fix(bootstrap): implement collection for-in lowering
- Export typeToTypeExpr from sema and preserve generic type args - Derive loop variable type from collection element type in sema - Substitute generic struct type params on field access - Add getCollectionElementTypeExpr helper in hir_lower - Replace placeholder collection for-in lowering - Add Array/Iter lowering: Array_Iter_T / Iter_HasNext_T / Iter_Next_T - Add Channel lowering: Channel_Recv_Ok_T loop - Register loop variable in varTypeExprs before body lowering Fixes _test_forin_stdlib, _test_forin_channel, _test_generic_trait, _test_import, _test_mono
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# Fix `for ... in` Iterator Lowering in Bootstrap
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> **Date:** 2026-06-14
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> **Status:** Approved
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> **Scope:** Bootstrap compiler (`bootstrap/hir_lower.nim`) — `for <var> in <collection>` loops
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## 1. Problem Statement
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The `for ... in` collection iterator is already parsed and sema-accepted, but the HIR→C lowering produces broken code for concrete generic collections such as `Array<int>` or `Channel<int>`.
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Five integration tests fail because of this:
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- `_test_forin_stdlib`: C compilation error `‘x’ undeclared` inside `Main`.
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- `_test_forin_channel`: C compilation error `‘msg’ undeclared` inside `Main`.
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- `_test_generic_trait`: sema error `cannot assign float32 to int at 12:9` — the loop variable `x` is typed as `float32` instead of `int`.
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- `_test_import`: sema error `cannot assign float32 to int at 15:9` — same issue.
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- `_test_mono`: sema error `cannot assign float32 to int at 38:9` — same issue.
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Two independent bugs are at play:
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1. **Missing C declaration:** the loop variable is not emitted as a local variable in the generated C function.
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2. **Wrong loop-variable type:** generic `Iter_Next_T` returns an unsubstituted type parameter `T`, which sema treats as `float32` by default.
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## 2. Goals
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1. Make `_test_forin_stdlib` pass.
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2. Make `_test_forin_channel` pass.
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3. Make `_test_generic_trait`, `_test_import`, and `_test_mono` pass (or at least no longer fail with the `float32` loop-variable error).
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4. Keep the fix minimal and confined to the bootstrap compiler.
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5. Ensure `make test` and `make selfhost-loop` continue to pass.
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## 3. Non-Goals
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1. Rewriting the `apps/` example applications (excluded per user request).
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2. Adding new language features such as `break`/`continue` in loops or iterator traits.
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3. Changing the selfhost compiler (`src/*.bux`) unless required for selfhost-loop parity.
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4. Fixing the unrelated `_test_slice` C typedef bug (tracked separately).
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5. Implementing full Destructors / Drop trait (next roadmap milestone after stabilization).
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## 4. Background: How `for ... in` Is Lowered
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The parser produces `stmtForIn(ident, expr, body)`.
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Sema checks the collection expression and infers the loop-variable type from it.
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HIR lowering currently desugars the loop to a placeholder infinite `hLoop` that ignores the iterator expression and never declares the loop variable.
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The intended lowering depends on the collection type:
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- **Array / Iter collections** (from `lib/Iter.bux`):
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```bux
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let __iter = Array_Iter_T(&collection);
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while (Iter_HasNext_T(&__iter)) {
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let x = Iter_Next_T(&__iter);
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// body
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}
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```
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- **Channel collections** (from `lib/Channel.bux`):
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```bux
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var x: T;
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while (true) {
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if (!Channel_Recv_Ok_T(&ch, &x)) { break; }
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// body
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}
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```
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## 5. Investigation Plan
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1. Read `_test_forin_stdlib/src/Main.bux`, `_test_forin_channel/src/Main.bux`, `_test_generic_trait/src/Main.bux`.
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2. Locate `stmtForIn` handling in `bootstrap/hir_lower.nim`.
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3. Verify how the iterator variable is introduced into HIR/C:
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- Is it registered in `ctx.varTypes` / `ctx.varTypeExprs`?
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- Is a `hirLet` emitted before the loop body?
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4. Verify how `Array_Iter_Next_T` is monomorphized:
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- Does the call get the `_int` suffix?
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- Does the returned `T` get substituted with `int`?
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5. Identify the minimal edit that fixes both symptoms.
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## 6. Hypothesis
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### 6.1 Missing Declaration
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The lowering creates the loop body with references to the loop variable, but the declaration site is omitted or not added to the function's local-variable list. As a result, the C backend never emits `int x;`, causing the `undeclared identifier` error.
