core: fix warnings when compiling without std (tokio-rs#2022)

Currently, compiling `tracing-core` with `default-features = false`
(i.e. for `no_std` targets) emits a few warnings. This is due to the
spinlock implementation's use of the deprecated `atomic::spin_loop_hint`
function (renamed to `hint::spin_loop`), and the use of deprecated
`compare_and_swap` instead of `compare_exchange` methods. Now that our
MSRV is 1.49 (the version in which `hint::spin_loop` was stabilized), we
can fix these warnings.

This branch replaces the deprecated APIs.

Also, I noticed that one of the tests emits unused-imports warnings with
`--no-default-features`. This is because the actual tests are feature
flagged to require `std`, but the module itself doesn't, so the imports
are just hanging out and not getting used for anything. I went ahead and
fixed that as well.

Signed-off-by: Eliza Weisman <[email protected]>

tracing-core/src/spin/mutex.rs

@@ -0,0 +1,118 @@
+use core::cell::UnsafeCell;
+use core::default::Default;
+use core::fmt;
+use core::hint;
+use core::marker::Sync;
+use core::ops::{Deref, DerefMut, Drop};
+use core::option::Option::{self, None, Some};
+use core::sync::atomic::{AtomicBool, Ordering};
+
+/// This type provides MUTual EXclusion based on spinning.
+pub(crate) struct Mutex<T: ?Sized> {
+    lock: AtomicBool,
+    data: UnsafeCell<T>,
+}
+
+/// A guard to which the protected data can be accessed
+///
+/// When the guard falls out of scope it will release the lock.
+#[derive(Debug)]
+pub(crate) struct MutexGuard<'a, T: ?Sized> {
+    lock: &'a AtomicBool,
+    data: &'a mut T,
+}
+
+// Same unsafe impls as `std::sync::Mutex`
+unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
+unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
+
+impl<T> Mutex<T> {
+    /// Creates a new spinlock wrapping the supplied data.
+    pub(crate) const fn new(user_data: T) -> Mutex<T> {
+        Mutex {
+            lock: AtomicBool::new(false),
+            data: UnsafeCell::new(user_data),
+        }
+    }
+}
+
+impl<T: ?Sized> Mutex<T> {
+    fn obtain_lock(&self) {
+        while self
+            .lock
+            .compare_exchange_weak(false, true, Ordering::Acquire, Ordering::Relaxed)
+            .is_err()
+        {
+            // Wait until the lock looks unlocked before retrying
+            while self.lock.load(Ordering::Relaxed) {
+                hint::spin_loop();
+            }
+        }
+    }
+
+    /// Locks the spinlock and returns a guard.
+    ///
+    /// The returned value may be dereferenced for data access
+    /// and the lock will be dropped when the guard falls out of scope.
+    pub(crate) fn lock(&self) -> MutexGuard<'_, T> {
+        self.obtain_lock();
+        MutexGuard {
+            lock: &self.lock,
+            data: unsafe { &mut *self.data.get() },
+        }
+    }
+
+    /// Tries to lock the mutex. If it is already locked, it will return None. Otherwise it returns
+    /// a guard within Some.
+    pub(crate) fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
+        if self
+            .lock
+            .compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed)
+            .is_ok()
+        {
+            Some(MutexGuard {
+                lock: &self.lock,
+                data: unsafe { &mut *self.data.get() },
+            })
+        } else {
+            None
+        }
+    }
+}
+
+impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.try_lock() {
+            Some(guard) => write!(f, "Mutex {{ data: ")
+                .and_then(|()| (&*guard).fmt(f))
+                .and_then(|()| write!(f, "}}")),
+            None => write!(f, "Mutex {{ <locked> }}"),
+        }
+    }
+}
+
+impl<T: ?Sized + Default> Default for Mutex<T> {
+    fn default() -> Mutex<T> {
+        Mutex::new(Default::default())
+    }
+}
+
+impl<'a, T: ?Sized> Deref for MutexGuard<'a, T> {
+    type Target = T;
+    fn deref<'b>(&'b self) -> &'b T {
+        &*self.data
+    }
+}
+
+impl<'a, T: ?Sized> DerefMut for MutexGuard<'a, T> {
+    fn deref_mut<'b>(&'b mut self) -> &'b mut T {
+        &mut *self.data
+    }
+}
+
+impl<'a, T: ?Sized> Drop for MutexGuard<'a, T> {
+    /// The dropping of the MutexGuard will release the lock it was created from.
+    fn drop(&mut self) {
+        self.lock.store(false, Ordering::Release);
+    }
+}

tracing-core/tests/dispatch.rs

@@ -6,7 +6,6 @@ use common::*;
 #[cfg(feature = "std")]
 use tracing_core::dispatch::*;
 
-#[cfg(feature = "std")]
 #[test]
 fn set_default_dispatch() {
     set_global_default(Dispatch::new(TestCollectorA)).expect("global dispatch set failed");
@@ -21,7 +20,6 @@ fn set_default_dispatch() {
     get_default(|current| assert!(current.is::<TestCollectorA>(), "global dispatch get failed"));
 }
 
-#[cfg(feature = "std")]
 #[test]
 fn nested_set_default() {
     let _guard = set_default(&Dispatch::new(TestCollectorA));