Rollup merge of #97647 - m-ou-se:lazy-box-locks, r=Amanieu

Lazily allocate and initialize pthread locks.

Lazily allocate and initialize pthread locks.

This allows {Mutex, Condvar, RwLock}::new() to be const, while still using the platform's native locks for features like priority inheritance and debug tooling. E.g. on macOS, we cannot directly use the (private) APIs that pthread's locks are implemented with, making it impossible for us to use anything other than pthread while still preserving priority inheritance, etc.

This PR doesn't yet make the public APIs const. That's for a separate PR with an FCP.

Tracking issue: https://github.com/rust-lang/rust/issues/93740

29 files changed, 184 insertions, 123 deletions

diff --git a/library/std/src/sys/hermit/condvar.rs b/library/std/src/sys/hermit/condvar.rs
index f6083530005..46f45b19771 100644
--- a/library/std/src/sys/hermit/condvar.rs
+++ b/library/std/src/sys/hermit/condvar.rs
@@ -70,9 +70,13 @@ impl Condvar {
         mutex.lock();
         res == 0
     }
+}
 
-    pub unsafe fn destroy(&self) {
-        let _ = abi::sem_destroy(self.sem1);
-        let _ = abi::sem_destroy(self.sem2);
+impl Drop for Condvar {
+    fn drop(&mut self) {
+        unsafe {
+            let _ = abi::sem_destroy(self.sem1);
+            let _ = abi::sem_destroy(self.sem2);
+        }
     }
 }
diff --git a/library/std/src/sys/hermit/mutex.rs b/library/std/src/sys/hermit/mutex.rs
index 97b4c49896f..ef44bf411fb 100644
--- a/library/std/src/sys/hermit/mutex.rs
+++ b/library/std/src/sys/hermit/mutex.rs
@@ -215,7 +215,4 @@ impl Mutex {
         }
         guard.locked
     }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {}
 }
diff --git a/library/std/src/sys/hermit/rwlock.rs b/library/std/src/sys/hermit/rwlock.rs
index 690bb155e1a..d43fa08a171 100644
--- a/library/std/src/sys/hermit/rwlock.rs
+++ b/library/std/src/sys/hermit/rwlock.rs
@@ -84,12 +84,6 @@ impl RwLock {
         // FIXME: should only wake up one of these some of the time
         self.cond.notify_all();
     }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {
-        self.lock.destroy();
-        self.cond.destroy();
-    }
 }
 
 impl State {
diff --git a/library/std/src/sys/itron/condvar.rs b/library/std/src/sys/itron/condvar.rs
index ed26c528027..008cd8fb1e3 100644
--- a/library/std/src/sys/itron/condvar.rs
+++ b/library/std/src/sys/itron/condvar.rs
@@ -117,8 +117,6 @@ impl Condvar {
         unsafe { mutex.lock() };
         success
     }
-
-    pub unsafe fn destroy(&self) {}
 }
 
 mod waiter_queue {
diff --git a/library/std/src/sys/itron/mutex.rs b/library/std/src/sys/itron/mutex.rs
index 5ee231882bb..2ba8454ff92 100644
--- a/library/std/src/sys/itron/mutex.rs
+++ b/library/std/src/sys/itron/mutex.rs
@@ -64,8 +64,10 @@ impl Mutex {
             }
         }
     }
+}
 
-    pub unsafe fn destroy(&self) {
+impl Drop for Mutex {
+    fn drop(&mut self) {
         if let Some(mtx) = self.mtx.get().map(|x| x.0) {
             expect_success_aborting(unsafe { abi::del_mtx(mtx) }, &"del_mtx");
         }
diff --git a/library/std/src/sys/sgx/condvar.rs b/library/std/src/sys/sgx/condvar.rs
index c9736880b08..36534e0eff3 100644
--- a/library/std/src/sys/sgx/condvar.rs
+++ b/library/std/src/sys/sgx/condvar.rs
@@ -1,4 +1,5 @@
 use crate::sys::locks::Mutex;
+use crate::sys_common::lazy_box::{LazyBox, LazyInit};
 use crate::time::Duration;
 
 use super::waitqueue::{SpinMutex, WaitQueue, WaitVariable};
@@ -7,7 +8,13 @@ pub struct Condvar {
     inner: SpinMutex<WaitVariable<()>>,
 }
 
