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iter mut.3: codegen + e2e — mut-local milestone end-to-end closed

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Replaces the iter mut.1 dispatch stubs in lower_term with real LLVM
lowering: mut-vars become alloca slots hoisted to the fn's entry
block, assign lowers to store, mut-var reads lower to load. Two
example fixtures run end-to-end printing 55 (Int via mut_counter,
Float via mut_sum_floats).

Concretely:

- Emitter struct gains three new fields: mut_var_allocas:
  BTreeMap<String, (String, Type)> for per-fn name→(alloca SSA,
  AIL type) tracking; pending_entry_allocas: String as a side
  buffer accumulating alloca instructions during body lowering;
  entry_block_end_marker: Option<usize> recording the byte position
  in self.body immediately after the entry: label.

- start_block(label) captures the marker when label == 'entry'.
  emit_fn resets all three new fields per fn body. At the end of
  emit_fn, String::insert_str splices pending_entry_allocas into
  self.body at the marker — alloca instructions land in the entry
  block regardless of where in the source tree Term::Mut was
  encountered (mem2reg eligibility preserved).

- Term::Mut arm: per var, fresh-name an alloca SSA, push the
  alloca instruction into pending_entry_allocas, lower the init at
  the current body position, emit a store to bind. The mut-var
  binding is NOT visible during init (matches typecheck-side
  ordering at lib.rs:3576). After all vars are bound, lower the
  body in the extended scope. On block exit, restore any prior
  bindings via a per-block save stack — supports nested
  shadowing correctly.

- Term::Assign arm: look up the alloca + type via
  mut_var_allocas.get(name); lower the value; emit store; yield
  the canonical Unit SSA per the existing Term::Lit { lit:
  Literal::Unit } convention.

- Term::Var arm: prepended with a mut-var lookup that emits a
  load <ty>, ptr <alloca> and returns the load SSA. Innermost-wins
  shadowing falls out of the BTreeMap's insert-overwrites
  semantics combined with the per-block save/restore.

- examples/mut_counter.ail + examples/mut_sum_floats.ail:
  recursive sum_helper accumulates 1..10 (Int) or 1.0..10.0 (Float)
  inside a mut block whose single var is assigned the helper's
  result. Both run end-to-end printing 55.

- crates/ail/tests/e2e.rs: mut_counter_prints_55 and
  mut_sum_floats_prints_55 #[test] fns using the existing
  build_and_run helper.

- DESIGN.md 'What is supported' subsection: 'Local mutable state'
  bullet describes the new construct, points at the two example
  fixtures, and reaffirms the seal-by-construction invariant under
  Decision 10.

Beyond-plan adjustments absorbed in this iter:
  - synth_with_extras's parallel Term::Var arm in
    ailang-codegen/src/lib.rs needed the same mut-var lookup as
    lower_term's. Plan only specified lower_term's arm.
  - crates/ailang-codegen/src/lambda.rs needed save/restore of all
    three new Emitter fields across the lambda-body boundary plus
    in-thunk splice of pending_entry_allocas at the lambda's own
    entry marker, so mut-blocks inside a closure body hoist into
    the closure's entry block (not the outer fn's).

mut-local milestone end-to-end status:
  - mut.1 (7b92719): AST + Form A surface.
  - mut.2 (b24718a): typecheck.
  - mut.3 (this commit): codegen + e2e.
  Audit follows.

Tests: 592 → 594 green; cargo build green; both fixtures execute
and print 55.

Journal: docs/journals/2026-05-15-iter-mut.3.md.

Refs: docs/specs/2026-05-15-mut-local.md, docs/plans/2026-05-15-iter-mut.3.md.
This commit is contained in:
Brummel
2026-05-15 01:56:26 +02:00
parent b057789b55
commit 03fb633d85
9 changed files with 454 additions and 10 deletions
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docs/DESIGN.md
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@@ -2831,6 +2831,26 @@ What **is** supported (and used as the smoke test for the pipeline):
arg types (ctor) or the scrutinee's `Type::Con.args` (match). An
unresolved `Type::Var` reaching `llvm_type` is a hard error
rather than a silent fallback to `ptr`.
- **Local mutable state.** `(mut (var <name> <type> <init>) ... <body>)`
declares lexically-scoped mutable bindings whose types are restricted
to the stack-resident scalars `Int`, `Float`, `Bool`, `Unit`. Inside
the block, `(assign <name> <value>)` updates a mut-var; references
to a mut-var name resolve to a `load` at codegen. Mut-vars are
alloca-resident — the `alloca` instructions are hoisted to the fn's
entry block via a per-fn side buffer spliced after `lower_term` so
mem2reg eligibility holds regardless of how deeply nested the
originating `Term::Mut` block sits. Exercised end-to-end by
`examples/mut_counter.ail` (Int) and `examples/mut_sum_floats.ail`
(Float). Mut-vars do not escape the enclosing mut block and do not
introduce a `!Mut` effect onto the surrounding fn signature.
Sealed-by-construction: the spec restricts var element types to
non-RC-managed primitives and forbids first-class references, so
Decision 10's "no shared mutable refs" invariant is preserved (each
mut-var is uniquely held by its enclosing block). Future milestones
on the Stateful-islands path lift the type restriction (heap-RC-
managed Str + ADTs), add escaping references (`ref a` + `!Mut`
effect), and introduce composable transducers (`Stateful a b` +
`pipe`). Spec: `docs/specs/2026-05-15-mut-local.md`.
Pipeline regression smoke tests: