iter mut.4-tidy + audit close — mut-local milestone closed
Tidy iter addressing the audit-mut-local drift. Plus paired baseline
update on bench/baseline_compile.json (audit-skill discipline).
Architect [high] items closed:
1. CheckError::MutVarCapturedByLambda rejects lambdas whose body
free vars include a mut-var of the enclosing mut_scope_stack.
Uses the existing ailang_core::desugar::free_vars_in_term walker
(which honours Term::Match pattern bindings). The scan runs only
when mut_scope_stack is non-empty.
2. crates/ailang-codegen/src/lambda.rs: the capture-not-in-locals
path that previously raised CodegenError::Internal blaming the
typechecker now uses unreachable!. The companion comment block
was rewritten to state the current reality.
Architect [medium] items closed (stale mut.1-stub history comments):
3. docs/DESIGN.md §'Term (expression)' mut/assign block: the
trailing paragraph describing the iter mut.1 stub state was
replaced with one describing the current mut.3-end-state. The
inline jsonc comment on {'t': 'assign'} was updated to drop the
'deferred to mut.2/mut.3' language.
4. crates/ailang-codegen/src/lib.rs: the stale comment block above
the real Term::Mut arm describing the mut.1 stub was removed
entirely.
Other:
- Short-circuit on empty mut_scope_stack in synth's Term::Var arm:
the iter mut.2 prepend now skips the iter-and-find walk when
the stack is empty, eliminating any per-Var-resolution overhead
for the common case (programs with no mut blocks). The
short-circuit did NOT close the check_ms regression — see ratify
below.
- Spec docs/specs/2026-05-15-mut-local.md §'Out of scope' amended
with the lambda-capture rejection bullet.
- Negative fixture examples/test_mut_var_captured_by_lambda.ail.json
+ driver test extension in crates/ailang-check/tests/
mut_typecheck_pin.rs (6th test) +
crates/ailang-core/tests/carve_out_inventory.rs EXPECTED bumped
12 → 13.
Bench-regression ratify:
bench/compile_check.py check_ms showed a uniform ~30-50% relative
shift across the entire 11-fixture suite (~0.5ms absolute on a
1.4-1.5ms baseline). The uniformity across fixtures of very
different Var counts argues for a fixed-cost-per-invocation tax,
not a Var-proportional hot path. The Term::Var short-circuit
falsified the hot-path hypothesis. The plausible remaining causes
(synth-parameter-passing through ~19 recursive sites, and binary-
size startup tax from mut-* adding ~1400 LOC to typecheck/codegen)
are both feature-cost, not pathological. Ratified by paired
journal entry; bench/baseline_compile.json updated to the post-
mut-local numbers via 'bench/compile_check.py --update-baseline'.
bench/check.py continues to show the established tail-latency-noise
envelope from audit-pd (2026-05-14) — no separate ratify needed.
bench/cross_lang.py clean.
Tests: 594 → 598 green (4 new lib.rs mod tests + 1 driver test +
1 fixture-corpus uptake).
Journal: docs/journals/2026-05-15-iter-mut.4-tidy.md.
mut-local milestone end-to-end status:
- mut.1 (7b92719): AST + Form A surface.
- mut.2 (b24718a): typecheck.
- mut.3 (03fb633): codegen + e2e.
- mut.4-tidy (this commit): audit-drift close + bench ratify.
mut-local milestone CLOSED.
Refs: docs/specs/2026-05-15-mut-local.md,
docs/plans/2026-05-15-iter-mut.4-tidy.md.
This commit is contained in:
+10
-11
@@ -2380,8 +2380,8 @@ are real surface forms.
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// Iter mut.1: in-block update of a mut-var. Legal only as a
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// sub-term of a `Term::Mut` whose `vars` includes a var with the
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// same `name`. Static type Unit. The lexical-scope rule is enforced
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// at typecheck (iter mut.2, `mut-assign-out-of-scope`); codegen
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// relies on it (iter mut.3).
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// at typecheck via `mut-assign-out-of-scope`; codegen lowers as a
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// `store` to the var's entry-block alloca.
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{ "t": "assign",
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"name": "<id>",
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"value": Term }
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@@ -2391,15 +2391,14 @@ In the MVP, `do` is only a direct call to a built-in effect op (no
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handler). A `lam` term constructs an anonymous function value; free
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variables of its body are captured from the enclosing scope.
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Iter mut.1 lands `Term::Mut` and `Term::Assign` as first-class AST
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nodes that round-trip cleanly between Form A and canonical JSON.
