Block indices

[rtjoa]

Add explicit indices into a block, gated by layouts_beta. A quick example follows below. For full documentation, see jane/doc/extensions/_02-unboxed-types/block-indices.md.

open Stdlib_beta

type pt = { x : int; y : int }
type line = { p : pt#; q : pt# }

let mk_idx () : (line, int) idx_imm = (.q.#y)

let get_coord (line : line) (i : (line, int) idx_imm) : int =
  Idx_imm.unsafe_get line i

let first_x (): (pt# array, int) idx_mut = (.(0).#x)

let inc_coord (pts : 'a) (i : ('a, int) idx_mut) =
  Idx_mut.unsafe_set pts i (Idx_mut.unsafe_get pts i + 1)

Testing

• Generated tests by enumerating records/arrays supporting block indices in testsuite/tests/records-and-block-indices
  • Compared get/set using block indices against normal get/set
  • Checked that each block index (<prefix><suffix>) is equivalent to the deepening operation (.idx(<prefix>)<suffix>)
• Typing and handwritten runtime tests in testsuite/tests/typing-layouts-block-indices

Reviewing

• @ccasin to "whole-feature" review
• @mshinwell to do targeted review of the middle-end, runtime, and runtime tests (backend/, bytecomp/, middle_end/, testsuite/tests/typing-layouts-block-indices/block_indices.ml, testsuite/tests/records-and-block-indices/test_generation.ml)
• Note that much of the diff is due to generated tests (beginning with generated_), which generates tests of the same shape for many types. The best way to review these is to review the file that does the generation, then look at the tests for just a few types.

[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).