It sounds like computer time is used now.
> So the default behaviour now is to nonetheless "catch" infinite loops by default? Why not make the default behaviour to just notify a user by emitting a message after some time (as described)?
Because lots of builds don't have a user. Having the user have to time out rustc because of this would be frustrating.
> If rustc "catches" infinite loops by default, how is backwards compatibility preserved with respect to `const` functions?
As long as the time is much larger than it takes to handle the old statement limit it is preserved.
> If I split one statement into two in `const fn`, do I break BC?
The old version might have prevented your code from compiling as mentioned in the notes.
The new version should in theory allow it if the unsplit version worked since it is based off compilation time.
Also there is an opt flag where it will go infinite