Add prototype 'opam build' command

[AltGr]

Provides tools for using a project directory from CWD.

Will auto-init and create a local switch unless specified otherwise.

Scans for local opam files and builds based on them.

Can install by copying the artefacts from the local switch to a given prefix.

Handling a local conf file that can give the specifics of how to create the
switch would be a nice addition.

[avsm]

This is exciting! Once merged, I can try include it in the CI and try it on the larger set of MirageOS opam packages. Does it require that the opam-version be OPAM2, or can it transparently pin to the existing OPAM1-format files that all the repos have?

[avsm]

It would be useful to be able to setup custom remotes and pins for this, as our master branches quite habitually require some customisation of the OPAM universe before things compile. How about an .opamrc that executes the local build commands after the init? That could also be interpreted by CI systems for their setup as well.

e.g. .opamrc:

opam remote add dev https://github.com/mirage/mirage-dev
opam pin add prometheus --dev

[AltGr]

Indeed. We already have .opamrc files, but these are only useful for the original opam init command (they can define global variables, e.g. sys-ocaml-version, remotes, a formula to choose the default compiler, etc.) We also have export files that can be used to define a given package selection, pins,etc.

None of those will be automatically used by opam build at the moment, though.

• an export file with a certain pre-defined name could be used automatically to initialise the switch (possibly depending on e.g. OS). Think "lock files".
• a switch-init file format could be defined, similar to opamrc, but tailored for specific switch initialisation: allowing to define switch-local variables, pre-select repositories, choose compiler, etc. There may be some overlap with the above export files, which could be included or selected using a formula.
• for depending on specific pins, see the proposal at [Feature Request] Specify exact git commit of a dependency #2762

Once properly defined, either wouldn't pose specific difficulties to implement, for interpretation when opam build is initially called and creates the local switch. However, what exactly should be done on further calls, if the files changed, isn't obvious and needs careful thought, and, maybe, careful handling.