runtime: complete Array.prototype.concat variadic and non-mutating semantics

[andrewtdiz]

Summary

Array.prototype.concat should accept any number of arguments, return a new array, leave the receiver unchanged, and follow Symbol.isConcatSpreadable / array-like spreadability rules. Perry currently handles only the single-argument static path, returns the receiver unchanged for multi-arg static calls, and dynamic dispatch calls the internal mutating concat helper.

Node parity probe

Run with Node v25.9.0:

for (const [label, fn] of [
  ["concat one array", () => [1].concat([2, 3])],
  ["concat multiple", () => [1].concat([2], [3, 4], 5)],
  ["concat preserves original", () => { const a = [1]; const out = a.concat([2]); return [out, a, out === a]; }],
  ["concat spreadable false", () => { const x = [2, 3]; x[Symbol.isConcatSpreadable] = false; return [1].concat(x); }],
  ["concat array-like spreadable true", () => { const x = {0: "a", 1: "b", length: 2, [Symbol.isConcatSpreadable]: true}; return [1].concat(x); }],
]) {
  try {
    console.log(label + ": " + JSON.stringify(fn()));
  } catch (e) {
    console.log(label + ": throws " + e.name + ": " + e.message);
  }
}

Output:

concat one array: [1,2,3]
concat multiple: [1,2,3,4,5]
concat preserves original: [[1,2],[1],false]
concat spreadable false: [1,[2,3]]
concat array-like spreadable true: [1,"a","b"]

Current Perry behavior in source

The current static path is intentionally single-argument only:

• crates/perry-codegen/src/lower_array_method.rs:106-132 routes one-arg concat to js_array_concat_new, but the comment says "For simplicity we only handle single-argument concat." and args.len() != 1 returns recv_box unchanged.
• crates/perry-runtime/src/array/concat_reverse.rs:94-124 implements js_array_concat_new(arr, other) for exactly one other array.

Dynamic dispatch uses the mutating helper:

• crates/perry-runtime/src/object/native_call_method.rs:1255-1260 handles dynamic "concat" by reading only the first argument and calling crate::array::js_array_concat(arr, other_ptr).
• crates/perry-runtime/src/array/concat_reverse.rs:5-92 documents js_array_concat as an append-into-destination helper used by spread/desugaring, while js_array_concat_new is the JS-semantic non-mutating helper.

The fixture comments still call out multi-arg concat as a known divergence:

• test-files/test_compat_arrays_iterators.ts:5-8 excludes multi-arg concat from parity coverage, while line 40 covers only the single-argument case.

Expected fix shape

Implement full JS concat behavior for static and dynamic paths:

• Accept zero or more concat arguments.
• Always return a new array and leave the receiver unchanged.
• Append each argument in order.
• Spread arrays by default.
• Respect Symbol.isConcatSpreadable === false for arrays and === true for array-like objects.
• Preserve non-array/non-spreadable values as single elements.

Please add regression coverage for zero args, one arg, multiple args, primitive/non-array arguments, receiver immutability, Symbol.isConcatSpreadable false on arrays, and spreadable array-like objects.