-pub type MovableCondvar = Box<Condvar>;
+pub(crate) type MovableCondvar = LazyBox<Condvar>;
+
+impl LazyInit for Condvar {
+    fn init() -> Box<Self> {
+        Box::new(Self::new())
+    }
+}
 
 impl Condvar {
     pub const fn new() -> Condvar {
@@ -15,9 +22,6 @@ impl Condvar {
     }
 
     #[inline]
-    pub unsafe fn init(&mut self) {}
-
-    #[inline]
     pub unsafe fn notify_one(&self) {
         let _ = WaitQueue::notify_one(self.inner.lock());
     }
@@ -38,7 +42,4 @@ impl Condvar {
         unsafe { mutex.lock() };
         success
     }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {}
 }
diff --git a/library/std/src/sys/sgx/mutex.rs b/library/std/src/sys/sgx/mutex.rs
index 98a390c4c2b..513cd77fd2a 100644
--- a/library/std/src/sys/sgx/mutex.rs
+++ b/library/std/src/sys/sgx/mutex.rs
@@ -1,11 +1,18 @@
 use super::waitqueue::{try_lock_or_false, SpinMutex, WaitQueue, WaitVariable};
+use crate::sys_common::lazy_box::{LazyBox, LazyInit};
 
 pub struct Mutex {
     inner: SpinMutex<WaitVariable<bool>>,
 }
 
 // not movable: see UnsafeList implementation
-pub type MovableMutex = Box<Mutex>;
+pub(crate) type MovableMutex = LazyBox<Mutex>;
+
+impl LazyInit for Mutex {
+    fn init() -> Box<Self> {
+        Box::new(Self::new())
+    }
+}
 
 // Implementation according to “Operating Systems: Three Easy Pieces”, chapter 28
 impl Mutex {
@@ -52,7 +59,4 @@ impl Mutex {
             true
         }
     }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {}
 }
diff --git a/library/std/src/sys/sgx/rwlock.rs b/library/std/src/sys/sgx/rwlock.rs
index 47be4c006ec..a97fb9ab026 100644
--- a/library/std/src/sys/sgx/rwlock.rs
+++ b/library/std/src/sys/sgx/rwlock.rs
@@ -2,6 +2,7 @@
 mod tests;
 
 use crate::num::NonZeroUsize;
+use crate::sys_common::lazy_box::{LazyBox, LazyInit};
 
 use super::waitqueue::{
     try_lock_or_false, NotifiedTcs, SpinMutex, SpinMutexGuard, WaitQueue, WaitVariable,
@@ -13,7 +14,13 @@ pub struct RwLock {
     writer: SpinMutex<WaitVariable<bool>>,
 }
 
-pub type MovableRwLock = Box<RwLock>;
+pub(crate) type MovableRwLock = LazyBox<RwLock>;
+
+impl LazyInit for RwLock {
+    fn init() -> Box<Self> {
+        Box::new(Self::new())
+    }
+}
 
 // Check at compile time that RwLock size matches C definition (see test_c_rwlock_initializer below)
 //
@@ -168,9 +175,6 @@ impl RwLock {
             unsafe { self.__read_unlock(rguard, wguard) };
         }
     }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {}
 }
 
 // The following functions are needed by libunwind. These symbols are named
diff --git a/library/std/src/sys/solid/rwlock.rs b/library/std/src/sys/solid/rwlock.rs
index df16cc680ad..433abc895f5 100644
--- a/library/std/src/sys/solid/rwlock.rs
+++ b/library/std/src/sys/solid/rwlock.rs
@@ -82,9 +82,11 @@ impl RwLock {
         let rwl = self.raw();
         expect_success_aborting(unsafe { abi::rwl_unl_rwl(rwl) }, &"rwl_unl_rwl");
     }
+}
 