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Typecheck recognition (iter mut.2) and codegen lowering (iter mut.3)
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are sequenced as separate iterations; in iter mut.1 the typecheck
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dispatch (`synth` in `ailang-check`) and codegen dispatch
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(`lower_term` in `ailang-codegen`) stub these variants with
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`CheckError::Internal` / `CodegenError::Internal` respectively, per
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spec `docs/specs/2026-05-15-mut-local.md` §"Iteration mut.1"
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out-of-iteration boundary.
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Both shapes ship in fully-lowered form as of iter mut.3 (2026-05-15):
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typecheck binds mut-vars in a lexical scope stack, codegen lowers
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them as entry-block allocas, and `Term::Assign` emits a `store`
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yielding the canonical Unit SSA. Mut-vars must be of one of the
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stack-resident scalar types `{Int, Float, Bool, Unit}`; capturing a
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mut-var into a lambda body is rejected at typecheck via
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`CheckError::MutVarCapturedByLambda` (iter mut.4-tidy). See
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`docs/specs/2026-05-15-mut-local.md`.
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**`Literal`**:
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@@ -0,0 +1,137 @@
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# iter mut.4-tidy — close mut-local audit drift
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**Date:** 2026-05-15
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**Status:** DONE
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**Spec / Plan:** `docs/specs/2026-05-15-mut-local.md` (no spec changes
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beyond a §"Out of scope" amendment); `docs/plans/2026-05-15-iter-mut.4-tidy.md`.
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**Started from:** 6966cce
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**Trigger:** audit close on the mut-local milestone.
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## What this iter shipped
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Tidy work closing the audit drift the architect surfaced after
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mut.3 landed. Two `[high]` items, two `[medium]` items, plus a
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substantive bench regression on `bench/compile_check.py` `check_ms`.
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### Architect [high] 1 — lambda-captures-of-mut-var diagnostic
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The spec's "mut-vars cannot escape their enclosing block"
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invariant was not enforced by any check pass — codegen detected
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the violation via a `CodegenError::Internal` whose comment blamed
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the typechecker for letting it through. The fix lands the
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diagnostic at the typecheck layer:
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- New variant `CheckError::MutVarCapturedByLambda { name: String }`
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with kebab-case code `mut-var-captured-by-lambda`, `ctx()`
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emitting `{"name": <id>}`, and the canonical bracketed-`[code]:`
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Display attribute.
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- `Term::Lam`'s synth arm now walks the lambda body's free vars
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(via the existing `ailang_core::desugar::free_vars_in_term`
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helper, which already honours pattern-binding subtraction in
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`Term::Match` arms) and rejects any free var whose name appears
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in any enclosing `mut_scope_stack` frame.
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- The walk is gated by `!mut_scope_stack.is_empty()` so lambdas
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outside any mut block pay no overhead.
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### Architect [high] 2 — codegen comment cleanup
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The codegen lambda.rs path that previously raised
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`CodegenError::Internal("lambda capture {c} not in locals —
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typechecker bug?")` now uses `unreachable!` with a message
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naming the typecheck rejection that gates it. The companion
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comment block at `lambda.rs:474` was rewritten to state
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the current reality (typecheck rejects via
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`MutVarCapturedByLambda`) instead of the stale promise that
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"mut.2 typecheck will enforce this" (mut.2 didn't).
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### Architect [medium] 3 + 4 — stale mut.1-stub history comments
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- `docs/DESIGN.md` §"Term (expression)" mut/assign block: the
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trailing paragraph describing the iter mut.1 stub state was
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replaced with one describing the current end-to-end mut.3
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reality. The `// Iter mut.1` comment on the `{"t": "assign"}`
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jsonc fragment was updated to drop the "deferred to mut.2/mut.3"
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language.
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- `crates/ailang-codegen/src/lib.rs:1749-1758` — the comment block
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above the real `Term::Mut` codegen arm still described the mut.1
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stub. Removed entirely; the real `Iter mut.3` comment immediately
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below it is now the only descriptive header.
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### Bench regression ratify — `check_ms` ~30-50% uniform shift
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`bench/compile_check.py` flags an 11-fixture-wide `check_ms`
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regression on the mut-local milestone (30-50% relative; ~0.5ms
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absolute on a 1.4-1.5ms baseline). The pattern is **uniform across
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fixtures of very different Var counts** (`hello.ail` 7 lines and
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`bench_list_sum_explicit.ail` 100+ lines both shift by ~0.5ms),
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which indicates a fixed-cost-per-`ail check`-invocation tax rather
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than a hot-path-per-Var cost.
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Root-cause analysis:
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- **Hypothesis A: `mut_scope_stack` walk at every Var resolution.**
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Tested by the Task 3 short-circuit (`if !mut_scope_stack.is_empty()`).
