Wasm of ocaml support

[vouillon]

This add the following:

• a (js_of_ocaml (wasm_files (<files-list>)) option in library and executable stanzas to specify wasm_of_ocaml JavaScript and Wasm runtime files.
• a (js_of_ocaml (submodes <submodes>)) option where each submode is either js or wasm in the environment and in library and executable stanzas to specify whether to generate JavaScript or Wasm code.

When compiling with wasm_of_ocaml, the output is a launcher file name.bc.wasm.js as well as a directory name.bc.wasm.asset containing the Wasm code and possibly some source maps.

When compiling to Wasm but not to JavaScript, a .bc.js file can also be produced for compatibility. It is just a copy of the
bc.wasm.js file.

[rgrinberg]

I'm not sure how to add tests since they would require wasm_of_ocaml, which is a bit complicated to install at the moment...

Nix would be a good option

[vouillon]

@hhugo Can you have a look?

[christinerose]

I've made some suggestions throughout. Mostly formatting stuff for consistency, but there is one area where the sentence was confusing, so I suggested an alternative. If it doesn't work for you, let me know and we'll work on a solution together.

I'm not able to make suggetsions on anything other than the changes made in this PR, as I don't have edit access.

[rgrinberg]

gentle ping @hhugo

[maiste]

I don't have the proper knowledge to review this PR. However, wouldn't it be easier to review / merge it if the PR was split in two: one with the documentation and one with the code itself?

[nojb]

Instead of having this, which controls JS vs WASM generation globally, couldn't we add a wasm constructor to the (modes) field in executable stanzas (similar to how there is a js constructor)?

[hhugo]

I've been unhappy about submodes but never had enough time to think about it.
I wonder if we could do something with modes of the form (<compilation-mode> <binary-kind>).
We could have (jsoo js) and (jsoo wasm), or something.

IIRC, you want to be able to compile both js and wasm without touching all dune files. We could imagine a new env field that define the semantic of the 'js' mode.

[vouillon]

This directive can also be included in executable stanzas.

I'm not completely happy with the design either. It might be better to have a separate wasm_of_ocaml field, for instance, to be able to pass different flags to the compilers.

Still, I think we need a global way to specify whether we want to generate JavaScript and/or Wasm code.

[vouillon]

Maybe we could have these in env stanzas:

(js_of_ocaml (by_default_if <blang expression>))
(wasm_of_ocaml (by_default_if <blang expression>))

and a (jsoo default) executable mode.

And the mode (jsoo js) and (jsoo wasm) could be used to generate explicitly some Wasm or JavaScript code.

[rgrinberg]

Agreed that this submodes stuff doesn't seem satisfactory. Making it more similar to regular modes would be preferable.

[rgrinberg]

Continued the PR here #11029 since I'm unable to push here

[rgrinberg]

CI is failing @vouillon

--- a/_build/.sandbox/6975479b7ea39eedbfcff9056c4b3b15/default/test/blackbox-tests/test-cases/wasmoo/inline-tests.t/run.t
+++ b/_build/.sandbox/6975479b7ea39eedbfcff9056c4b3b15/default/test/blackbox-tests/test-cases/wasmoo/inline-tests.t/run.t.corrected
@@ -15,8 +15,17 @@ Run inline tests using Node.js
   $ dune runtest --profile release
   inline tests (Native)
   inline tests (Native)
-  inline tests (Wasm)
-  inline tests (Wasm)
+  File "wasm/dune", lines 1-6, characters 0-142:
+  1 | (library
+  2 |  (name inline_tests_wasm)
+  3 |  (js_of_ocaml (wasm_files wasm.wat) (submodes wasm))
+  4 |  (inline_tests
+  5 |   (modes js)
+  6 |   (backend fake_backend)))
+  wasm_of_ocaml: unknown option '--linkall'.
m  Usage: wasm_of_ocaml [COMMAND] …
+  Try 'wasm_of_ocaml --help' for more information.
+  [1]