Proposal: Per unit environment

[saagraa]

Usecase

(* Assume this exists *)
module Gen : sig
  val v : ?doc:string -> ?meta:B0_meta.t -> prog:B0_unit.t -> Fpath.t -> B0_unit.t
  val file : u : B0_srscs.sel
end

(* Compile this with Metaocaml. A metaocaml program that prints out regular ocaml code *)
let meta_gen = B0_ocaml.exe "meta_gen" ~srcs:[ `File (Fpath.v "some/source/file.ml")]

let gen_file = Gen.v ~prog:meta_gen Fpath.(v "gen_file.ml")

(* Compile this with normal Ocaml *)
let mylib = B0_ocaml.libname "mylib"
let mylib_lib = B0_ocaml.lib mylib ~srcs:[ Gen.file gen_file ]

Implementation

Based on my current understanding of how things work, this could be implemented in a few ways:

1. Add an ~env argument to B0_ocaml.{lib,exe} They would then be responsible for passing those along to the tool spawns. The type should probably be something like (B0_build.t -> string String.Map.t) so that a tool spawn can generate the environment, such as a call to opam env
2. Add an env key to the meta that gets passed to B0_ocaml.lib and B0_ocaml.exe. Again, B0_ocaml.{lib,exe} would be responsible for looking up that value and passing it to the tool spaws
3. Add a wrapper around B0_unit.t to build that unit with a set of env vars. I'm not entirely clear how or if this would work, I need to look more that the unit definition/registration code.
4. Probably some mechanism with B00_ocaml. From what I can tell, its only possible to mess with the environment from inside a (B0_build.t -> ...), and theres no mechanism to do that with the high level B0_ocaml.{lib,exe}

Or this is already possible and I completely missed it.

If you like any of these I could take a stab at implementing and submitting a pr.