std: use os_unfair_lock for Mutex on Apple platforms

library/std/src/lib.rs

@@ -351,6 +351,7 @@
 #![feature(str_internals)]
 #![feature(strict_provenance)]
 #![feature(strict_provenance_atomic_ptr)]
+#![feature(sync_unsafe_cell)]
 // tidy-alphabetical-end
 //
 // Library features (alloc):

library/std/src/sys/locks/mutex/apple.rs

@@ -0,0 +1,47 @@
+//! Mutex for Apple platforms.
+//!
+//! On Apple platforms, priority inheritance is the default for locks. To avoid
+//! having to use pthread's mutex, which needs some tricks to work correctly, we
+//! instead use `os_unfair_lock`, which is small, movable and supports priority-
+//! inheritance and appeared with macOS 10.12, which is exactly our minimum
+//! supported version.
+
+use crate::cell::SyncUnsafeCell;
+
+// FIXME: move these definitions to libc
+
+#[allow(non_camel_case_types)]
+#[repr(C)]
+struct os_unfair_lock {
+    _opaque: u32,
+}
+
+const OS_UNFAIR_LOCK_INIT: os_unfair_lock = os_unfair_lock { _opaque: 0 };
+
+extern "C" {
+    fn os_unfair_lock_lock(lock: *mut os_unfair_lock);
+    fn os_unfair_lock_trylock(lock: *mut os_unfair_lock) -> bool;
+    fn os_unfair_lock_unlock(lock: *mut os_unfair_lock);
+}
+
+pub struct Mutex {
+    lock: SyncUnsafeCell<os_unfair_lock>,
+}
+
+impl Mutex {
+    pub const fn new() -> Mutex {
+        Mutex { lock: SyncUnsafeCell::new(OS_UNFAIR_LOCK_INIT) }
+    }
+
+    pub fn lock(&self) {
+        unsafe { os_unfair_lock_lock(self.lock.get()) }
+    }
+
+    pub fn try_lock(&self) -> bool {
+        unsafe { os_unfair_lock_trylock(self.lock.get()) }
+    }
+
+    pub unsafe fn unlock(&self) {
+        unsafe { os_unfair_lock_unlock(self.lock.get()) }
+    }
+}

library/std/src/sys/locks/mutex/mod.rs

@@ -11,6 +11,14 @@ cfg_if::cfg_if! {
     ))] {
         mod futex;
         pub use futex::Mutex;
+    } else if #[cfg(any(
+        target_os = "macos",
+        target_os = "ios",
+        target_os = "tvos",
+        target_os = "watchos",
+    ))] {
+        mod apple;
+        pub use apple::Mutex;
     } else if #[cfg(target_os = "fuchsia")] {
         mod fuchsia;
         pub use fuchsia::Mutex;