fix interactions with global names, function scopes & repl

[davidhewitt]

Fixes #183

I will probably extend this to also cover #423

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Summary by cubic

Reworks global name resolution to be slot-safe and Python-correct without adding opcodes. Prepare eagerly allocates module slots for all global references; the VM falls back to builtins when a slot is Undefined, and NameLookup always caches into the correct slot. Fixes OOB panics, module-scope NameError semantics, and late binding in functions and the REPL.

• Refactors

  • Module-scope names are always Global; prepare allocates/reuses a real module slot for every global reference (including unresolved).
  • Function scope never does parse-time builtin substitution; module scope only substitutes when the name isn’t already bound (including from prior REPL snippets).
  • Compiler stops emitting LoadLocalCallable; callable loads use LoadGlobalCallable. Legacy opcodes remain for decode compatibility.
  • VM adds builtin fallback for LoadGlobal and LoadGlobalCallable; delete now raises NameError when the slot is Undefined.
  • NameLookup always carries a real slot; resume caches the value into that slot (globals or frame locals).

• Bug Fixes

  • Fixed OOB access from global X inside functions when X had no prior module slot.
  • Module-level reads now consistently raise NameError (never UnboundLocalError) and respect non-executed assignments.
  • Late binding works across functions and REPL: later def shadows builtins for both callable and non-callable loads.
  • Name lookups cache into their own slots and persist across calls.

^{Written for commit 607fe80. Summary will update on new commits.}

Review in cubic

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 15 skipped benchmarks¹

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_{Comparing dh/load-global-by-name (824de9d) with main (0039ec3)}

Footnotes

1. 15 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

Codecov Results 📊

❌ Patch coverage is 68.98%. Project has 24136 uncovered lines.
❌ Project coverage is 66.16%. Comparing base (base) to head (head).

Files with missing lines (5)

File	Patch %	Lines
crates/monty/src/prepare.rs	63.57%	⚠️ 51 Missing and 10 partials
crates/monty/src/bytecode/vm/mod.rs	86.21%	⚠️ 4 Missing and 6 partials
crates/monty/src/bytecode/compiler.rs	66.67%	⚠️ 1 Missing
crates/monty/src/repl.rs	85.71%	⚠️ 1 Missing
crates/monty/src/run_progress.rs	85.71%	⚠️ 1 Missing

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
- Coverage    66.19%    66.16%    -0.03%
==========================================
  Files          279       279         —
  Lines        71052     71319      +267
  Branches    152853    153176      +323
==========================================
+ Hits         47030     47183      +153
- Misses       24022     24136      +114
- Partials      3993      4028       +35

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Generated by Codecov Action

[codecov]

Codecov Report

❌ Patch coverage is 85.35354% with 29 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/monty/src/prepare.rs	87.14%	5 Missing and 13 partials ⚠️
crates/monty/src/bytecode/vm/mod.rs	72.41%	2 Missing and 6 partials ⚠️
crates/monty/src/bytecode/compiler.rs	66.66%	1 Missing ⚠️
crates/monty/src/repl.rs	85.71%	1 Missing ⚠️
crates/monty/src/run_progress.rs	85.71%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[cubic-dev-ai]

1 issue found

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="crates/monty/src/intern.rs">

<violation number="1" location="crates/monty/src/intern.rs:913">
P1: New serialized field `global_slots` is missing `#[serde(default)]`, which breaks deserialization of previously dumped runners/snapshots.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

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[cubic-dev-ai]

1 issue found

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

2 issues found

<sub>Reply with feedback, questions, or to request a fix.

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