debug: add verbose logging and toolchain inspection for Windows

Added comprehensive debugging to understand why rustc can't find wasm-component-ld.exe:

1. Transition changes:
   - Added -vv flag for verbose rustc output on Windows
   - Will show search paths and command lines

2. New debug_rust_toolchain rule:
   - Inspects rustc location and sysroot
   - Lists files in rustlib directories
   - Checks if wasm-component-ld.exe is visible
   - Uses no-sandbox execution to see actual filesystem

Usage in CI:
  bazel build //rust/debug:windows_toolchain_debug

This will produce a _debug.txt file showing exactly what files are
accessible to rustc in the Bazel sandbox on Windows.

Author: avrabe

rust/debug/BUILD.bazel

@@ -0,0 +1,13 @@
+"""Debug targets for investigating Windows issues"""
+
+load("//rust:debug_windows.bzl", "debug_rust_toolchain")
+
+# Debug target that will show us what's visible in the Bazel sandbox on Windows
+debug_rust_toolchain(
+    name = "windows_toolchain_debug",
+    is_windows = select({
+        "@platforms//os:windows": True,
+        "//conditions:default": False,
+    }),
+    visibility = ["//visibility:public"],
+)

rust/debug_windows.bzl

@@ -0,0 +1,140 @@
+"""Debug rules for investigating Windows Rust toolchain issues"""
+
+def _debug_rust_toolchain_impl(ctx):
+    """Debug rule that dumps information about the Rust toolchain on Windows"""
+
+    output = ctx.actions.declare_file(ctx.label.name + "_debug.txt")
+
+    # Get rustc from the toolchain
+    rust_toolchain = ctx.toolchains["@rules_rust//rust:toolchain_type"]
+    rustc = rust_toolchain.rustc
+
+    # Create a debug script
+    if ctx.attr.is_windows:
+        # Windows PowerShell script
+        script = """
+@echo off
+echo === Rust Toolchain Debug Info === > {output}
+echo. >> {output}
+echo Rustc path: {rustc} >> {output}
+echo. >> {output}
+
+REM Get rustc's sysroot
+{rustc} --print sysroot >> {output} 2>&1
+
+REM Check if lib directory exists relative to rustc
+FOR %%I IN ("{rustc}") DO SET "rustc_dir=%%~dpI"
+echo. >> {output}
+echo Rustc directory: %rustc_dir% >> {output}
+echo. >> {output}
+
+REM Try to find wasm-component-ld.exe
+echo Searching for wasm-component-ld.exe: >> {output}
+where wasm-component-ld.exe >> {output} 2>&1 || echo Not found in PATH >> {output}
+echo. >> {output}
+
+REM List what's in the rustc bin directory
+echo Contents of rustc bin directory: >> {output}
+dir /B "%rustc_dir%" >> {output} 2>&1
+
+REM Check if lib/rustlib exists
+echo. >> {output}
+echo Checking lib/rustlib structure: >> {output}
+dir /B "%rustc_dir%..\\lib\\rustlib" >> {output} 2>&1
+
+REM Check x86_64-pc-windows-msvc bin directory
+echo. >> {output}
+echo Checking x86_64-pc-windows-msvc bin directory: >> {output}
+dir /B "%rustc_dir%..\\lib\\rustlib\\x86_64-pc-windows-msvc\\bin" >> {output} 2>&1
+""".format(
+            output = output.path,
+            rustc = rustc.path,
+        )
+
+        ctx.actions.run_shell(
+            outputs = [output],
+            inputs = [rustc],
+            command = script,
+            mnemonic = "DebugRustToolchain",
+            execution_requirements = {
+                "no-sandbox": "1",  # Disable sandbox to see actual file system
+            },
+        )
+    else:
+        # Linux/Mac bash script
+        script = """
+#!/bin/bash
+{{
+    echo "=== Rust Toolchain Debug Info ==="
+    echo
+    echo "Rustc path: {rustc}"
+    echo
+
+    # Get rustc's sysroot
+    {rustc} --print sysroot
+
+    # Get rustc directory
+    rustc_dir=$(dirname "{rustc}")
+    echo
+    echo "Rustc directory: $rustc_dir"
+    echo
+
+    # Try to find wasm-component-ld
+    echo "Searching for wasm-component-ld:"
+    which wasm-component-ld || echo "Not found in PATH"
+    echo
+
+    # List what's in the rustc bin directory
+    echo "Contents of rustc bin directory:"
+    ls -la "$rustc_dir" || echo "Directory not accessible"
+
+    # Check if lib/rustlib exists
+    echo
+    echo "Checking lib/rustlib structure:"
+    ls -la "$rustc_dir/../lib/rustlib" || echo "Directory not accessible"
+
+}} > {output} 2>&1
+""".format(
+            output = output.path,
+            rustc = rustc.path,
+        )
+
+        ctx.actions.run_shell(
+            outputs = [output],
+            inputs = [rustc],
+            command = script,
+            mnemonic = "DebugRustToolchain",
+        )
+
+    return [DefaultInfo(files = depset([output]))]
+
+debug_rust_toolchain = rule(
+    implementation = _debug_rust_toolchain_impl,
+    attrs = {
+        "is_windows": attr.bool(
+            default = False,
+            doc = "Whether this is running on Windows",
+        ),
+    },
+    toolchains = [
+        "@rules_rust//rust:toolchain_type",
+    ],
+    doc = """
+    Debug rule that dumps information about the Rust toolchain.
+
+    On Windows, this will show:
+    - Where rustc is located
+    - What rustc's sysroot is
+    - Whether wasm-component-ld.exe can be found
+    - What files are visible in the rustlib directory
+
+    Example:
+        debug_rust_toolchain(
+            name = "debug_windows",
+            is_windows = select({
+                "@platforms//os:windows": True,
+                "//conditions:default": False,
+            }),
+        )
+    """,
+)

rust/transitions.bzl

@@ -39,6 +39,10 @@ def _wasm_transition_impl(settings, attr):
     #   rustc/lib/rustlib/x86_64-pc-windows-msvc/bin/wasm-component-ld.exe
     # But rustc can't find it by name alone. We need to help it.
     if is_windows:
+        # DEBUG: Add verbose output to see what rustc is doing
+        # -vv shows command lines and search paths
+        current_flags.extend(["-vv"])
+
         # Try using -Clink-self-contained=yes to force rustc to use its own linker tools
         # This should make it search in lib/rustlib/{target}/bin/
         current_flags.extend(["-Clink-self-contained=yes"])