Windows: wasm-component-ld.exe missing from rustc_lib filegroup causes wasm32-wasip2 builds to fail

[avrabe]

Summary

Windows builds targeting wasm32-wasip2 fail with error: linker 'wasm-component-ld.exe' not found because the rustc filegroup patterns in rust/private/repository_utils.bzl don't include wasm-component-ld{binary_ext}.

Root Cause

The rustc_lib filegroup in repository_utils.bzl currently declares:

"lib/rustlib/{target_triple}/bin/gcc-ld/*",
"lib/rustlib/{target_triple}/bin/rust-lld{binary_ext}",

However, the actual Windows rustc distribution (rustc-1.90.0-x86_64-pc-windows-msvc.tar.xz) contains additional binaries in that directory:

What's in lib/rustlib/x86_64-pc-windows-msvc/bin/:

• ✅ rust-lld.exe (95MB) - Declared in filegroup
• ❌ wasm-component-ld.exe (5.1MB) - Missing from filegroup
• ❌ rust-objcopy.exe (4.2MB) - Missing from filegroup
• ❌ *.pdb debug symbols - Missing from filegroup

What's in gcc-ld/ subdirectory (all included via wildcard):

• ✅ wasm-ld.exe, ld.lld.exe, ld64.lld.exe, lld-link.exe + .pdb files

Impact

When Bazel creates the sandbox on Windows:

1. It copies only files declared in the filegroup
2. wasm-component-ld.exe is excluded because it's not in the pattern
3. rustc tries to find the linker at {sysroot}/lib/rustlib/{target}/bin/wasm-component-ld.exe
4. File doesn't exist in sandbox → Error

This blocks all Rust wasm32-wasip2 component builds on Windows.

Evidence

Test Case

BCR Compatibility Testing for rules_wasm_component:

• Linux/macOS: ✅ All tests passing
• Windows: ❌ Fails with linker 'wasm-component-ld.exe' not found

CI logs: https://github.com/pulseengine/rules_wasm_component/actions

Verified Tool Exists

Downloaded and extracted rustc-1.90.0-x86_64-pc-windows-msvc.tar.xz:

$ ls -lh rustc/lib/rustlib/x86_64-pc-windows-msvc/bin/
total 243328
-rwxr-xr-x  1 r  wheel   95M Sep 14 17:05 rust-lld.exe
-rwxr-xr-x  1 r  wheel  4.2M Sep 14 17:05 rust-objcopy.exe
-rwxr-xr-x  1 r  wheel  5.1M Sep 14 17:05 wasm-component-ld.exe  ← EXISTS!
-rw-r--r--  1 r  wheel  5.7M Sep 14 17:05 wasm-component-ld.pdb

Tool is present in official distribution but excluded from Bazel sandbox.

Proposed Fix

Option 1: Add Explicit Declarations (Conservative)

"lib/rustlib/{target_triple}/bin/wasm-component-ld{binary_ext}",
"lib/rustlib/{target_triple}/bin/rust-objcopy{binary_ext}",

Option 2: Use Wildcard (Future-Proof)

"lib/rustlib/{target_triple}/bin/*{binary_ext}",

Wildcard ensures any future tools added to this directory are automatically included.

Why Linux/macOS Might Not Be Affected

Need to verify if:

• Linux/macOS sandboxing is less strict
• Different linker discovery mechanisms on Unix
• Pattern matching differs with/without .exe extension

But Windows is definitely broken without this fix.

Related Work

• ✅ Issue Add support for WASI Preview 2 and 3 platform selection bazelbuild/rules_rust#3502 - Added WASI Preview 2 platform support (merged, works on Linux/macOS)
• ❌ Windows wasm32-wasip2 support blocked by this missing filegroup entry

Environment

• rules_rust version: 0.65.0 (from BCR)
• Bazel version: 8.x
• Platform: Windows (windows-latest CI runner)
• Rust version: 1.90.0
• Target: wasm32-wasip2

Additional Context

Investigation documented in:

• https://github.com/pulseengine/rules_wasm_component (see docs/WINDOWS_SUPPORT.md)
• Downloaded Windows rustc distribution to verify tool existence
• Created debug infrastructure to inspect Bazel sandbox on Windows

The tool definitely exists - it's just not being copied into the sandbox due to incomplete filegroup patterns.

[avrabe]

Resolution

Fixed in PR bazelbuild#3727

Root Cause

The issue was caused by different Bazel sandbox implementations:

• Windows: Copies files into sandbox - only explicitly listed files available
• Linux/macOS: Uses symlinks - can access unlisted files in same directory

Fix

Added rust-objcopy and wasm-component-ld to the rustc_lib filegroup in repository_utils.bzl.

These binaries are present in recent Rust versions:

• wasm-component-ld since 1.82.0
• rust-objcopy since 1.84.0

Investigation Details

Full investigation artifacts available in /tmp/rustc_investigation/ including:

• Binary verification across Rust 1.80.0 through 1.90.0
• Platform comparison (Linux, macOS, Windows)
• Detailed root cause analysis

Branch: https://github.com/avrabe/rules_rust/tree/fix/add-missing-rustc-binaries