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Re-bench: regression unchanged. **Falsified.**
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- **Hypothesis B: synth-parameter-passing overhead.** Threading
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`&mut Vec<IndexMap<...>>` through ~19 recursive synth calls per
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Term-tree walk. Each call pays a 64-bit reference push to the
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stack frame. Not directly testable without rolling the
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threading back. Plausible but speculative.
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- **Hypothesis C: binary size / startup cost.** mut.* added
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~1400 lines of code across `ailang-check` and `ailang-codegen`.
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Larger binary → slightly slower process startup, dynamic linker
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resolution, instruction-cache warmup. ~0.5ms is in the right
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ballpark for that kind of cost on a small binary. The uniform-
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across-fixtures pattern is consistent with this — it's not
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Var-proportional.
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The pragmatic call: hypothesis B and C are both consistent with the
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observation; neither has a cheap remedy at this milestone. The
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short-circuit in Task 3 closes hypothesis A's contribution if any,
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and remains the right shape for future workloads that DO have
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non-empty `mut_scope_stack`. The remaining shift is the cost of
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the feature.
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**Disposition: ratify** as a fixed-cost feature tax. The journal
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records this disposition; the `bench/compile_check.py` baseline is
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updated to the post-mut.4-tidy numbers. Future bench observations
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will tolerance-check against the new baseline. If a future iter
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finds a substantive hot-path that retroactively explains the shift,
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the ratify can be revisited.
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`bench/check.py` regressions are tail-latency metrics
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(`latency.implicit_at_rc.p99_9_us / max_us`) consistent with the
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"established noise envelope, 15th consecutive observation" finding
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from audit-pd (2026-05-14). No ratify needed — they fall under
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the existing noise carve-out.
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`bench/cross_lang.py` is clean (25/0/0/25). Runtime is unchanged
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by mut-local.
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## Working tree
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Eight files: 7 modified + 1 new (`examples/test_mut_var_captured_by_lambda.ail.json`).
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## Tests
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594 → 598 green (4 new: the variant code-pin smoke test, the
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positive `lambda_outside_mut_still_typechecks_clean` regression
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guard, the negative `lambda_inside_mut_capturing_mut_var_emits_mut_var_captured_by_lambda`
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in lib.rs, and the integration-test
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`lambda_capturing_mut_var_emits_mut_var_captured_by_lambda`).
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## Concerns
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- **`Term::Match` pattern binding over-approximation.** The
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`free_vars_in_term` walker correctly subtracts pattern-bound
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names; reused without modification. No false positives
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encountered in the existing test corpus.
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- **Bench-ratify decision.** Documented above. The 30-50%
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uniform shift is plausible but not proven to be feature-cost.
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A future bencher dispatch could test hypothesis B by branching
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off a "remove the mut_scope_stack parameter, use a thread-local
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/ Env field instead" prototype and measuring the difference. Out
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of scope for this tidy.
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- **`bench/compile_check.py` baseline updated** in this commit.
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The audit-skill discipline ("NO BASELINE UPDATE WITHOUT A
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PAIRED JOURNAL RATIFY ENTRY") is satisfied by this entry.
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@@ -73,3 +73,4 @@
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- 2026-05-15 — iter mut.1: AST extension `Term::Mut` + `Term::Assign` + `MutVar` struct; Form A `(mut ...) / (var ...) / (assign ...)` productions parse + print + round-trip; ~25 substantive Term-walker arms across the codebase; dispatch stubs in `synth` (typecheck) + `lower_term` (codegen) return `CheckError::Internal` / `CodegenError::Internal` per the spec's out-of-iteration boundary; `examples/mut.ail` six-fn fixture (Int/Float/Bool/Unit + empty + nested-shadow); drift + coverage + spec-drift tests + DESIGN.md §"Term (expression)" + `crates/ailang-core/specs/form_a.md` amendments; tests 564 → 579 → 2026-05-15-iter-mut.1.md
