[feature tracking] Workflow for iterating on a compiler development branch?

[gasche]

Currently the opam-compiler README says:

To iterate on a compiler feature and try opam packages at the same time, it supports to ways to reinstall the compiler: either a safe and slow technique that will reinstall all packages, or a quick way that will just overwrite the compiler in place.

but as far as I understand the "quick way" is not implemented yet. This would be a convenient feature when doing compiler development that requires testing on opam packages -- it was the central feature of opam-compiler-conf. We discussed this briefly on Discuss.

I'm opening the present issue to track progress on this feature (if there is interest among opam-compiler developers, no pressure!), as suggested by @avsm in ocaml/ocaml#9632 (comment).

Note: I should point out that this feature is very useful for certain cases of working with the compiler, but that these use-cases are pretty rare. It matters for problems where we need to iterate on the compiler, test on an opam package, and we are confident that the hard-reinstall will preserve the semantics. I've done this in the past to debug and fix compiler issues that break an opam package (a typing issue, a compile-time blowup, a runtime failure, etc.), but I haven't used it myself in the last couple years; other people have (@lthls has developed his own script for this in ocaml/ocaml#9632 ). In the more common cases, we can isolate the issue on a short reproduction case without dependencies, and iterate without needing an opam environment. (And in the bad cases, we need the compiler changes to affect all compiler files, so we need a "full" reinstall anyway.)
I think that it's about a handful of people using it sporadically over years -- but when we need this workflow it is very useful.

[avsm]

Thanks for creating this issue @gasche. I'll selfishly bump this higher up the priority stack since it's extremely useful for multicore compiler development (where recompiling opam universes happens all the time with compiler changes).