+impl Drop for RwLock {
     #[inline]
-    pub unsafe fn destroy(&self) {
+    fn drop(&mut self) {
         if let Some(rwl) = self.rwl.get().map(|x| x.0) {
             expect_success_aborting(unsafe { abi::rwl_del_rwl(rwl) }, &"rwl_del_rwl");
         }
diff --git a/library/std/src/sys/unix/locks/futex.rs b/library/std/src/sys/unix/locks/futex.rs
index 7a63af1ad7c..a9a1a32c5af 100644
--- a/library/std/src/sys/unix/locks/futex.rs
+++ b/library/std/src/sys/unix/locks/futex.rs
@@ -25,9 +25,6 @@ impl Mutex {
     pub unsafe fn init(&mut self) {}
 
     #[inline]
-    pub unsafe fn destroy(&self) {}
-
-    #[inline]
     pub unsafe fn try_lock(&self) -> bool {
         self.futex.compare_exchange(0, 1, Acquire, Relaxed).is_ok()
     }
@@ -118,12 +115,6 @@ impl Condvar {
         Self { futex: AtomicU32::new(0) }
     }
 
-    #[inline]
-    pub unsafe fn init(&mut self) {}
-
-    #[inline]
-    pub unsafe fn destroy(&self) {}
-
     // All the memory orderings here are `Relaxed`,
     // because synchronization is done by unlocking and locking the mutex.
 
diff --git a/library/std/src/sys/unix/locks/futex_rwlock.rs b/library/std/src/sys/unix/locks/futex_rwlock.rs
index 5ff1aba7974..1f902f50587 100644
--- a/library/std/src/sys/unix/locks/futex_rwlock.rs
+++ b/library/std/src/sys/unix/locks/futex_rwlock.rs
@@ -64,9 +64,6 @@ impl RwLock {
     }
 
     #[inline]
-    pub unsafe fn destroy(&self) {}
-
-    #[inline]
     pub unsafe fn try_read(&self) -> bool {
         self.state
             .fetch_update(Acquire, Relaxed, |s| is_read_lockable(s).then(|| s + READ_LOCKED))
diff --git a/library/std/src/sys/unix/locks/mod.rs b/library/std/src/sys/unix/locks/mod.rs
index 04c5c489fc9..03400efa3c9 100644
--- a/library/std/src/sys/unix/locks/mod.rs
+++ b/library/std/src/sys/unix/locks/mod.rs
@@ -9,14 +9,14 @@ cfg_if::cfg_if! {
     ))] {
         mod futex;
         mod futex_rwlock;
-        pub use futex::{Mutex, MovableMutex, Condvar, MovableCondvar};
-        pub use futex_rwlock::{RwLock, MovableRwLock};
+        pub(crate) use futex::{Mutex, MovableMutex, MovableCondvar};
+        pub(crate) use futex_rwlock::{RwLock, MovableRwLock};
     } else {
         mod pthread_mutex;
         mod pthread_rwlock;
         mod pthread_condvar;
-        pub use pthread_mutex::{Mutex, MovableMutex};
-        pub use pthread_rwlock::{RwLock, MovableRwLock};
-        pub use pthread_condvar::{Condvar, MovableCondvar};
+        pub(crate) use pthread_mutex::{Mutex, MovableMutex};
+        pub(crate) use pthread_rwlock::{RwLock, MovableRwLock};
+        pub(crate) use pthread_condvar::MovableCondvar;
     }
 }
diff --git a/library/std/src/sys/unix/locks/pthread_condvar.rs b/library/std/src/sys/unix/locks/pthread_condvar.rs
index 099aa68706f..61c28d696bc 100644
--- a/library/std/src/sys/unix/locks/pthread_condvar.rs
+++ b/library/std/src/sys/unix/locks/pthread_condvar.rs
@@ -1,12 +1,13 @@
 use crate::cell::UnsafeCell;
 use crate::sys::locks::{pthread_mutex, Mutex};
+use crate::sys_common::lazy_box::{LazyBox, LazyInit};
 use crate::time::Duration;
 
 pub struct Condvar {
     inner: UnsafeCell<libc::pthread_cond_t>,
 }
 
-pub type MovableCondvar = Box<Condvar>;
+pub(crate) type MovableCondvar = LazyBox<Condvar>;
 
 unsafe impl Send for Condvar {}
 unsafe impl Sync for Condvar {}
@@ -18,6 +19,14 @@ fn saturating_cast_to_time_t(value: u64) -> libc::time_t {
     if value > <libc::time_t>::MAX as u64 { <libc::time_t>::MAX } else { value as libc::time_t }
 }
 
