fix(indexing): classify string receivers behind array claims (#7891)

[proggeramlug]

Closes #7891.

What changed

• keep an SSO receiver boxed when a string-valued key is read through an erased Array declaration
• classify heap String receivers by their GC header before the array by-name fallback
• leave the numeric guarded-array tier unchanged
• add IR gates for the string-key and numeric-key siblings plus a runtime heap-string unit test
• add source parity coverage for literal/dynamic keys, heap/SSO strings, arrays, objects, and the existing direct-string behavior

Reproduction

On current main, the new fixture printed undefined/s for the declared receiver while Node printed s/s. The direct any control already printed s/s in both.

Validation

• focused codegen tests: 2 passed
• focused runtime test: 1 passed
• cargo test -p perry-codegen: passed, including integration and doc tests
• exact wrapper build: -p perry -p perry-runtime-static -p perry-stdlib-static
• fixture: Perry exit 0, Node exit 0, byte-exact output
• test-registration check: 197 files across 4 registries
• file-size gate: passed

Summary by CodeRabbit

• Bug Fixes

  • Fixed numeric string-key access when values declared as arrays contain runtime strings.
  • Preserved expected string indexing and length property behavior across declared, nullable, parsed, and dynamic values.
  • Maintained correct behavior for ordinary arrays and object property lookups.

• Tests

  • Added regression coverage for string receivers, numeric and non-numeric keys, short parsed strings, and fallback scenarios.

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Configuration used: defaults

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Run ID: 348e3a7e-cd80-443c-87ed-6935fc372c51

📥 Commits

Reviewing files that changed from the base of the PR and between 6d9f12e and 1237c12.

📒 Files selected for processing (7)

• changelog.d/7904-declared-array-string-index.md
• crates/perry-codegen/src/expr/index_get.rs
• crates/perry-codegen/src/expr/index_get_claim_tests.rs
• crates/perry-codegen/src/expr/mod.rs
• crates/perry-runtime/src/array/indexing.rs
• test-files/test_gap_7890_declared_array_receiver_element_read.ts
• test-files/test_gap_7891_string_receiver_numeric_string_key.ts

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📝 Walkthrough

Walkthrough

The change fixes numeric string-key reads when a runtime string passes through an erased array declaration. Code generation preserves boxed SSO string handling, runtime lookup classifies heap strings, and regression tests cover both compiler IR and end-to-end behavior.

Changes

Declared array string-key indexing

Layer / File(s)	Summary
Runtime string receiver classification crates/perry-runtime/src/array/indexing.rs	Heap string receivers now resolve numeric keys through string indexing while preserving length and ordinary property fallback.
Claimed-array dispatch crates/perry-codegen/src/expr/index_get.rs, crates/perry-codegen/src/expr/index_get_claim_tests.rs, crates/perry-codegen/src/expr/mod.rs	Declared-array lowering excludes static string and symbol keys from numeric claims and uses tag-aware boxed dispatch for non-numeric keys. IR tests verify string and numeric paths.
Regression coverage and changelog test-files/test_gap_7891_string_receiver_numeric_string_key.ts, test-files/test_gap_7890_declared_array_receiver_element_read.ts, changelog.d/7904-declared-array-string-index.md	End-to-end tests cover string receivers, nullable values, arrays, direct strings, and ordinary objects. Comments and the changelog describe the handling.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant DeclaredArrayRead
  participant CodegenDispatch
  participant RuntimeArrayLookup
  participant RuntimeStringIndexing
  DeclaredArrayRead->>CodegenDispatch: Lower declared-array key read
  CodegenDispatch->>RuntimeStringIndexing: SSO receiver with numeric string key
  CodegenDispatch->>RuntimeArrayLookup: Other receiver or fallback property lookup
  RuntimeArrayLookup->>RuntimeStringIndexing: Heap string with numeric key

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Possibly related PRs

• PerryTS/perry#7890: Adds the declared-array claim handling that this change extends for string-key dispatch.
• PerryTS/perry#7893: Establishes related declared-array key classification in index_get.rs.
• PerryTS/perry#6888: Adds the SSO-safe boxed string-indexing behavior extended here.

Suggested reviewers: thehypnoo

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes fix string-key reads on runtime strings behind erased array declarations while preserving numeric-key behavior and adding the required regression coverage.
Out of Scope Changes check	✅ Passed	All changes support issue #7891, including code fixes, regression tests, source coverage, and a changelog entry.
Title check	✅ Passed	The title clearly and concisely identifies the primary fix: classifying string receivers behind erased array declarations.
Description check	✅ Passed	The description covers the fix, related issue, implementation changes, reproduction, and detailed validation; omitted template headings are non-critical.

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/7891-runtime-string-index

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