Document use of mem::transmute

src/lib.rs

@@ -331,7 +331,10 @@ pub enum Level {
     /// The "error" level.
     ///
     /// Designates very serious errors.
-    Error = 1, // This way these line up with the discriminants for LevelFilter below
+    // This way these line up with the discriminants for LevelFilter below
+    // This works because Rust treats field-less enums the same way as C does:
+    // https://doc.rust-lang.org/reference/items/enumerations.html#custom-discriminant-values-for-field-less-enumerations
+    Error = 1,
     /// The "warn" level.
     ///
     /// Designates hazardous situations.
@@ -1173,6 +1176,12 @@ pub fn set_max_level(level: LevelFilter) {
 /// [`set_max_level`]: fn.set_max_level.html
 #[inline(always)]
 pub fn max_level() -> LevelFilter {
+    // Since `LevelFilter` is `repr(usize)`,
+    // this transmute is sound if and only if `MAX_LOG_LEVEL_FILTER`
+    // is set to a usize that is a valid discriminant for `LevelFilter`.
+    // Since `MAX_LOG_LEVEL_FILTER` is private, the only time it's set
+    // is by `set_max_level` above, i.e. by casting a `LevelFilter` to `usize`.
+    // So any usize stored in `MAX_LOG_LEVEL_FILTER` is a valid discriminant.
     unsafe { mem::transmute(MAX_LOG_LEVEL_FILTER.load(Ordering::Relaxed)) }
 }
 
@@ -1257,19 +1266,21 @@ fn set_logger_inner<F>(make_logger: F) -> Result<(), SetLoggerError>
 where
     F: FnOnce() -> &'static dyn Log,
 {
-    unsafe {
-        match STATE.compare_and_swap(UNINITIALIZED, INITIALIZING, Ordering::SeqCst) {
-            UNINITIALIZED => {
+    match STATE.compare_and_swap(UNINITIALIZED, INITIALIZING, Ordering::SeqCst) {
+        UNINITIALIZED => {
+            unsafe {
                 LOGGER = make_logger();
-                STATE.store(INITIALIZED, Ordering::SeqCst);
-                Ok(())
             }
-            INITIALIZING => {
-                while STATE.load(Ordering::SeqCst) == INITIALIZING {}
-                Err(SetLoggerError(()))
+            STATE.store(INITIALIZED, Ordering::SeqCst);
+            Ok(())
+        }
+        INITIALIZING => {
+            while STATE.load(Ordering::SeqCst) == INITIALIZING {
+                std::sync::atomic::spin_loop_hint();
             }
-            _ => Err(SetLoggerError(())),
+            Err(SetLoggerError(()))
         }
+        _ => Err(SetLoggerError(())),
     }
 }
 
@@ -1345,13 +1356,11 @@ impl error::Error for ParseLevelError {}
 ///
 /// If a logger has not been set, a no-op implementation is returned.
 pub fn logger() -> &'static dyn Log {
-    unsafe {
-        if STATE.load(Ordering::SeqCst) != INITIALIZED {
-            static NOP: NopLogger = NopLogger;
-            &NOP
-        } else {
-            LOGGER
-        }
+    if STATE.load(Ordering::SeqCst) != INITIALIZED {
+        static NOP: NopLogger = NopLogger;
+        &NOP
+    } else {
+        unsafe { LOGGER }
     }
 }