+impl LazyInit for Condvar {
+    fn init() -> Box<Self> {
+        let mut condvar = Box::new(Self::new());
+        unsafe { condvar.init() };
+        condvar
+    }
+}
+
 impl Condvar {
     pub const fn new() -> Condvar {
         // Might be moved and address is changing it is better to avoid
@@ -32,14 +41,14 @@ impl Condvar {
         target_os = "android",
         target_os = "redox"
     ))]
-    pub unsafe fn init(&mut self) {}
+    unsafe fn init(&mut self) {}
 
     // NOTE: ESP-IDF's PTHREAD_COND_INITIALIZER support is not released yet
     // So on that platform, init() should always be called
     // Moreover, that platform does not have pthread_condattr_setclock support,
     // hence that initialization should be skipped as well
     #[cfg(target_os = "espidf")]
-    pub unsafe fn init(&mut self) {
+    unsafe fn init(&mut self) {
         let r = libc::pthread_cond_init(self.inner.get(), crate::ptr::null());
         assert_eq!(r, 0);
     }
@@ -52,7 +61,7 @@ impl Condvar {
         target_os = "redox",
         target_os = "espidf"
     )))]
-    pub unsafe fn init(&mut self) {
+    unsafe fn init(&mut self) {
         use crate::mem::MaybeUninit;
         let mut attr = MaybeUninit::<libc::pthread_condattr_t>::uninit();
         let r = libc::pthread_condattr_init(attr.as_mut_ptr());
@@ -179,14 +188,14 @@ impl Condvar {
 
     #[inline]
     #[cfg(not(target_os = "dragonfly"))]
-    pub unsafe fn destroy(&self) {
+    unsafe fn destroy(&mut self) {
         let r = libc::pthread_cond_destroy(self.inner.get());
         debug_assert_eq!(r, 0);
     }
 
     #[inline]
     #[cfg(target_os = "dragonfly")]
-    pub unsafe fn destroy(&self) {
+    unsafe fn destroy(&mut self) {
         let r = libc::pthread_cond_destroy(self.inner.get());
         // On DragonFly pthread_cond_destroy() returns EINVAL if called on
         // a condvar that was just initialized with
@@ -195,3 +204,10 @@ impl Condvar {
         debug_assert!(r == 0 || r == libc::EINVAL);
     }
 }
+
+impl Drop for Condvar {
+    #[inline]
+    fn drop(&mut self) {
+        unsafe { self.destroy() };
+    }
+}
diff --git a/library/std/src/sys/unix/locks/pthread_mutex.rs b/library/std/src/sys/unix/locks/pthread_mutex.rs
index 76840ce74dd..916e898d890 100644
--- a/library/std/src/sys/unix/locks/pthread_mutex.rs
+++ b/library/std/src/sys/unix/locks/pthread_mutex.rs
@@ -1,12 +1,13 @@
 use crate::cell::UnsafeCell;
 use crate::mem::MaybeUninit;
 use crate::sys::cvt_nz;
+use crate::sys_common::lazy_box::{LazyBox, LazyInit};
 
 pub struct Mutex {
     inner: UnsafeCell<libc::pthread_mutex_t>,
 }
 
-pub type MovableMutex = Box<Mutex>;
+pub(crate) type MovableMutex = LazyBox<Mutex>;
 
 #[inline]
 pub unsafe fn raw(m: &Mutex) -> *mut libc::pthread_mutex_t {
@@ -16,6 +17,14 @@ pub unsafe fn raw(m: &Mutex) -> *mut libc::pthread_mutex_t {
 unsafe impl Send for Mutex {}
 unsafe impl Sync for Mutex {}
 
