Change !cfg!(..) to cfg!(not(..)) for better readability

mlua-sys/build/main_inner.rs

@@ -24,7 +24,7 @@ fn main() {
     println!("cargo:rerun-if-changed=build");
 
     // Check if compilation and linking is handled by external crate
-    if !cfg!(feature = "external") {
+    if cfg!(not(feature = "external")) {
         let target_os = env::var("CARGO_CFG_TARGET_OS").unwrap();
         if target_os == "windows" && cfg!(feature = "module") {
             if !std::env::var("LUA_LIB_NAME").unwrap_or_default().is_empty() {

src/userdata/ref.rs

@@ -63,7 +63,7 @@ impl<T> TryFrom<UserDataVariant<T>> for UserDataRef<T> {
 
     #[inline]
     fn try_from(variant: UserDataVariant<T>) -> Result<Self> {
-        let guard = if !cfg!(feature = "send") || is_sync::<T>() {
+        let guard = if cfg!(not(feature = "send")) || is_sync::<T>() {
             variant.raw_lock().try_lock_shared_guarded()
         } else {
             variant.raw_lock().try_lock_exclusive_guarded()