fix(windows): detect Windows host in transition and set linker to wasm-component-ld.exe

The previous select() approach didn't work because it evaluated in the target
platform context (wasm32-wasip2) rather than the execution platform context
(Windows). Transitions have access to both platforms through the settings
parameter, so we can detect the Windows host platform and conditionally set
the linker in the transition itself.

This approach:
- Reads the host_platform from transition settings
- Detects 'windows' in the platform string
- Adds '-C linker=wasm-component-ld.exe' to extra_rustc_flags on Windows
- Works because transitions can modify build configuration based on exec platform

The transition now explicitly handles the @rules_rust//:extra_rustc_flags
output, preserving existing flags and adding Windows-specific ones when needed.

Author: avrabe

rust/rust_wasm_component.bzl

@@ -311,15 +311,6 @@ def rust_wasm_component(
 
         profile_rustc_flags = rustc_flags + config["rustc_flags"]
 
-        # Add Windows-specific linker flag for wasm-component-ld.exe
-        # On Windows, executables need .exe extension but Rust target spec doesn't add it
-        # This is a workaround for https://github.com/rust-lang/rust/issues/...
-        windows_linker_flag = select({
-            "@platforms//os:windows": ["-C", "linker=wasm-component-ld.exe"],
-            "//conditions:default": [],
-        })
-        profile_rustc_flags = profile_rustc_flags + windows_linker_flag
-
         # Add wit-bindgen generated code if specified
         all_srcs = list(srcs)
 

rust/transitions.bzl

@@ -17,16 +17,33 @@
 def _wasm_transition_impl(settings, attr):
     """Transition to WASM platform for component builds"""
 
-    # Use WASI Preview 2 - now Tier 2 support in Rust 1.82+
+    # Detect Windows execution platform for wasm-component-ld.exe workaround
+    # On Windows, the linker needs .exe extension but Rust's wasm32-wasip2 target spec doesn't add it
+    host_platform = str(settings["//command_line_option:host_platform"])
+    is_windows = "windows" in host_platform
+
+    # Get current rustc flags
+    rustc_flags = list(settings.get("@rules_rust//:extra_rustc_flags", []))
+
+    # Add Windows-specific linker configuration
+    if is_windows:
+        # Override the linker for wasm32-wasip2 on Windows hosts
+        rustc_flags.extend(["-C", "linker=wasm-component-ld.exe"])
+
     return {
         "//command_line_option:platforms": "//platforms:wasm32-wasip2",
+        "@rules_rust//:extra_rustc_flags": rustc_flags,
     }
 
 wasm_transition = transition(
     implementation = _wasm_transition_impl,
-    inputs = [],
+    inputs = [
+        "//command_line_option:host_platform",
+        "@rules_rust//:extra_rustc_flags",
+    ],
     outputs = [
         "//command_line_option:platforms",
+        "@rules_rust//:extra_rustc_flags",
     ],
 )