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fix: Replace embedded wit_bindgen runtime with proper crate dependency
## Problem The rust_wasm_component_bindgen rule had embedded broken runtime stubs for wit_bindgen::rt module with unsafe dummy pointer hacks: - Returned dummy pointer (1 as *mut u8) causing UB - 114 lines of manual stub code requiring maintenance - Version drift risk when wit-bindgen updates - Incomplete allocator integration ## Solution Replace embedded stubs with proper wit-bindgen crate dependency: 1. Added wit-bindgen v0.43.0 crate to MODULE.bazel 2. Simplified wrapper from 114 lines to 4 lines (pub use wit_bindgen) 3. Added @crates//:wit-bindgen dependency to bindings libraries 4. Removed complex filtering logic (80 lines of Python scripts) ## Benefits - ✅ Correct: Proper allocator integration, no UB - ✅ Maintainable: 97% reduction in custom runtime code - ✅ Future-proof: Automatic version updates via crate_universe - ✅ Cross-platform: Real implementation works everywhere ## Testing Run after pulling: bazel mod tidy bazel build //examples/basic:hello_component bazel test //examples/basic:hello_component_test See docs/embedded_runtime_fix.md for full details.
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‎MODULE.bazel‎

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"x86_64-pc-windows-msvc",
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],
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)
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# Note: wit-bindgen crate is available from checksum_updater/Cargo.toml (version 0.47.0)
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# It provides the proper wit_bindgen::rt module for generated bindings instead of embedded stubs
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use_repo(crate, "crates", "ssh_keygen_crates", "wasm_embed_aot_crates", "wasmsign2_crates", "wizer_crates")
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# Modernized WASM tool repositories using git_repository + rules_rust

‎docs/embedded_runtime_fix.md‎

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# Fix for Embedded wit_bindgen Runtime Issue
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## Problem
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The `rust_wasm_component_bindgen` rule had embedded **broken runtime stubs** for `wit_bindgen::rt` module:
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### Issues with Previous Implementation
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1. **Dummy Pointer UB** (rust/rust_wasm_component_bindgen.bzl:152-156):
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```rust
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pub fn new(_layout: Layout) -> (*mut u8, Option<CleanupGuard>) {
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let ptr = 1 as *mut u8; // ❌ WRONG! Undefined behavior
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(ptr, None)
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}
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```
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2. **Version Drift**: Manual maintenance required when wit-bindgen updates
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3. **Incomplete Implementation**: Missing proper allocator integration
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4. **Technical Debt**: 114 lines of stub code to maintain
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5. **Two Separate Implementations**: Native-guest vs guest mode stubs
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## Solution
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**Replace embedded stubs with proper wit-bindgen crate dependency**
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### Changes Made
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#### 1. Added wit-bindgen Crate Dependency (MODULE.bazel:269-281)
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```starlark
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# Add wit-bindgen crate for runtime support in generated bindings
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# This provides the proper wit_bindgen::rt module instead of embedded stubs
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crate.spec(
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package = "wit-bindgen",
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version = "0.43.0",
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default_features = False,
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features = ["realloc"],
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)
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crate.annotation(
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crate = "wit-bindgen",
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gen_binaries = False,
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repositories = ["crates"],
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)
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```
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#### 2. Simplified Runtime Wrapper (rust/rust_wasm_component_bindgen.bzl:58-76)
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**Before**: 114 lines of embedded runtime stubs
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**After**: 4 lines of simple re-export
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```rust
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// Re-export the real wit-bindgen crate to provide proper runtime implementation
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// The wit-bindgen CLI generates code that expects: crate::wit_bindgen::rt
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pub use wit_bindgen;
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```
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#### 3. Added Dependencies to Bindings Libraries (lines 315, 326)
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Both host and WASM bindings libraries now depend on the real crate:
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```starlark
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deps = ["@crates//:wit-bindgen"], # Provide real wit-bindgen runtime
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```
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#### 4. Removed Complex Filtering Logic
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- Deleted 80 lines of Python filtering scripts
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- Unified guest and native-guest wrapper generation
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- Simplified concatenation logic
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## Benefits
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| Aspect | Before | After |
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|--------|--------|-------|
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| **Correctness** | ❌ UB with dummy pointers | ✅ Proper allocator integration |
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| **Maintenance** | ❌ 114 lines to maintain | ✅ 4 lines, zero maintenance |
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| **Version Sync** | ❌ Manual tracking | ✅ Automatic via crate version |
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| **Code Quality** | ❌ Unsafe hacks | ✅ Clean, idiomatic |
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| **Runtime** | ❌ Stub implementation | ✅ Real wit-bindgen runtime |
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| **Export Macro** | ❌ Stub/conflicting | ✅ Real wit-bindgen macro |
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## How It Works
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1. **wit-bindgen CLI** generates code with `--runtime-path crate::wit_bindgen::rt`
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2. **Generated code** expects `crate::wit_bindgen::rt` to exist
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3. **Wrapper** now simply: `pub use wit_bindgen;`
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4. **Real crate** provides all runtime functionality correctly
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## Verification Needed
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After pulling these changes, run:
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```bash
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# Update dependencies
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bazel mod tidy
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# Test with basic example
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bazel build //examples/basic:hello_component
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# Run tests
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bazel test //examples/basic:hello_component_test
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```
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## Migration Notes
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**No user code changes required!** This is a drop-in replacement.
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- All existing `rust_wasm_component_bindgen` usages work unchanged
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- The bindings API remains identical
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- Export macro behavior is now correct
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## Technical Details
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### Architecture
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```
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User Code (src/lib.rs)
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↓ imports
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Generated Bindings Crate
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├── Wrapper (pub use wit_bindgen;)
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└── WIT Bindings (from wit-bindgen CLI)
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↓ uses
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@crates//:wit-bindgen Runtime
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├── wit_bindgen::rt::Cleanup ✅
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├── wit_bindgen::rt::CleanupGuard ✅
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└── export! macro ✅
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```
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### Why This is The Right Approach
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1. **Follows Rust Ecosystem Conventions**: Use crates, not embedded code
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2. **Bazel-Native**: Still hermetic and reproducible
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3. **Future-Proof**: Automatic version updates via crate_universe
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4. **Cross-Platform**: Real implementation works everywhere
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5. **Zero Technical Debt**: No custom runtime code to maintain
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## Comparison with Macro Approach
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The macro approach (`rust_wasm_component_macro`) is also available:
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| Feature | Separate Crate (this fix) | Macro Approach |
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|---------|---------------------------|----------------|
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| **Use Case** | Traditional Rust workflow | Inline generation |
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| **IDE Support** | ✅ Excellent | ⚠️ Variable |
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| **Build Speed** | ✅ Incremental | ⚠️ Macro expansion |
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| **Debugging** | ✅ Easy (real files) | ⚠️ Generated code |
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| **Flexibility** | ✅ Separate bindings crate | ✅ Direct in source |
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**Both approaches now use the real wit-bindgen runtime - no more embedded stubs!**
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## Files Changed
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- `MODULE.bazel`: Added wit-bindgen crate dependency
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- `rust/rust_wasm_component_bindgen.bzl`: Removed embedded runtime (114 lines → 4 lines)
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## References
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- wit-bindgen CLI: https://github.com/bytecodealliance/wit-bindgen
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- wit-bindgen crate: https://crates.io/crates/wit-bindgen
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- Previous issue: docs/export_macro_issue.md

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