+impl LazyInit for Mutex {
+    fn init() -> Box<Self> {
+        let mut mutex = Box::new(Self::new());
+        unsafe { mutex.init() };
+        mutex
+    }
+}
+
 impl Mutex {
     pub const fn new() -> Mutex {
         // Might be moved to a different address, so it is better to avoid
@@ -73,13 +82,13 @@ impl Mutex {
     }
     #[inline]
     #[cfg(not(target_os = "dragonfly"))]
-    pub unsafe fn destroy(&self) {
+    unsafe fn destroy(&mut self) {
         let r = libc::pthread_mutex_destroy(self.inner.get());
         debug_assert_eq!(r, 0);
     }
     #[inline]
     #[cfg(target_os = "dragonfly")]
-    pub unsafe fn destroy(&self) {
+    unsafe fn destroy(&mut self) {
         let r = libc::pthread_mutex_destroy(self.inner.get());
         // On DragonFly pthread_mutex_destroy() returns EINVAL if called on a
         // mutex that was just initialized with libc::PTHREAD_MUTEX_INITIALIZER.
@@ -89,6 +98,13 @@ impl Mutex {
     }
 }
 
+impl Drop for Mutex {
+    #[inline]
+    fn drop(&mut self) {
+        unsafe { self.destroy() };
+    }
+}
+
 pub(super) struct PthreadMutexAttr<'a>(pub &'a mut MaybeUninit<libc::pthread_mutexattr_t>);
 
 impl Drop for PthreadMutexAttr<'_> {
diff --git a/library/std/src/sys/unix/locks/pthread_rwlock.rs b/library/std/src/sys/unix/locks/pthread_rwlock.rs
index 11a0c0457cd..75e5759c787 100644
--- a/library/std/src/sys/unix/locks/pthread_rwlock.rs
+++ b/library/std/src/sys/unix/locks/pthread_rwlock.rs
@@ -1,5 +1,6 @@
 use crate::cell::UnsafeCell;
 use crate::sync::atomic::{AtomicUsize, Ordering};
+use crate::sys_common::lazy_box::{LazyBox, LazyInit};
 
 pub struct RwLock {
     inner: UnsafeCell<libc::pthread_rwlock_t>,
@@ -7,11 +8,17 @@ pub struct RwLock {
     num_readers: AtomicUsize,
 }
 
-pub type MovableRwLock = Box<RwLock>;
+pub(crate) type MovableRwLock = LazyBox<RwLock>;
 
 unsafe impl Send for RwLock {}
 unsafe impl Sync for RwLock {}
 
+impl LazyInit for RwLock {
+    fn init() -> Box<Self> {
+        Box::new(Self::new())
+    }
+}
+
 impl RwLock {
     pub const fn new() -> RwLock {
         RwLock {
@@ -128,7 +135,7 @@ impl RwLock {
         self.raw_unlock();
     }
     #[inline]
-    pub unsafe fn destroy(&self) {
+    unsafe fn destroy(&mut self) {
         let r = libc::pthread_rwlock_destroy(self.inner.get());
         // On DragonFly pthread_rwlock_destroy() returns EINVAL if called on a
         // rwlock that was just initialized with
@@ -141,3 +148,10 @@ impl RwLock {
         }
     }
 }
+
+impl Drop for RwLock {
+    #[inline]
+    fn drop(&mut self) {
+        unsafe { self.destroy() };
+    }
+}
diff --git a/library/std/src/sys/unsupported/locks/condvar.rs b/library/std/src/sys/unsupported/locks/condvar.rs
index 8dbe03bad9b..f27bf2b26bd 100644
--- a/library/std/src/sys/unsupported/locks/condvar.rs
+++ b/library/std/src/sys/unsupported/locks/condvar.rs
@@ -11,9 +11,6 @@ impl Condvar {
     }
 
     #[inline]
-    pub unsafe fn init(&mut self) {}
-
-    #[inline]
     pub unsafe fn notify_one(&self) {}
 
