Prepare for 5.2 AST bump

[patricoferris]

This PR is a patch for an upcoming release of ppxlib where the internal AST is bumped from 4.14 to 5.2. Until that is merged and released, there is no reason to merge this PR.

To test these changes, you can use the following opam-based workflow. I've made releases of most of these patches to an opam-repository overlay.

opam switch create ppxlib-bump --repos=ppxlib=git+https://github.com/patricoferris/opam-repository#5.2-ast-bump
opam install <your-package>

The following describes the most notable changes to the AST.

Note: no update has been made of the opam file, but ppxlib will likely need a lower-bound before merging/releasing.

Functions

---

Currently

In the parsetree currently, functions like:

fun x y z -> ...

Are represented roughly as

Pexp_fun(x, Pexp_fun (y, Pexp_fun(z, ...)))

Functions like:

function A -> ... | B -> ...

Are represented roughly as

Pexp_function ([ case A; case B ])

Since 5.2

All of these functions now map to a single AST node Pexp_function (note, this is the same name as the old cases function). The first argument is a list of parameters meaning:

fun x y z -> ...

Now looks like:

Pexp_function([x; y; z], _constraint, body)

And the body is where we can either have more expressions (Pfunction_body _) or cases (Pfunction_cases _). That means:

function A -> ... | B -> ...

Has an empty list of parameters:

Pexp_function([], _, Pfunction_cases ([case A; case B]))

Local Module Opens for Types

Another feature added in 5.2 was the ability to locally open modules in type definitions.

module M = struct
  type t = A | B | C
end

type t = Local_open_coming of M.(t)

This has a Ptyp_open (module_identifier, core_type) AST node. Just like normal module opens this does create some syntactic ambiguity about where things come from inside the parentheses.

[raphael-proust]

I've tested after

• rebased on top of recent master
• installed ppxlib.0.36.0

The only issue is that ppxlib.0.36.0 is incompatible with ppx_let (which is used in tests, to make sure that lwt exports the values that ppx_let needs).

Ultimately, the ppx_let test could be run in a different, separate environment. It's a test of lwt only. But this is a bit difficult to do from a purely technical pov…

Maybe we can wait until ppx_let releases a version compatible with ppxlib.36.0

[patricoferris]

PR for ppx_let janestreet/ppx_let#16 (which also needs the PR to ppxlib_jane janestreet/ppxlib_jane#5). Hopefully these get reviewed/merged promptly and we can get a release of the library.

[kit-ty-kate]

Now that the janestreet packages have been updated and merged into opam-repository, could this PR be looked at again?
This is required for lwt_ppx to be compatible with OCaml 5.4

[smorimoto]

We have to send a patch to bisect_ppx as well, I guess.

[patricoferris]

I'm taking a look at bisect_ppx today

[kit-ty-kate]

Is bisect_ppx really necessary to merge this? I really don't think such CI tool should block the release of such a key ecosystem package.

[smorimoto]

Ref: #1063