fix(rust): keep SystemRoot for Windows test and run processes

crates/once-frontend/prelude/rust.star

@@ -545,11 +545,14 @@ def _rust_unix_tool_dirs():
 # inheriting the host's PATH would make the action depend on whatever happens
 # to be installed. The standard system directories are the same everywhere the
 # platform runs, so they stay part of the action key without importing the
-# host's configuration.
-def _rust_process_path():
+# host's configuration. Windows processes also need `SystemRoot`: without it
+# Winsock cannot initialize and every socket fails with os error 10106.
+def _rust_process_env():
     if host_os() == "windows":
-        return _rust_windows_tool_env().get("PATH") or ""
-    return ":".join(_rust_unix_tool_dirs())
+        tools = _rust_windows_tool_env()
+        return {key: tools[key] for key in ["PATH", "SystemRoot"] if tools.get(key)}
+    path = ":".join(_rust_unix_tool_dirs())
+    return {"PATH": path} if path else {}
 
 def _rust_tool_path(tools):
     dirs = []
@@ -2032,9 +2035,8 @@ def _rust_run_args(ctx):
 
 def _rust_binary_run_env(ctx, run_dir):
     env = {"HOME": execution_path(run_dir + "/home")}
-    process_path = _rust_process_path()
-    if process_path:
-        env["PATH"] = process_path
+    for key, value in _rust_process_env().items():
+        env[key] = value
     _rust_merge_env_lower_precedence(env, _rust_cargo_config_env(ctx))
     for key, value in _rust_host_env(_rust_attr(ctx, "env_inherit", [])).items():
         env[key] = value
@@ -2102,9 +2104,8 @@ def _rust_cargo_runtime_env(ctx):
 
 def _rust_test_env(ctx, test_dir):
     env = {"HOME": execution_path(_rust_scratch_dir(ctx) + "/test-home")}
-    process_path = _rust_process_path()
-    if process_path:
-        env["PATH"] = process_path
+    for key, value in _rust_process_env().items():
+        env[key] = value
     for key, value in _rust_cargo_runtime_env(ctx).items():
         env[key] = value
     for key, value in _rust_bin_exe_env(ctx).items():

crates/once-frontend/tests/prelude/rust_build_script.rs

@@ -165,3 +165,37 @@ result = repr("ok")
         .iter()
         .any(|arg| arg == "-Ctarget-cpu=cortex-a76"));
 }
+
+#[test]
+fn processes_launched_from_rust_artifacts_keep_windows_system_root() {
+    let env = |os: &str| {
+        let source = format!(
+            r#"{prelude}
+def host_os():
+    return {os:?}
+def host_env(name):
+    return {{"Path": "C:\\Windows\\system32", "SystemRoot": "C:\\Windows", "INCLUDE": "C:\\sdk\\include"}}.get(name, "")
+ctx = {{
+    "label": {{"package": "", "name": "app", "id": "app"}},
+    "attr": {{"crate_root": "main.rs"}},
+    "deps": [],
+    "srcs": ["*.rs"],
+}}
+result = repr([
+    {{key: value for key, value in env.items() if key in ["PATH", "SystemRoot", "INCLUDE"]}}
+    for env in [_rust_test_env(ctx, ""), _rust_binary_run_env(ctx, ".once/run")]
+])
+"#,
+            prelude = all_prelude_source(),
+        );
+        let workspace = TempDir::new().unwrap();
+        let store = store_for(workspace.path(), "");
+        with_active_store(store, || eval_prelude_source_to_repr(source))
+            .1
+            .unwrap()
+    };
+    let windows = r#"{"PATH": "C:\\Windows\\system32", "SystemRoot": "C:\\Windows"}"#;
+    assert_eq!(env("windows"), format!("[{windows}, {windows}]"));
+    let linux = r#"{"PATH": "/usr/bin:/bin:/usr/sbin:/sbin"}"#;
+    assert_eq!(env("linux"), format!("[{linux}, {linux}]"));
+}