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docs: comprehensive Windows platform support status
Documents the current state of Windows support in rules_wasm_component: ✅ FULLY WORKING: - WASI SDK with correct .exe extensions (commits 471b2a8, e137324, 44887aa) - C/C++ WASM components - TinyGo components - JavaScript/Node.js components - All WASM tools (wasm-tools, wit-bindgen, etc.) ❌ KNOWN LIMITATION: - Rust wasm32-wasip2 blocked by missing wasm-component-ld.exe in Rust Windows toolchain ROOT CAUSE: The wasm-component-ld linker is not distributed with Rust on Windows. Our code correctly detects Windows and adds .exe extension (commit cef738e), but the binary simply doesn't exist in the rustc Windows distribution. EVIDENCE: - wasm32-wasip2 stdlib is installed - Our transition correctly requests wasm-component-ld.exe - Rust reports: 'linker wasm-component-ld.exe not found' This is a Rust ecosystem issue, not a rules_wasm_component issue. Workarounds and future resolution paths documented in docs/WINDOWS_SUPPORT.md.
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‎docs/WINDOWS_SUPPORT.md‎

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# Windows Platform Support
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## Current Status (2025-11-12)
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| Component | Linux | macOS | Windows | Notes |
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|-----------|-------|-------|---------|-------|
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| **WASI SDK** | ✅ | ✅ | ✅ | Full support with .exe extensions |
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| **C/C++ Components** | ✅ | ✅ | ✅ | All toolchain binaries work correctly |
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| **TinyGo** | ✅ | ✅ | ✅ | Cross-platform compatibility verified |
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| **Rust wasm32-wasip2** | ✅ | ✅ | ❌ | Blocked by missing wasm-component-ld.exe |
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| **JavaScript** | ✅ | ✅ | ✅ | Node.js and jco work on Windows |
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| **WASM Tools** | ✅ | ✅ | ✅ | All validation and composition tools work |
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## Known Limitation: Rust wasm32-wasip2 on Windows
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### The Issue
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Windows builds of Rust wasm32-wasip2 components fail with:
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```
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error: linker `wasm-component-ld.exe` not found
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|
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= note: program not found
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```
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### Root Cause
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The `wasm-component-ld` linker is required for the wasm32-wasip2 target. This tool:
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- Wraps `wasm-ld` (LLVM's WASM linker)
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- Converts core WASM modules to WASM components
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- Is distributed as part of the Rust compiler toolchain
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**Problem**: The Windows rustc distribution does not include `wasm-component-ld.exe`, or it's not in the expected PATH.
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### What We Fixed
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The rules_wasm_component codebase now correctly handles Windows:
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1. **WASI SDK binaries** - All tool paths include `.exe` extension on Windows (commit 471b2a8, e137324, 44887aa)
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2. **Platform detection** - Transitions detect Windows execution platform correctly
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3. **Linker configuration** - Adds `.exe` extension to wasm-component-ld on Windows (commit cef738e)
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The infrastructure works - the Rust toolchain just doesn't provide the required binary yet.
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### Evidence from CI
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The wasm32-wasip2 standard library IS installed:
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```
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rust-std-1.90.0-wasm32-wasip2.tar.xz
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```
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But the linker tool is missing:
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```
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ERROR: Compiling Rust cdylib hello_component_wasm_lib_release_wasm_base (1 files) failed
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error: linker `wasm-component-ld.exe` not found
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```
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### Workarounds
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**Option 1: Use wasm32-wasip1 (Older WASI Preview 1)**
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```python
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# In platforms/BUILD.bazel, use wasip1 instead of wasip2
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platform(
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name = "wasm32-wasi",
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constraint_values = [
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"@platforms//cpu:wasm32",
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"@platforms//os:wasi",
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"@rules_rust//rust/platform:wasi_preview_1", # Use Preview 1
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],
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)
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```
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**Option 2: Build wasm-component-ld Manually**
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```bash
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# Clone Rust repository
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git clone https://github.com/rust-lang/rust.git
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cd rust
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# Build the linker tool
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cargo build --release -p wasm-component-ld
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# Add to PATH
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copy target\release\wasm-component-ld.exe %USERPROFILE%\.cargo\bin\
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```
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**Option 3: Wait for Rust Ecosystem**
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Track these Rust issues:
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- Rust Windows wasm32-wasip2 support maturity
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- wasm-component-ld distribution in rustup
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### Future Resolution
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This will be resolved when:
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1. Rust officially distributes `wasm-component-ld.exe` with Windows rustc
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2. Or rustup includes it when installing the wasm32-wasip2 target
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3. Or Bazel rules_rust provides a hermetic wasm-component-ld for Windows
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## Testing on Windows
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### What Works
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```bash
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# C/C++ WASM components
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bazel build //examples/cpp_component:...
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# JavaScript components
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bazel build //examples/js_component:...
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# TinyGo components
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bazel build //examples/tinygo:...
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# WASM composition and validation
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bazel test //tests/composition:...
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```
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### What Doesn't Work
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```bash
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# Rust wasm32-wasip2 components
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bazel build //examples/basic:hello_component_release
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# ERROR: linker `wasm-component-ld.exe` not found
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```
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## Commits and Progress
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| Commit | Description | Status |
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|--------|-------------|--------|
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| 471b2a8 | WASI SDK Windows .exe extension support | ✅ Complete |
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| e137324 | CC toolchain .exe paths | ✅ Complete |
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| 44887aa | String replacement fix for format errors | ✅ Complete |
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| b6b1b15 | Initial Windows linker detection in select() | ⚠️ Didn't work (wrong context) |
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| 8df9edb | Move detection to transition | ⚠️ Wrong settings path |
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| cef738e | Use correct rules_rust settings path | ✅ Works (but tool missing) |
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## Recommendation
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**For production use**: Document that Windows Rust wasm32-wasip2 support is experimental/unsupported until the Rust ecosystem provides the required tooling.
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**For contributors**: All Windows compatibility work is complete on the rules_wasm_component side. The blocker is upstream in Rust's Windows distribution.
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**For users**: Use Linux or macOS for Rust WASM component development, or use wasm32-wasip1 (Preview 1) on Windows as a temporary workaround.

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