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### 6.2 Wrong Type
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When the collection is `Array<int>`, the lowering calls `Array_Iter_Next` (without `_int`) or calls a monomorphized `Array_Iter_Next_int` but assigns its result to a variable whose type was resolved from the generic signature (`T`). Because sema does not substitute `T` for the loop variable, the variable defaults to `float32`.
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The fix is likely to:
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- Explicitly declare the loop variable with the concrete element type derived from the collection type.
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- Ensure `Array_Iter` / `Array_Iter_HasNext` / `Array_Iter_Next` are monomorphized with the collection's type arguments.
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## 7. Proposed Fix
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The fix spans sema and HIR lowering:
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1. **Sema fixes**:
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- In `skFor`, derive the loop-variable type from the collection's element type (`Array<T>` / `Channel<T>` inner type).
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- In `ekField` for structs, build a type-parameter substitution map from the object's concrete type args and apply it to the field type, so `arr.data` on `Array<int>` resolves to `*int`.
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- Fix `typeToTypeExpr` to preserve generic type arguments on named types.
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2. **HIR lowering**:
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- Replace the placeholder `skFor` collection branch with real lowering that selects by collection type.
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- **Array / Iter collections**:
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- Extract element type from `Array<T>` or `Iter<T>`.
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- Generate or reuse `Iter_T` struct instance.
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- Generate function instances `Array_Iter_T`, `Iter_HasNext_T`, `Iter_Next_T`.
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- If the collection expression is not a simple identifier, spill it to a temporary `Array_T` variable first.
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- Emit `alloca __iter`, store `Array_Iter_T(&collection)`, then `while (Iter_HasNext_T(&__iter)) { let x = Iter_Next_T(&__iter); body }`.
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- Register `x` in `ctx.varTypeExprs` before lowering the body.
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- **Channel collections**:
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- Extract element type from `Channel<T>`.
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- Generate function instance `Channel_Recv_Ok_T`.
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- Emit `alloca x`, then `while (true) { if (!Channel_Recv_Ok_T(&ch, &x)) break; body }`.
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## 8. Affected Files
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| File | Expected Change |
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|------|-----------------|
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| `bootstrap/sema.nim` | Derive loop var type; substitute struct type params on field access; preserve type args in `typeToTypeExpr`. |
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| `bootstrap/hir_lower.nim` | Replace placeholder collection `skFor` lowering with Array/Iter and Channel lowerings. |
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| `_test_forin_stdlib/src/Main.bux` | No changes. |
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| `_test_forin_channel/src/Main.bux` | No changes. |
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| `_test_generic_trait/src/Main.bux` | No changes. |
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| `_test_import/src/Main.bux` | No changes. |
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| `_test_mono/src/Main.bux` | No changes. |
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## 9. Testing Plan
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### 9.1 Target Tests
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```bash
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cd _test_forin_stdlib && /home/ziko/z-git/bux/bux/buxc run
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cd _test_forin_channel && /home/ziko/z-git/bux/bux/buxc run
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cd _test_generic_trait && /home/ziko/z-git/bux/bux/buxc run
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cd _test_import && /home/ziko/z-git/bux/bux/buxc run
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cd _test_mono && /home/ziko/z-git/bux/bux/buxc run
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```
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Expected: all exit 0.
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### 9.2 Regression Tests
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```bash
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cd /home/ziko/z-git/bux/bux
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make test
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make selfhost-loop
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```
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Expected: all pass / no changes in selfhost-loop output.
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## 10. Success Criteria
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- `_test_forin_stdlib` reports PASS.
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- `_test_forin_channel` reports PASS.
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- `_test_generic_trait`, `_test_import`, `_test_mono` no longer fail with the `float32` loop-variable error.
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- `make test` reports no new failures.
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- `make selfhost-loop` remains deterministic.
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## 11. Risks & Mitigations
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| Risk | Mitigation |
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|------|------------|
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| Fix changes loop lowering and breaks range-based `for i in 0..10` | Keep range-based path separate; only touch `stmtForIn` collection path. |
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| Same bug exists in selfhost compiler | Selfhost-loop will catch output differences; fix selfhost only if required. |
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| Root cause is deeper than loop lowering | Time-box investigation; report findings if not resolved quickly. |
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## 12. Relation to Other Work
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- Previous commit `06db492` fixed generic `operator []` resolution. This work continues generic-type propagation into iterator loops.
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- `_test_slice` has a separate C typedef bug and is out of scope for this task.
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