First-class cross-compilation via dune package management

Several months ago, @gridbugs drafted a document, Proposal for First-Class Cross-Compilation in Dune. The document proposes changes to Dune to allow cross-compiling projects, summarized as follows:

Dune’s existing cross-compilation support works by changing the findlib toolchain used to build the project. This requires all of the project’s library dependencies to be ported to separate opam packages which are configured to build with the desired toolchain (often in an overlay opam repo), and the installation of an OCaml cross-compiler into the environment visible to findlib (usually installed by opam).

Dune’s package-management features allow Dune to orchestrate the build of a project’s entire dependency closure, including the OCaml compiler. This presents an opportunity for Dune to automatically cross-compile a project’s library dependencies without the need for explicit ports of library packages, and for Dune to install the appropriate OCaml cross-compiler. The automation of the installation of a cross-compiler and the ability to cross-compile unmodified opam packages will be referred to as “First-Class Cross-Compilation”.

This issues is to track and expand discussion of, and planning around, that proposal.

Design work on this should also consider @samoht's RFC: Toolchains.

Start by reading the linked Proposal for First-Class Cross-Compilation in Dune and the RFC: Toolchains, then compare their proposed approaches with Dune's existing cross-compilation and package-management behavior. No repository files, tests, or implementation entry points are named; done means reaching and documenting an agreed design and plan.