feat(selfhost): auto-drop for interface-based Drop implementations

- Extend Lcx_BuildAutoDropFree to detect TypeName_Drop methods registered by sema for extend Type for Drop impl blocks. - Preserve existing @[Drop] attribute path and move semantics. - Add design doc and implementation plan. - Mark Destructors/Drop roadmap item done in selfhost.
2026-06-14 17:44:35 +03:00
parent 2c8223f94e
commit e5e490605a
5 changed files with 406 additions and 12 deletions
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+# Drop Interface Auto-Drop Design Document
+
+> **Date:** 2026-06-14  
+> **Status:** Approved  
+> **Scope:** Selfhost compiler (`src/*.bux`)
+
+## 1. Problem Statement
+
+Bux already has two ways to declare that a type needs automatic cleanup:
+
+1. **`@[Drop]` attribute** on a struct — triggers auto-drop lookup of `TypeName_Drop`.
+2. **`extend Type for Drop { ... }`** — implements the `Drop` interface from `lib/Drop.bux`.
+
+The parser and sema already support both forms, but the HIR lowering step only recognizes the `@[Drop]` attribute. As a result, a type that implements `Drop` via the interface is **not** automatically cleaned up when its variables go out of scope.
+
+Example (`_test_interface_drop/src/Main.bux`):
+
+```bux
+import Drop
+
+struct Buffer {
+    ptr: *int
+}
+
+extend Buffer for Drop {
+    func Drop(self: *Buffer) {
+        bux_free(self.ptr as *void);
+    }
+}
+
+func main() -> int {
+    let buf: Buffer = Buffer { ptr: bux_alloc(10 as uint * sizeof(int)) as *int };
+    return 0;  // Buffer_Drop(&buf) should be emitted here, but currently is not
+}
+```
+
+## 2. Goals
+
+1. **Universal Drop recognition**: Auto-drop fires for any type that implements `Drop`, whether via `@[Drop]` or via `extend ... for Drop`.
+2. **No syntax changes**: The existing `interface Drop` and `extend ... for Drop` syntax remains unchanged.
+3. **Preserve move semantics**: Variables that have been moved are still skipped by the existing `CBE_IsMoved` logic.
+4. **Selfhost loop compatibility**: The compiler must still bootstrap deterministically.
+
+## 3. Non-Goals
+
+1. **`own T` type**: Adding `own T` to selfhost is a separate, larger feature.
+2. **Scoped defers on break/continue**: Tracked separately.
+3. **Interface vtable dispatch for auto-drop**: Auto-drop remains static lookup of `TypeName_Drop`; no runtime interface cost.
+
+## 4. Architecture
+
+### 4.1 Before (Current)
+
+In `src/hir_lower.bux`, `Lcx_BuildAutoDropFree` only returns a drop function name when `decl.isDrop != 0`:
+
+```bux
+// User-defined types with @[Drop]: look for TypeName_Drop function
+if typeSym.kind == skType && typeSym.decl != null as *Decl && typeSym.decl.isDrop != 0 {
+    let dropName: String = String_Concat(typeName, "_Drop");
+    // ... generic monomorphization ...
+    return dropName;
+}
+```
+
+### 4.2 After (Proposed)
+
+Extend the user-type branch to also detect interface-based implementations:
+
+```bux
+// User-defined types with @[Drop] OR with an explicit TypeName_Drop method
+if typeSym.kind == skType && typeSym.decl != null as *Decl {
+    let dropName: String = String_Concat(typeName, "_Drop");
+    let hasAttr: bool = typeSym.decl.isDrop != 0;
+    let dropSym: Symbol = Scope_Lookup(ctx.scope, dropName);
+    let hasMethod: bool = dropSym.kind == skFunc;
+
+    if hasAttr || hasMethod {
+        // ... generic monomorphization ...
+        return dropName;
+    }
+}
+```
+
+Sema already registers methods from `extend ... for Drop` as `TypeName_Drop` function symbols in `ctx.scope` (see `src/sema.bux:1197-1207`). Therefore, looking up the method by name is sufficient to detect an interface-based Drop implementation.
+
+### 4.3 Generic Types
+
+For generic `extend Box<T> for Drop { func Drop(self: *Box<T>) { ... } }`, the symbol registered is `Box_Drop` (generic). The existing `Lcx_FindGenericFunc` / `Lcx_GenerateFuncInstance` path in `Lcx_BuildAutoDropFree` handles monomorphization when the concrete type is a mangled generic instance (e.g., `Box_int`). The proposed change reuses that path unchanged.
+
+## 5. Affected Files
+
+| File | Change |
+|------|--------|
+| `src/hir_lower.bux` | Extend `Lcx_BuildAutoDropFree` to detect `TypeName_Drop` method symbols even without `@[Drop]` attribute. |
+| `_test_interface_drop/src/Main.bux` | Verify that `Buffer_Drop(&buf)` is emitted. |
+
+## 6. Testing Plan
+
+### 6.1 Existing Tests
+
+| Test | Expected |
+|------|----------|
+| `_test_drop_user` | Still passes; `@[Drop]` path unchanged. |
+| `_test_drop_move` | Still skips drop for moved vars. |
+| `_test_drop_trait` | Still passes; Array auto-drop unchanged. |
+| `make selfhost-loop` | C output remains identical. |
+
+### 6.2 Target Test
+
+`_test_interface_drop/src/Main.bux` should compile and the generated `build/main.c` should contain a call to `Buffer_Drop(&buf)` before `return 0`.
+
+## 7. Risks & Mitigations
+
+| Risk | Mitigation |
+|------|------------|
+| Any method named `Drop` triggers auto-drop | Acceptable because `Drop` is a privileged interface name. If stricter checking is needed later, sema can record explicit `extend ... for Drop` implementations in a dedicated table. |
+| Generic monomorphization missed | Reuse existing generic Drop instance generation path already used for `@[Drop]` types. |
+| Selfhost loop breaks | Run `make selfhost-loop` before committing. |
+
+## 8. Future Work
+
+1. **`own T` in selfhost**: Add `own T` type expression + move-on-pass semantics.
+2. **Scoped defers on break/continue**: Match bootstrap behavior.
+3. **Explicit interface-implementation table**: If the simple method-name lookup becomes too permissive, record `extend ... for Drop` facts in sema and expose them to HIR lowering.
+
+## 9. Placeholder Scan
+
+- No TBD/TODO placeholders.
+- All file paths are exact.
+- Code snippets match the current selfhost source.