Formatter drops comments in dict patterns, variant spread patterns and dict literal spreads #8820

Found by adversarial experiments on semantic analysts.

What is broken

The formatter deletes comments in these positions:

• Dict pattern key: | dict{"a" /* k */: b} → | dict{"a": b}
• Dict pattern value: | dict{"a": /* c */ a} → | dict{"a": a} (the record pattern {a: /* c */ a} keeps its comment)
• Empty dict pattern: | dict{/* e */} → | dict{}
• Variant spread pattern: | .../* c */ t → | ...t
• Polymorphic variant type pattern: | #.../* c */ t → | #...t
• Dict literal with a spread: an own-line comment before the first row after a spread is dropped (/* x */ on the same line is kept)

Reproduction

cm.res:

let f = d =>
  switch d {
  | dict{"a" /* k */: b} => b
  | dict{"a": /* c */ a} => a
  | dict{/* e */} => 0
  | _ => 1
  }
type t = A | B
type u = | ...t | C
let g = x =>
  switch x {
  | .../* c */ t => 0
  | #.../* c */ t => 0
  | C => 1
  }
let b = dict{
  ...base,
  // first
  "a": 1,
}

./_build/default/compiler/syntax/cli/res_cli.exe cm.res

Observed: every comment above is missing from the output. Expected: each comment is printed.

Where

• Ppat_type and Ppat_variant_spread attach no comments:
  https://github.com/rescript-lang/rescript/blob/8d51b9ce7/compiler/syntax/src/res_comments_table.ml#L2056
• Dict pattern rows are walked as record rows, so key comments are attached to the key location:
  https://github.com/rescript-lang/rescript/blob/8d51b9ce7/compiler/syntax/src/res_comments_table.ml#L2057-L2063
• print_pattern_dict_row prints the key with print_longident, which does not consult the comment table. The record row printer uses print_lident_path ... cmt_tbl.
  https://github.com/rescript-lang/rescript/blob/8d51b9ce7/compiler/syntax/src/res_printer.ml#L3051-L3076
• The Ppat_dict case has no print_comments_inside:
  https://github.com/rescript-lang/rescript/blob/8d51b9ce7/compiler/syntax/src/res_printer.ml#L2845-L2864
• In a dict literal with spreads, each run of rows becomes a synthetic expression. Leading comments attach to that expression (https://github.com/rescript-lang/rescript/blob/8d51b9ce7/compiler/syntax/src/res_comments_table.ml#L1402-L1410), but print_spread_dict_expr prints the run without printing them (https://github.com/rescript-lang/rescript/blob/8d51b9ce7/compiler/syntax/src/res_printer.ml#L1782-L1786). This cause is from reading the code.

No comment fixture covers dict patterns or variant spread patterns (tests/syntax_tests/data/printer/comments/). The roundtrip test compares the first formatting with the second, so a comment lost on the first print passes. The fix should add these cases to the comment fixtures.

Introduced by

The same output is produced at ece8b14, before the dict pattern and variant spread nodes existed. The current code comes from b081cd1 (#8749) and bffd18e (#8742).

Impact

Running the formatter silently deletes user comments.