Convert OCaml parsetrees between different major versions

This library converts between parsetrees of different OCaml versions.

Supported versions are 4.02, 4.03, 4.04, 4.05, 4.06, 4.07, 4.08, 4.09,
4.10, 4.11, 4.12 and 4.13. For each version, there is a snapshot of the
parsetree and conversion functions to the next and/or previous version.

Asts

module Ast_{402,403,404,405,406,407,408,409,410,411,412,413} : sig

  (* Version specific copy of AST *)
  module Asttypes
  module Parsetree

  (* Magic numbers used for marshalling *)
  module Config : sig
    val ast_impl_magic_number : string
    val ast_intf_magic_number : string
  end
end

These embed copies of AST definitions for each supported OCaml major version.

The AST matching the version of the OCaml toolchain will contain
equalities relating the copy of types to the definitions from
compiler-libs. For instance, when installed with OCaml 4.04.x the
type Ast_404.Parsetree.expression is equal to the type
Parsetree.expression from the compiler libraries.

Migration modules

For each pair of versions $(n) and $(n+1), the two modules
Migrate_parsetree_$(n)_$(n+1) and Migrate_parsetree_$(n+1)_$(n)
convert the AST forward and backward.

The forward conversion is total while the backward conversion is
partial: when a feature is not available in a previous version of the
parsetree, a Migrate_parsetree_def.Migration_error exception is
raised detailing the failure case.

Adding a new OCaml version

We use Cinaps to generate
boilerplate. You can install it via opam: opam install cinaps.

Add the new version in
src/cinaps_helpers/cinaps_helpers.ml
supported_versions.

Copy the last src/ast_xxx.ml file to src/ast_<new_version>.ml,
then go over the file and update each sub-module by replacing its
signature and implementation with the code from the compiler. For the
Config sub-module, update the two variables with the values in
utils/config.mlp in the compiler source tree.

Once this is done, call:

$ dune exec tools/add_special_comments.exe src/ast_<new_version>.ml

Then diff the src/ast_xxx.ml and src/ast_<new_version>.ml and go
over the diff to make sure the difference are relevant. The ast_...
files require some adjustments which should pop up when you do this
diff. Port the old adjustments to the new file as required.

Add migration functions. In the commands below, set $OLD and $NEW to the
appropriate version numbers, e.g. 408 and 409:

• Manually compile the asts (ocamlc -c src/ast_{$NEW,$OLD}.ml -I +compiler-libs -I _build/default/src/.migrate_parsetree.objs/byte/)

• Using tools/gencopy.exe (dune build tools/gencopy.exe), generate copy code to and from previous version:

_build/default/tools/gencopy.exe -I . -I src/ -I +compiler-libs -map Ast_${NEW}:Ast_${OLD} Ast_${NEW}.Parsetree.{expression,pattern,core_type,toplevel_phrase} > src/migrate_${NEW}_${OLD}.ml
_build/default/tools/gencopy.exe -I . -I src/ -I +compiler-libs -map Ast_${OLD}:Ast_${NEW} Ast_${OLD}.Parsetree.{expression,pattern,core_type,toplevel_phrase} > src/migrate_${OLD}_${NEW}.ml

• Fix the generated code by implementing new cases

TODO: specialize and improve gencopy for these cases

At any time, you can expand boilerplate code by running make cinaps.

Update build system:

• make sure make cinaps reaches a fixed point :)

• make should succeed