Support rescript.json

[cometkim]

Resolves #5278
Closes #6193

[cometkim]

Note: This is essential to supporting the rescript.json. We need to discuss some existing issues.

1. Backporting

Do we need it for v10?

2. Simplifying config

The config is currently over-bloated. There are many variants of config structure, recursive definitions, deprecated features, and even undocumented features no one uses.

It adds a lot of complexity to the codebase, making refactoring and third-party replication difficult.

Here's a list we can point out:

• reason (legacy jsx support)
• generators, cut-generators (undocumented)
• pp-flags (I think it is a deprecated one)
• entries (???)
• sources[].generators (undocumented)
• sources[].group (undocumented)
• sources[].resources (undocumented)
• sources[].internal-depends (undocumented)
• sources[].subdirs (recursive definition, it's too complex)

And maybe we can remove all the bs- prefixes from fields.

3. Refactoring plan

In the current codebase, the loading and parsing of rescript.json (or bsconfig.json) tends to be repetitive. This is because building config is an IO-heavy operation so difficult to reuse elsewhere.

I suggested separating "config" and "manifest" in my previous PR.

• config: a program context
• manifest: a parsed user input

Making manifest input pure logic can make it easier to reuse, and perhaps we can publish it to opam so third-party tools can use it.

4. Migration

This PR simply adds the printing message "bsconfig.json is deprecated"

It may be sufficient since bsconfig.json is still supported, but a more complex strategy may be needed to achieve "Simplifying config" and "Refactoring plan" together.

5. Further works

• Documents, website, and comments need to change.
• Blog post about it.
• Interacting with library authors

[cometkim]

Just rebased PR.

I think this can be included in v11.0, we can then release the new configuration in v11.x in "compat mode"

No matter how we change the configuration, it should be in v12. I'll open a new issue for that.

[cknitt]

Thanks a lot for this! Looks good to me.
Also tested it against the project that I am currently working on.

@cristianoc @zth Anything to add from your side?