fix(ir,codegen): autovivify missing parents in nested array writes

[mirchaemanuel]

fix(ir,codegen): autovivify missing parents in nested array writes

Fixes #555.

Symptom

$a = ['x'];
$a[7][1] = 'patched';   // PHP: autovivifies $a[7] as an array, no warning
echo $a[7][1];          // elephc: empty line, "Undefined array key 7" twice,
                        // and the dropped value leaked

Root cause

The parent chain of a nested assignment was lowered as a plain read. A missing element produced a detached Mixed(null) box (with an undefined-key warning), and __rt_mixed_array_set released the value in its incompatible-target path. Nothing installed a container into the missing slot: there were no write-context lookup semantics for intermediate elements. Additionally, a string key on an indexed payload branched to the same drop path, losing legal writes like $a[0]['new'] = 'v'.

Fix

The parent chain now lowers with fetch-for-write semantics:

• New __rt_mixed_array_get_for_write autovivifies missing indexed elements (grow, zero-fill, length update), null gap slots, boxed-null slots, and missing hash keys, installing an empty-array cell into the parent storage and returning the STORED cell (retained) so the three-operand set writes through the parent. Indexed payloads are first normalized via __rt_array_to_mixed (COW split + boxed slots, republished into the cell) — which also lands writes through concrete intermediates of 3+ level chains, the residual limitation documented in fix(ir): write nested array assignments through the parent cell (#529) #553.
• Concrete array<mixed>/assoc locals go through a new __rt_array_ensure_elem_for_write wrapper plus a local storeback; a literal string key on an indexed local promotes it via Op::ArrayToHash.
• __rt_mixed_array_set now promotes indexed payloads to hash storage for string keys (new __rt_mixed_cell_promote_to_hash) instead of dropping the write, matching PHP array-key semantics.
• Write-context reads emit no undefined-key warning, matching PHP.
• Both AArch64 and x86_64 emitters implemented symmetrically.

Tests

tests/codegen/arrays/nested_autovivify.rs (14 tests): the exact issue repro, append position, null gap and boxed-null parents, assoc missing key, multi-level int and string chains from [], Mixed indexed/assoc receivers, string-key child promotion, existing-parent and shared-array COW controls, and function-return visibility; all assert no warning and heap-debug cleanliness. (The json_decode variant asserts stdout/warnings only — Mixed builtin results stored into locals leak one owner reference on current main independently of nested writes, reproducible with a bare read.) 13/13 of the original tests failed before the fix.

Limitations (documented)

• Incompatible scalar intermediates are intentionally not converted (the write is still dropped; PHP raises Error: Cannot use a scalar value as an array — no error machinery added here).
• Negative indexes on dense indexed storage stay unsupported (pre-existing storage model).
• Statically-Mixed key expressions on concrete locals and ArrayAccess-object intermediates keep the previous plain-read path.
• Autovivified gaps remain dense (zero-filled) per elephc's array model, so count() can differ from PHP's sparse arrays (pre-existing).

QA

• Issue repro prints patched, zero warnings, leak summary: clean, under --ir-opt=on and =off.
• After rebasing onto current main (post-fix(ir): write nested array assignments through the parent cell (#529) #553, including its extended nested_mixed_write suite): nested_autovivify 14, nested_mixed_write 8, arrays:: 366, mixed 262 — all green. Pre-rebase full slices: nested 155, json 445, cargo test --lib 930 (one pre-existing environment-dependent DNS test failure unrelated to this change, fails identically on pristine main). No new clippy warnings.

https://claude.ai/code/session_01QyzntRke1Cgxgq1ueuz4X9