bux-lang/docs/superpowers/plans/2026-06-14-drop-interface-auto-drop.md

Drop Interface Auto-Drop Implementation Plan

For agentic workers: REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (- [ ]) syntax for tracking.

Goal: Make the selfhost compiler emit automatic destructor calls for types that implement Drop via extend Type for Drop, not only for types marked with @[Drop].

Architecture: Extend Lcx_BuildAutoDropFree in src/hir_lower.bux to detect a TypeName_Drop function symbol registered by sema for interface-based implementations. Reuse the existing generic monomorphization and C-backend defer paths.

Tech Stack: Bux selfhost compiler (Bux language), Nim bootstrap compiler, C backend, GNU make.

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File map

File	Responsibility
src/hir_lower.bux	Detects TypeName_Drop methods and wires them into auto-drop defers.
_test_interface_drop/src/Main.bux	Existing integration test that should now emit Buffer_Drop(&buf).
_test_drop_user/src/Main.bux	Existing test using @[Drop]; must keep working.
_test_drop_move/src/Main.bux	Existing test verifying moved vars are not double-dropped; must keep working.
_test_drop_trait/src/Main.bux	Existing test for Array<int> auto-drop; must keep working.

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Task 1: Verify the current failure

Files:

• Read: src/hir_lower.bux:2745-2760

• Read: _test_interface_drop/build/main.c (after build)

• Step 1: Build the bootstrap compiler

cd /home/ziko/z-git/bux/bux
make build

Expected: buxc is created at project root, build succeeds.

• Step 2: Compile _test_interface_drop and inspect generated C

cd /home/ziko/z-git/bux/bux/_test_interface_drop
../buxc run 2>&1 | tail -5
grep -n "Buffer_Drop" build/main.c

Expected: Buffer_Drop function is declared/defined, but main() does not call it before return 0. This confirms the bug.

• Step 3: Commit the baseline observation (optional)

No file changes yet; skip commit or note the finding in a scratch file.

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Task 2: Extend Lcx_BuildAutoDropFree to detect interface Drop

Files:

• Modify: src/hir_lower.bux:2745-2760

• Step 1: Read the exact current code

cd /home/ziko/z-git/bux/bux
sed -n '2740,2765p' src/hir_lower.bux

Current code (approximate):

    // 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");
        // Ensure inner free is also monomorphized since Drop calls Free
        if String_StartsWith(dropName, "Array_Drop_") {
            let elemType: String = Lcx_GetMangledSuffix(dropName, "Array_Drop_");
            let genDrop: *Decl = Lcx_FindGenericFunc(ctx, "Array_Drop");
            if genDrop != null as *Decl {
                Lcx_GenerateFuncInstance(ctx, genDrop, elemType, "", 1);
            }
        }
        return dropName;
    }

• Step 2: Modify the check to include interface-based Drop

Replace the block with:

    // 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 {
            // Ensure inner free is also monomorphized since Drop calls Free
            if String_StartsWith(dropName, "Array_Drop_") {
                let elemType: String = Lcx_GetMangledSuffix(dropName, "Array_Drop_");
                let genDrop: *Decl = Lcx_FindGenericFunc(ctx, "Array_Drop");
                if genDrop != null as *Decl {
                    Lcx_GenerateFuncInstance(ctx, genDrop, elemType, "", 1);
                }
            }
            return dropName;
        }
    }

This preserves the @[Drop] path and adds the interface path.

• Step 3: Rebuild the bootstrap compiler

cd /home/ziko/z-git/bux/bux
make build

Expected: build succeeds.

• Step 4: Verify _test_interface_drop now emits the drop call

cd /home/ziko/z-git/bux/bux/_test_interface_drop
rm -rf build
../buxc run 2>&1 | tail -5
grep -n "Buffer_Drop" build/main.c

Expected: main() now contains Buffer_Drop(&buf); before return 0;.

• Step 5: Commit the implementation

cd /home/ziko/z-git/bux/bux
git add src/hir_lower.bux
git commit -m "feat(selfhost): auto-drop for interface-based Drop implementations"

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Task 3: Regression-test existing Drop tests

Files:

• Test: _test_drop_user/src/Main.bux

• Test: _test_drop_move/src/Main.bux

• Test: _test_drop_trait/src/Main.bux

• Step 1: Compile _test_drop_user

cd /home/ziko/z-git/bux/bux/_test_drop_user
rm -rf build
../buxc run 2>&1 | tail -5

Expected: builds and runs without error.

• Step 2: Compile _test_drop_move

cd /home/ziko/z-git/bux/bux/_test_drop_move
rm -rf build
../buxc run 2>&1 | tail -5

Expected: builds and runs without double-free / crash.

• Step 3: Compile _test_drop_trait

cd /home/ziko/z-git/bux/bux/_test_drop_trait
rm -rf build
../buxc run 2>&1 | tail -5

Expected: prints 30 and exits cleanly.

• Step 4: Commit if all pass

If any fail, debug before committing. If all pass:

cd /home/ziko/z-git/bux/bux
git add -A
git commit -m "test: verify existing Drop tests still pass"

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Task 4: Selfhost-loop determinism check

Files:

• All src/*.bux

• Step 1: Run the selfhost loop

cd /home/ziko/z-git/bux/bux
make selfhost-loop

Expected output:

Selfhost loop passed: C output is identical.

• Step 2: If it fails, diff the C outputs

diff -u build/selfhost-loop-a/build/main.c build/selfhost-loop-b/build/main.c | head -80

Investigate any difference. Likely causes:

• Variable naming drift — not expected from this change.

• New monomorphization triggered by the change — acceptable only if deterministic.

• Step 3: Commit after passing

cd /home/ziko/z-git/bux/bux
git add -A
git commit -m "ci: selfhost-loop passes with Drop interface auto-drop"

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Spec coverage check

Spec requirement	Implementing task
Universal Drop recognition	Task 2
No syntax changes	No task needed
Preserve move semantics	Task 2 reuses existing path; Task 3 verifies _test_drop_move
Selfhost loop compatibility	Task 4

Placeholder scan

• No TBD/TODO/"implement later" markers.
• Each step includes exact file paths and commands.
• Code blocks contain the actual change.

Type consistency check

• Scope_Lookup returns Symbol — used correctly.
• Symbol.kind == skFunc matches existing patterns in the codebase.
• typeSym.decl.isDrop != 0 preserved from original code.