fix: Allow to build with autostart and without systemd feature

Before this change, import of `systemd::is_systemd_used` was hidden
behind `cfg(feature = "systemd")` predicate. But that function is
expected to be present in the autostart block even if `systemd` feature
is disabled.

Fix that by importing `is_systemd_used` unconditionally.

Provide an implementation of `is_systemd_used` for
`cfg(not(feature = "systemd"))` which just returns `false`.

Fixes: #136

Author: vadorovsky

src/main.rs

@@ -30,8 +30,9 @@ use std::{
 	os::fd::{AsRawFd, OwnedFd},
 	sync::Arc,
 };
+use systemd::is_systemd_used;
 #[cfg(feature = "systemd")]
-use systemd::{get_systemd_env, is_systemd_used, spawn_scope};
+use systemd::{get_systemd_env, spawn_scope};
 use tokio::{
 	net::UnixStream,
 	sync::{

src/systemd.rs

@@ -52,10 +52,15 @@ pub fn stop_systemd_target() {
 
 /// Determine if systemd is used as the init system. This should work on all
 /// linux distributions.
+#[cfg(feature = "systemd")]
 pub fn is_systemd_used() -> &'static bool {
 	static IS_SYSTEMD_USED: OnceLock<bool> = OnceLock::new();
 	IS_SYSTEMD_USED.get_or_init(|| Path::new("/run/systemd/system").exists())
 }
+#[cfg(not(feature = "systemd"))]
+pub fn is_systemd_used() -> &'static bool {
+	&false
+}
 
 #[cfg(feature = "systemd")]
 pub async fn get_systemd_env() -> Result<Vec<EnvVar>, zbus::Error> {