     #[inline]
@@ -26,7 +23,4 @@ impl Condvar {
     pub unsafe fn wait_timeout(&self, _mutex: &Mutex, _dur: Duration) -> bool {
         panic!("condvar wait not supported");
     }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {}
 }
diff --git a/library/std/src/sys/unsupported/locks/mutex.rs b/library/std/src/sys/unsupported/locks/mutex.rs
index cad991aae5e..56bad71b189 100644
--- a/library/std/src/sys/unsupported/locks/mutex.rs
+++ b/library/std/src/sys/unsupported/locks/mutex.rs
@@ -32,7 +32,4 @@ impl Mutex {
     pub unsafe fn try_lock(&self) -> bool {
         self.locked.replace(true) == false
     }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {}
 }
diff --git a/library/std/src/sys/unsupported/locks/rwlock.rs b/library/std/src/sys/unsupported/locks/rwlock.rs
index 14fd351314c..bf6e2d3d080 100644
--- a/library/std/src/sys/unsupported/locks/rwlock.rs
+++ b/library/std/src/sys/unsupported/locks/rwlock.rs
@@ -62,7 +62,4 @@ impl RwLock {
     pub unsafe fn write_unlock(&self) {
         assert_eq!(self.mode.replace(0), -1);
     }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {}
 }
diff --git a/library/std/src/sys/wasm/mod.rs b/library/std/src/sys/wasm/mod.rs
index 9992e44b0e7..55b5ad314da 100644
--- a/library/std/src/sys/wasm/mod.rs
+++ b/library/std/src/sys/wasm/mod.rs
@@ -54,8 +54,8 @@ cfg_if::cfg_if! {
             #![allow(unsafe_op_in_unsafe_fn)]
             mod futex;
             mod futex_rwlock;
-            pub use futex::{Mutex, MovableMutex, Condvar, MovableCondvar};
-            pub use futex_rwlock::{RwLock, MovableRwLock};
+            pub(crate) use futex::{Mutex, MovableMutex, Condvar, MovableCondvar};
+            pub(crate) use futex_rwlock::{RwLock, MovableRwLock};
         }
         #[path = "atomics/futex.rs"]
         pub mod futex;
diff --git a/library/std/src/sys/windows/locks/condvar.rs b/library/std/src/sys/windows/locks/condvar.rs
index dfd8cfdceee..59e2c1be0f0 100644
--- a/library/std/src/sys/windows/locks/condvar.rs
+++ b/library/std/src/sys/windows/locks/condvar.rs
@@ -19,9 +19,6 @@ impl Condvar {
     }
 
     #[inline]
-    pub unsafe fn init(&mut self) {}
-
-    #[inline]
     pub unsafe fn wait(&self, mutex: &Mutex) {
         let r = c::SleepConditionVariableSRW(self.inner.get(), mutex::raw(mutex), c::INFINITE, 0);
         debug_assert!(r != 0);
@@ -51,8 +48,4 @@ impl Condvar {
     pub unsafe fn notify_all(&self) {
         c::WakeAllConditionVariable(self.inner.get())
     }
-
-    pub unsafe fn destroy(&self) {
-        // ...
-    }
 }
diff --git a/library/std/src/sys/windows/locks/mutex.rs b/library/std/src/sys/windows/locks/mutex.rs
index 9fa280b8b76..08f55844a0e 100644
--- a/library/std/src/sys/windows/locks/mutex.rs
+++ b/library/std/src/sys/windows/locks/mutex.rs
@@ -53,9 +53,4 @@ impl Mutex {
     pub unsafe fn unlock(&self) {
         c::ReleaseSRWLockExclusive(raw(self));
     }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {
-        // SRWLock does not need to be destroyed.
-    }
 }
diff --git a/library/std/src/sys/windows/locks/rwlock.rs b/library/std/src/sys/windows/locks/rwlock.rs
index 12906652e0b..a32df85e2f6 100644
--- a/library/std/src/sys/windows/locks/rwlock.rs
+++ b/library/std/src/sys/windows/locks/rwlock.rs
@@ -38,9 +38,4 @@ impl RwLock {
     pub unsafe fn write_unlock(&self) {
         c::ReleaseSRWLockExclusive(self.inner.get())
     }
-
-    #[inline]
-    pub unsafe fn destroy(&self) {
-        // ...
-    }
 }
diff --git a/library/std/src/sys_common/condvar.rs b/library/std/src/sys_common/condvar.rs
index 67d4b126209..1def0518e0a 100644
--- a/library/std/src/sys_common/condvar.rs
+++ b/library/std/src/sys_common/condvar.rs
@@ -15,9 +15,7 @@ pub struct Condvar {
 impl Condvar {
     /// Creates a new condition variable for use.
     pub fn new() -> Self {
-        let mut c = imp::MovableCondvar::from(imp::Condvar::new());
-        unsafe { c.init() };
-        Self { inner: c, check: CondvarCheck::new() }
+        Self { inner: imp::MovableCondvar::new(), check: CondvarCheck::new() }
     }
 
