only evidence themselves in a transient manner, particularly with malformed code. People try to file tickets that can no longer be reproduced.

Anything even resembling telemetry is always a contentious topic, so I want to clarify that I am not proposing any kind of telemetry (specially for end users, although I would expect us to have a telemetry service for project tools like crater and perf that would use these metrics).

I've written the above post both as a way to start a conversation on the matter, and as a signal both to the project and the community at large on where I believe we should stand, what are the clear lines that we should not cross.