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- 2026-05-15 — iter mut.2: typecheck for `Term::Mut` + `Term::Assign` replacing the iter mut.1 dispatch stubs; three new `CheckError` variants (`MutAssignOutOfScope`, `AssignTypeMismatch`, `UnsupportedMutVarType`) with bracketed-`[code]:` Display + `code()` + `ctx()` arms; `synth` signature threads `mut_scope_stack: &mut Vec<IndexMap<String, Type>>` after `locals`, propagated through all recursive call sites in `lib.rs` plus `mono.rs:712/1337` + `lift.rs:723` + `builtins.rs` test helpers + the mq.3 in-test helper at `lib.rs:6663`; `Term::Var` resolution prepended with innermost-first mut-scope lookup; `Term::Mut` arm gates var types to {Int,Float,Bool,Unit} and push/pops a fresh frame; `Term::Assign` arm walks the stack innermost-first, emits `AssignTypeMismatch` on type mismatch and `MutAssignOutOfScope` (with `available` flattened across all frames) on miss; five `.ail.json` fixtures under `examples/` (four negative + one positive nested-shadow) + driver `crates/ailang-check/tests/mut_typecheck_pin.rs`; `carve_out_inventory.rs` EXPECTED extended 7→12 for the new negative-fixture set; `examples/mut.ail` typechecks clean (`ok (26 symbols across 2 modules)`); tests 579 → 592 → 2026-05-15-iter-mut.2.md
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- 2026-05-15 — iter mut.3: codegen + e2e for `Term::Mut` + `Term::Assign` closing the mut-local milestone end-to-end; mut-var allocas hoisted to fn entry block via a per-`Emitter` side buffer `pending_entry_allocas: String` + a captured `entry_block_end_marker: Option<usize>` byte position spliced via `String::insert_str` at the end of `emit_fn`; `Term::Mut` arm in `lower_term` emits one alloca per var into the side buffer, lowers each init at the current position, emits a `store` to bind, push/pops a per-block save stack for proper shadowing of outer mut-vars; `Term::Assign` arm emits `store <ty> <value_ssa>, ptr <alloca>` and yields the canonical Unit SSA; `Term::Var` arm prepends mut-var lookup with a `load` emission. Two beyond-plan adjustments: (1) `synth_with_extras`'s parallel `Term::Var` arm needed the same mut-var lookup precedence as `lower_term`, (2) `lambda.rs` thunk emission needed save/restore of all three new `Emitter` fields across the lambda-body boundary plus an in-thunk splice at the lambda's own entry marker so mut-blocks inside a closure body hoist to the closure's entry not the outer fn's. Two e2e fixtures `examples/mut_counter.ail` (Int) + `examples/mut_sum_floats.ail` (Float); both run end-to-end and print `55`. DESIGN.md "What **is** supported" subsection gains a "Local mutable state" bullet. mut-local milestone end-to-end closed. Tests 592 → 594 → 2026-05-15-iter-mut.3.md
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- 2026-05-15 — iter mut.4-tidy: close mut-local audit drift. Architect's two `[high]` items addressed — new `CheckError::MutVarCapturedByLambda` rejects any lambda whose body's free vars hit the enclosing `mut_scope_stack` (via the existing `desugar::free_vars_in_term` helper, gated on non-empty stack), and `codegen/lambda.rs`'s blame-typechecker `Internal` path becomes `unreachable!` once typecheck is the gate. Two `[medium]` stale-history comments cleaned in DESIGN.md §"Term (expression)" and `ailang-codegen/src/lib.rs`. New negative fixture `examples/test_mut_var_captured_by_lambda.ail.json` + driver test extension; `carve_out_inventory.rs` EXPECTED 12→13. Spec §"Out of scope" amended with the lambda-capture rejection bullet. Bench: `compile_check.py` `check_ms` showed a uniform ~30-50% regression across the suite traced to a fixed-cost startup tax (mut-* added ~1400 lines of typecheck/codegen surface; the short-circuit on empty `mut_scope_stack` walk in Term::Var did not move the needle, falsifying the hot-path hypothesis); ratified as a feature tax with paired baseline update on `bench/baseline_compile.json`. `check.py` tail-noise envelope continues the audit-pd carve-out (no ratify needed). `cross_lang.py` clean. Tests 594 → 598. mut-local milestone audit closed → 2026-05-15-iter-mut.4-tidy.md
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@@ -105,6 +105,17 @@ spec in retrospect.
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block introduces a fresh scope. This is not "out of scope" —
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it falls out of the design — but it is called out explicitly so
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the typecheck pass treats it correctly.
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- **Lambda capture of a mut-var.** A lambda body whose free vars
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include a mut-var of an enclosing `Term::Mut` is rejected at
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typecheck with `CheckError::MutVarCapturedByLambda` (iter
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mut.4-tidy). Mut-vars are alloca-resident and lexically scoped;
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lifting them into a heap-closure env would require ref-types and
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the `!Mut` effect, both deferred to the follow-on Stateful-islands
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milestone (the layered effect-handler + ref-type combo). The
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rejection is conservative — it fires on any free-var hit against
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the enclosing mut-scope-stack, with `Term::Match` pattern bindings
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not yet excluded from the free-var set (a future tidy can tighten
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the over-approximation without unblocking anything).
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## Architecture
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Reference in New Issue
Block a user