     /// Signals one waiter on this condition variable to wake up.
@@ -55,9 +53,3 @@ impl Condvar {
         self.inner.wait_timeout(mutex.raw(), dur)
     }
 }
-
-impl Drop for Condvar {
-    fn drop(&mut self) {
-        unsafe { self.inner.destroy() };
-    }
-}
diff --git a/library/std/src/sys_common/condvar/check.rs b/library/std/src/sys_common/condvar/check.rs
index d0d0d596518..ce8f3670487 100644
--- a/library/std/src/sys_common/condvar/check.rs
+++ b/library/std/src/sys_common/condvar/check.rs
@@ -1,6 +1,7 @@
 use crate::ptr;
 use crate::sync::atomic::{AtomicPtr, Ordering};
 use crate::sys::locks as imp;
+use crate::sys_common::lazy_box::{LazyBox, LazyInit};
 use crate::sys_common::mutex::MovableMutex;
 
 pub trait CondvarCheck {
@@ -9,7 +10,7 @@ pub trait CondvarCheck {
 
 /// For boxed mutexes, a `Condvar` will check it's only ever used with the same
 /// mutex, based on its (stable) address.
-impl CondvarCheck for Box<imp::Mutex> {
+impl<T: LazyInit> CondvarCheck for LazyBox<T> {
     type Check = SameMutexCheck;
 }
 
