Allow lock guards to be sent to other threads

But only if deadlock detection is not enabled.

Fixes Amanieu#197

Cargo.toml

@@ -29,6 +29,7 @@ deadlock_detection = ["parking_lot_core/deadlock_detection"]
 serde = ["lock_api/serde"]
 stdweb = ["instant/stdweb"]
 wasm-bindgen = ["instant/wasm-bindgen"]
+send_guard = []
 
 [workspace]
 exclude = ["benchmark"]

README.md

@@ -72,6 +72,8 @@ in the Rust standard library:
 18. Optional support for [serde](https://docs.serde.rs/serde/).  Enable via the
     feature `serde`.  **NOTE!** this support is for `Mutex`, `ReentrantMutex`,
     and `RwLock` only; `Condvar` and `Once` are not currently supported.
+19. Lock guards can be sent to other threads when the `send_guard` geature is
+    enabled.
 
 ## The parking lot
 
@@ -118,6 +120,12 @@ parking_lot = { version = "0.10", features = ["nightly"] }
 The experimental deadlock detector can be enabled with the
 `deadlock_detection` Cargo feature.
 
+To allow sending `MutexGuard`s and `RwLock*Guard`s to other threads, enable the
+`send_guard` option.
+
+Note that the `deadlock_detection` and `send_guard` features are incompatible
+and cannot be used together.
+
 The core parking lot API is provided by the `parking_lot_core` crate. It is
 separate from the synchronization primitives in the `parking_lot` crate so that
 changes to the core API do not cause breaking changes for users of `parking_lot`.

src/lib.rs

@@ -30,6 +30,15 @@ pub mod deadlock;
 #[cfg(not(feature = "deadlock_detection"))]
 mod deadlock;
 
+// If deadlock detection is enabled, we cannot allow lock guards to be sent to
+// other threads.
+#[cfg(all(feature = "send_guard", feature = "deadlock_detection"))]
+compile_error!("the `send_guard` and `deadlock_detection` features cannot be used together");
+#[cfg(feature = "send_guard")]
+type GuardMarker = lock_api::GuardSend;
+#[cfg(not(feature = "send_guard"))]
+type GuardMarker = lock_api::GuardNoSend;
+
 pub use self::condvar::{Condvar, WaitTimeoutResult};
 pub use self::fair_mutex::{const_fair_mutex, FairMutex, FairMutexGuard, MappedFairMutexGuard};
 pub use self::mutex::{const_mutex, MappedMutexGuard, Mutex, MutexGuard};

src/raw_mutex.rs

@@ -11,7 +11,7 @@ use core::{
     time::Duration,
 };
 use instant::Instant;
-use lock_api::{GuardNoSend, RawMutex as RawMutex_};
+use lock_api::RawMutex as RawMutex_;
 use parking_lot_core::{self, ParkResult, SpinWait, UnparkResult, UnparkToken, DEFAULT_PARK_TOKEN};
 
 // UnparkToken used to indicate that that the target thread should attempt to
@@ -60,7 +60,7 @@ unsafe impl lock_api::RawMutex for RawMutex {
         state: AtomicU8::new(0),
     };
 
-    type GuardMarker = GuardNoSend;
+    type GuardMarker = crate::GuardMarker;
 
     #[inline]
     fn lock(&self) {

src/raw_rwlock.rs

@@ -13,7 +13,7 @@ use core::{
     sync::atomic::{AtomicUsize, Ordering},
 };
 use instant::Instant;
-use lock_api::{GuardNoSend, RawRwLock as RawRwLock_, RawRwLockUpgrade};
+use lock_api::{RawRwLock as RawRwLock_, RawRwLockUpgrade};
 use parking_lot_core::{
     self, deadlock, FilterOp, ParkResult, ParkToken, SpinWait, UnparkResult, UnparkToken,
 };
@@ -62,7 +62,7 @@ unsafe impl lock_api::RawRwLock for RawRwLock {
         state: AtomicUsize::new(0),
     };
 
-    type GuardMarker = GuardNoSend;
+    type GuardMarker = crate::GuardMarker;
 
     #[inline]
     fn lock_exclusive(&self) {