Suggested workflow for vscode check-on-save can lead to surprisingly expensive check builds #141955
Description
With the stage 0 redesign, ./x check now (sometimes?) defaults to stage 1. If a downloaded rustc cannot be used, this command is quite expensive: it does a full rustc build, and then checks everything! That's not what users expect from a check build (which is supposed to be quick). It's also bad when you are on battery -- it's a nasty surprise when you realize that just opening vscode sucked up 8% of your battery as it did a full rustc build.
I don't know what the best fix here, but I think it is a hard requirement for the suggested check-on-save command that it never does an actual full compiler build. This was true with the old staging, and we should make sure it becomes true again.