diff --git a/library/std/src/sys_common/lazy_box.rs b/library/std/src/sys_common/lazy_box.rs
new file mode 100644
index 00000000000..647c13d2437
--- /dev/null
+++ b/library/std/src/sys_common/lazy_box.rs
@@ -0,0 +1,77 @@
+#![allow(dead_code)] // Only used on some platforms.
+
+// This is used to wrap pthread {Mutex, Condvar, RwLock} in.
+
+use crate::marker::PhantomData;
+use crate::ops::{Deref, DerefMut};
+use crate::ptr::null_mut;
+use crate::sync::atomic::{
+    AtomicPtr,
+    Ordering::{AcqRel, Acquire},
+};
+
+pub(crate) struct LazyBox<T: LazyInit> {
+    ptr: AtomicPtr<T>,
+    _phantom: PhantomData<T>,
+}
+
+pub(crate) trait LazyInit {
+    /// This is called before the box is allocated, to provide the value to
+    /// move into the new box.
+    ///
+    /// It might be called more than once per LazyBox, as multiple threads
+    /// might race to initialize it concurrently, each constructing and initializing
+    /// their own box. (All but one of them will be destroyed right after.)
+    fn init() -> Box<Self>;
+}
+
+impl<T: LazyInit> LazyBox<T> {
+    #[inline]
+    pub const fn new() -> Self {
+        Self { ptr: AtomicPtr::new(null_mut()), _phantom: PhantomData }
+    }
+
+    #[inline]
+    fn get_pointer(&self) -> *mut T {
+        let ptr = self.ptr.load(Acquire);
+        if ptr.is_null() { self.initialize() } else { ptr }
+    }
+
+    #[cold]
+    fn initialize(&self) -> *mut T {
+        let new_ptr = Box::into_raw(T::init());
+        match self.ptr.compare_exchange(null_mut(), new_ptr, AcqRel, Acquire) {
+            Ok(_) => new_ptr,
+            Err(ptr) => {
+                // Lost the race to another thread.
+                // Drop the box we created, and use the one from the other thread instead.
+                drop(unsafe { Box::from_raw(new_ptr) });
+                ptr
+            }
+        }
+    }
+}
+
+impl<T: LazyInit> Deref for LazyBox<T> {
+    type Target = T;
+    #[inline]
+    fn deref(&self) -> &T {
+        unsafe { &*self.get_pointer() }
+    }
+}
+
+impl<T: LazyInit> DerefMut for LazyBox<T> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut T {
+        unsafe { &mut *self.get_pointer() }
+    }
+}
+
+impl<T: LazyInit> Drop for LazyBox<T> {
+    fn drop(&mut self) {
+        let ptr = *self.ptr.get_mut();
+        if !ptr.is_null() {
+            drop(unsafe { Box::from_raw(ptr) });
+        }
+    }
+}
diff --git a/library/std/src/sys_common/mod.rs b/library/std/src/sys_common/mod.rs
index 804727fbc54..80f56bf7522 100644
--- a/library/std/src/sys_common/mod.rs
+++ b/library/std/src/sys_common/mod.rs
@@ -24,6 +24,7 @@ pub mod backtrace;
 pub mod condvar;
 pub mod fs;
 pub mod io;
+pub mod lazy_box;
 pub mod memchr;
 pub mod mutex;
 pub mod process;
diff --git a/library/std/src/sys_common/mutex.rs b/library/std/src/sys_common/mutex.rs
index 12a09c98605..36ea888d8de 100644
--- a/library/std/src/sys_common/mutex.rs
+++ b/library/std/src/sys_common/mutex.rs
@@ -61,9 +61,7 @@ unsafe impl Sync for MovableMutex {}
 impl MovableMutex {
     /// Creates a new mutex.
     pub fn new() -> Self {
-        let mut mutex = imp::MovableMutex::from(imp::Mutex::new());
-        unsafe { mutex.init() };
-        Self(mutex)
+        Self(imp::MovableMutex::new())
     }
 
     pub(super) fn raw(&self) -> &imp::Mutex {
@@ -92,9 +90,3 @@ impl MovableMutex {
         self.0.unlock()
     }
 }
-
-impl Drop for MovableMutex {
-    fn drop(&mut self) {
-        unsafe { self.0.destroy() };
-    }
-}
diff --git a/library/std/src/sys_common/remutex.rs b/library/std/src/sys_common/remutex.rs
index 8f252308de7..8921af311d4 100644
--- a/library/std/src/sys_common/remutex.rs
+++ b/library/std/src/sys_common/remutex.rs
@@ -168,13 +168,6 @@ impl<T> ReentrantMutex<T> {
     }
 }
 
-impl<T> Drop for ReentrantMutex<T> {
-    fn drop(&mut self) {
-        // Safety: We're the unique owner of this mutex and not going to use it afterwards.
-        unsafe { self.mutex.destroy() }
-    }
-}
-
 impl<T> Deref for ReentrantMutexGuard<'_, T> {
     type Target = T;
 
diff --git a/library/std/src/sys_common/rwlock.rs b/library/std/src/sys_common/rwlock.rs
index 12e7a72a344..abc9fd561f1 100644
--- a/library/std/src/sys_common/rwlock.rs
+++ b/library/std/src/sys_common/rwlock.rs
@@ -74,7 +74,7 @@ pub struct MovableRwLock(imp::MovableRwLock);
 impl MovableRwLock {
     /// Creates a new reader-writer lock for use.
     pub fn new() -> Self {
-        Self(imp::MovableRwLock::from(imp::RwLock::new()))
+        Self(imp::MovableRwLock::new())
     }
 
     /// Acquires shared access to the underlying lock, blocking the current
@@ -126,9 +126,3 @@ impl MovableRwLock {
         self.0.write_unlock()
     }
 }
-
-impl Drop for MovableRwLock {
-    fn drop(&mut self) {
-        unsafe { self.0.destroy() };
-    }
-}