dimgigov
e5e490605a
- 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.
124 lines
4.4 KiB
Markdown
124 lines
4.4 KiB
Markdown
# Drop Trait / Destructors Design Document
|
|
|
|
> **Date:** 2026-06-10
|
|
> **Status:** Superseded — see [2026-06-14-drop-interface-auto-drop-design.md](2026-06-14-drop-interface-auto-drop-design.md)
|
|
> **Scope:** Selfhost compiler (`src/*.bux`)
|
|
|
|
## 1. Problem Statement
|
|
|
|
The `@[Drop]` attribute and auto-drop mechanism already exist in the selfhost compiler, but they are **restricted to `@[Checked]` functions only**. This means regular (unchecked) code does not receive automatic cleanup for resource-holding types. The programmer must manually call `TypeName_Drop()` or use `defer`.
|
|
|
|
This is inconsistent with the goal of making Bux "safe by choice" — if a type declares it needs cleanup (`@[Drop]`), that cleanup should happen regardless of whether the function is checked.
|
|
|
|
## 2. Goals
|
|
|
|
1. **Universal auto-drop**: `@[Drop]` types are automatically cleaned up when they go out of scope in **all** functions, not just `@[Checked]` ones.
|
|
2. **Preserve move semantics**: Variables that have been moved are NOT double-freed (existing `CBE_IsMoved` logic stays).
|
|
3. **No breaking changes**: Existing code continues to work; we only *add* cleanup where it was previously skipped.
|
|
4. **Selfhost loop compatibility**: The compiler must still bootstrap deterministically after these changes.
|
|
|
|
## 3. Non-Goals
|
|
|
|
1. **`own T` type**: Adding `own T` to the selfhost type system is a separate, larger feature. This design works with the existing value-semantics + `@[Drop]` model.
|
|
2. **Scoped defers on break/continue**: The selfhost C backend currently does not emit defers on `break`/`continue` (unlike bootstrap). Fixing this is tracked separately.
|
|
3. **Drop trait interface dispatch**: Auto-drop uses static lookup of `TypeName_Drop`, not interface vtables. This is intentional — zero runtime cost.
|
|
|
|
## 4. Architecture
|
|
|
|
### 4.1 Before (Current)
|
|
|
|
```bux
|
|
// hir_lower.bux:1118-1139
|
|
if ctx.checkedFunc && !String_Eq(alloca.typeName, "") {
|
|
// build auto-drop defer
|
|
}
|
|
```
|
|
|
|
Only checked functions get auto-drop.
|
|
|
|
### 4.2 After (Proposed)
|
|
|
|
```bux
|
|
// hir_lower.bux:1118-1139
|
|
if !String_Eq(alloca.typeName, "") {
|
|
// build auto-drop defer
|
|
}
|
|
```
|
|
|
|
All functions get auto-drop for `@[Drop]` types.
|
|
|
|
### 4.3 How It Works
|
|
|
|
1. **Parser**: `@[Drop]` attribute on struct → `decl.isDrop = 1`
|
|
2. **HIR Lowering** (`hir_lower.bux`):
|
|
- When lowering a `let` binding, after the `hStore`/ `hAlloca`:
|
|
- Call `Lcx_BuildAutoDropFree(ctx, typeName)` to find the drop function name
|
|
- If found, wrap it in `hDefer` and append to statement chain
|
|
- **Remove** the `ctx.checkedFunc` gate
|
|
3. **C Backend** (`c_backend.bux`):
|
|
- `CBE_EmitDefers` already handles:
|
|
- LIFO order
|
|
- Skip moved variables (`CBE_IsMoved`)
|
|
- No changes needed in C backend
|
|
|
|
## 5. Affected Files
|
|
|
|
| File | Change |
|
|
|------|--------|
|
|
| `src/hir_lower.bux` | Remove `ctx.checkedFunc &&` condition on line 1120 |
|
|
| `_test_drop_user/src/Main.bux` | Remove `@[Checked]` from test to verify universal drop |
|
|
| `_test_drop_trait/src/Main.bux` | Fix syntax (already broken — `module Main;` + indexing) |
|
|
|
|
## 6. Testing Plan
|
|
|
|
### 6.1 Existing Tests
|
|
|
|
| Test | Expected |
|
|
|------|----------|
|
|
| `_test_drop_user` | Should pass WITHOUT `@[Checked]` |
|
|
| `_test_drop_move` | Should still skip drop for moved vars |
|
|
| `selfhost-loop` | C output must remain identical |
|
|
|
|
### 6.2 New Test (Optional)
|
|
|
|
Create `_test_drop_universal/src/Main.bux`:
|
|
```bux
|
|
@[Drop]
|
|
struct Resource {
|
|
id: int
|
|
}
|
|
|
|
var dropCount: int = 0;
|
|
|
|
func Resource_Drop(self: *Resource) {
|
|
dropCount = dropCount + 1;
|
|
}
|
|
|
|
func Main() -> int {
|
|
// NOT @[Checked]
|
|
let r: Resource = Resource { id: 1 };
|
|
// r should be auto-dropped here
|
|
return dropCount; // expect 1
|
|
}
|
|
```
|
|
|
|
## 7. Risks & Mitigations
|
|
|
|
| Risk | Mitigation |
|
|
|------|------------|
|
|
| Existing unchecked code now gets implicit drops | This is the intended behavior. Types with `@[Drop]` explicitly opt-in. |
|
|
| Performance impact from extra defer calls | Minimal — defers are inlined as direct C calls. No runtime overhead. |
|
|
| Selfhost loop breaks | Test `make selfhost-loop` before committing. |
|
|
|
|
## 8. Future Work
|
|
|
|
1. **`own T` in selfhost**: Add `own T` type expression + move-on-pass semantics (like bootstrap).
|
|
2. **Scoped defers on break/continue**: Match bootstrap behavior.
|
|
3. **Drop trait interface check**: Verify at compile time that `@[Drop]` types actually implement `Drop` interface (has `TypeName_Drop` method).
|
|
|
|
## 9. Placeholder Scan
|
|
|
|
- No TBD/TODO placeholders.
|
|
- All file paths are exact.
|
|
- All code is copy-paste ready.
|