Conditionally compile disable_device_discovery

Skip all the relevant code where it is not supported, and it would
result in a no-op. Currently, this means libusb + non-freebsd.

Author: gtsiam

src/lib.rs

@@ -124,11 +124,15 @@ pub type HidResult<T> = Result<T, HidError>;
 pub const MAX_REPORT_DESCRIPTOR_SIZE: usize = 4096;
 
 enum InitState {
-    Uninit { device_discovery: bool },
+    Uninit {
+        #[cfg(all(libusb, not(target_os = "freebsd")))]
+        device_discovery: bool,
+    },
     Init,
 }
 
 static INIT_STATE: Mutex<InitState> = Mutex::new(InitState::Uninit {
+    #[cfg(all(libusb, not(target_os = "freebsd")))]
     device_discovery: true,
 });
 
@@ -151,6 +155,8 @@ impl HidApi {
     /// # Panics
     ///
     /// Panics if an hidapi context has already been initialized.
+    #[cfg(all(libusb, not(target_os = "freebsd")))]
+    #[cfg_attr(docsrs, doc(cfg(not(target_os = "freebsd"))))]
     pub fn disable_device_discovery() {
         if let InitState::Uninit { device_discovery } = &mut *INIT_STATE.lock().unwrap() {
             *device_discovery = false;
@@ -166,8 +172,11 @@ impl HidApi {
     pub fn new() -> HidResult<Self> {
         let mut init_state = INIT_STATE.lock().expect("HidApi context initialization");
 
-        #[allow(unused_variables)]
-        if let InitState::Uninit { device_discovery } = *init_state {
+        if let InitState::Uninit {
+            #[cfg(all(libusb, not(target_os = "freebsd")))]
+            device_discovery,
+        } = *init_state
+        {
             #[cfg(all(libusb, not(target_os = "freebsd")))]
             if !device_discovery {
                 // Do not scan for devices in libusb_init()