fix(unsafe): Remove unsafe code in favor of OnceLock (#20)

This also simplifies the code a bit

src/lib.rs

@@ -23,9 +23,8 @@
 //! ```
 #![allow(clippy::bool_to_int_with_if)]
 
-use std::cell::UnsafeCell;
 use std::env;
-use std::sync::Once;
+use std::sync::OnceLock;
 
 /// possible stream sources
 #[derive(Clone, Copy, Debug)]
@@ -159,16 +158,6 @@ pub fn on(stream: Stream) -> Option<ColorLevel> {
     translate_level(supports_color(stream))
 }
 
-struct CacheCell(UnsafeCell<Option<ColorLevel>>);
-
-unsafe impl Sync for CacheCell {}
-
-static INIT: [Once; 2] = [Once::new(), Once::new()];
-static ON_CACHE: [CacheCell; 2] = [
-    CacheCell(UnsafeCell::new(None)),
-    CacheCell(UnsafeCell::new(None)),
-];
-
 macro_rules! assert_stream_in_bounds {
     ($($variant:ident)*) => {
         $(
@@ -187,12 +176,10 @@ be returned from then on.
 If you expect your environment to change between calls, use [`on`]
 */
 pub fn on_cached(stream: Stream) -> Option<ColorLevel> {
-    let stream_index = stream as usize;
-    INIT[stream_index].call_once(|| unsafe {
-        *ON_CACHE[stream_index].0.get() = translate_level(supports_color(stream));
-    });
+    static CACHE: [OnceLock<Option<ColorLevel>>; 2] = [OnceLock::new(), OnceLock::new()];
 
-    unsafe { *ON_CACHE[stream_index].0.get() }
+    let stream_index = stream as usize;
+    *CACHE[stream_index].get_or_init(|| translate_level(supports_color(stream)))
 }
 
 /**
@@ -254,6 +241,22 @@ mod tests {
         assert_eq!(on(Stream::Stdout), None);
     }
 
+    #[test]
+    #[cfg_attr(miri, ignore)]
+    fn test_on_cached() {
+        let _test_guard = TEST_LOCK.lock().unwrap();
+        set_up();
+        env::set_var("IGNORE_IS_TERMINAL", "1");
+
+        env::set_var("CLICOLOR", "1");
+        assert!(on(Stream::Stdout).is_some());
+        assert!(on_cached(Stream::Stdout).is_some());
+
+        env::set_var("CLICOLOR", "0");
+        assert!(on(Stream::Stdout).is_none());
+        assert!(on_cached(Stream::Stdout).is_some());
+    }
+
     #[test]
     #[cfg_attr(miri, ignore)]
     fn test_clicolor_force_ansi() {