Unboxed records

[rtjoa]

Add support for unboxed records (construction, functional updates, projection, pattern matching), whose types have product jkinds. For example:

type 'a with_idx : value & immediate = #{ data : 'a ; i : int }
let idx #{ data = _ ; i } = i
let inc r = #{ r with i = r.#i + 1 }

The fields of unboxed records have a separate namespace from record fields. While fields can be mutable, we don't yet support updating them.

Implementation decisions

Unboxed record types are distinct from record types, rather than being yet another record_representation.

• Having separate codepaths is likely to be less error-prone than adding another record_representation, which we might not always match on in the right places.
• The majority of the duplication this causes is ameliorated by making label_description abstract over the type for record representations, which is now either record_representation or record_unboxed_product_representation. This allowed nearly all shared logic to be parameterized by the record "form," with the most notable exception being in lambda/matching.ml.

A (temporary) jkind-checking algorithm that interleaves looking through unboxed types and type expressions constructors.

• See the comment on type_jkind_deep in typing/ctype.ml.
• We could have instead updated constrain_type_jkind to return a jkind estimate, but this would require significant changes. Instead, we use this somewhat duplicative solution, as it will be removable once we support kind-polymorphic type declarations.

Future work

• Declaring a boxed record t also declares its unboxed version t#, when doing so would be well-founded.
• The signature item type t boxes value & value declares the kinds of both t and t# and indicates that they are linked.
• Unboxed record fields can be mutated (only when the unboxed record is nested within a boxed record).
• Infinite-size unboxed records give better type errors (see testsuite/tests/typing-layouts-unboxed-records/recursive.ml).

Review: @ccasin

[github-actions]

Parser Change Checklist

This PR modifies the parser. Please check that the following tests are updated:

• parsetree/source_jane_street.ml

This test should have examples of every new bit of syntax you are adding. Feel free to just check the box if your PR does not actually change the syntax (because it is refactoring the parser, say).