fix: reject stray commas in object, array, call and parameter lists

[He-Pin]

Motivation

{,} (and { , }) parsed as an empty object, while go-jsonnet and jrsonnet both report a parse error:

Program	sjsonnet (before)	sjsonnet (after)	go-jsonnet	jrsonnet
{,}	{ }	parse error	Unexpected: "," while parsing field definition	expected field name, got ','
{ , }	{ }	parse error	parse error	parse error
local f(a) = a; f(,)	runtime arity error	parse error	parse error	parse error
(function(,) 1)()	1	parse error	parse error	parse error
[1,,]	[1]	parse error	parse error	parse error

Root cause: the comma-list grammars combined rep(sep = ",") (accepts zero elements) with an optional trailing comma, so a lone comma was consumed as the "trailing" comma of an empty list. The Jsonnet grammar requires at least one element before a comma. The array grammar had a related hole: after the first comma, expr.rep(0, sep = ",") ~ ",".? accepted zero elements followed by a comma, so [1,,] parsed as [1].

Modification

sjsonnet/src/sjsonnet/Parser.scala — capture the trailing comma (,.!) in the four affected grammars (objinside, args, params, arrBody`) and fail with a descriptive parse error when a comma appears with zero preceding elements:

• Expected at least one object member before ','
• Expected at least one argument before ','
• Expected at least one parameter before ','
• Expected at least one array element before ','

Implementation note: the obvious rep(1, sep = ",") ~ ",".? wrapped in an outer .? was tried first and rejected — it triggers a Scala 2.13 lambda-lift compiler bug (Could not find proxy for case val x1: fastparse.ParsingRun) with fastparse's implicit plumbing. The capture-and-check shape keeps the original flat parser structure and compiles on all Scala versions. For arrBody, cut = true is set before the failure so the error propagates through the enclosing .? instead of being swallowed.

Result

• {,}, { , }, f(,), function(,), [1,,] are parse errors.
• Valid trailing commas unchanged (pinned by a new positive test): {a: 1,}, {a: 1, b: 2,}, [1, 2,], f(1, 2,), function(a,) e, function(x=1,) e, [1, for x in xs], plus object comprehensions, object locals and asserts with commas.
• Other comma misuses were already rejected and still are ({,a: 1}, {a: 1,, b: 2}, [,], [1,,2], {,,}).

Test plan

• New: new_test_suite/error.parse_object_lone_comma.jsonnet, error.parse_call_lone_comma.jsonnet, error.parse_params_lone_comma.jsonnet, error.parse_array_double_comma.jsonnet (+ goldens)
• New: new_test_suite/parse_trailing_commas_valid.jsonnet (+ golden) pinning the preserved valid forms
• ./mill 'sjsonnet.jvm[_].test' — all Scala versions (2.12.21 / 2.13.18 / 3.3.8) pass
• ./mill 'sjsonnet.js[_].compile' 'sjsonnet.native[_].compile' 'sjsonnet.wasm[_].compile' — pass
• ./mill __.checkFormat — clean

Found by four-way differential testing (sjsonnet vs go-jsonnet vs jrsonnet vs spec).