diff --git a/library/std/src/sys/unix/locks/pthread_mutex.rs b/library/std/src/sys/unix/locks/pthread_mutex.rs
index 916e898d8906e..98afee69ba622 100644
--- a/library/std/src/sys/unix/locks/pthread_mutex.rs
+++ b/library/std/src/sys/unix/locks/pthread_mutex.rs
@@ -1,5 +1,5 @@
 use crate::cell::UnsafeCell;
-use crate::mem::MaybeUninit;
+use crate::mem::{forget, MaybeUninit};
 use crate::sys::cvt_nz;
 use crate::sys_common::lazy_box::{LazyBox, LazyInit};
 
@@ -23,6 +23,24 @@ impl LazyInit for Mutex {
         unsafe { mutex.init() };
         mutex
     }
+
+    fn destroy(mutex: Box<Self>) {
+        // We're not allowed to pthread_mutex_destroy a locked mutex,
+        // so check first if it's unlocked.
+        if unsafe { mutex.try_lock() } {
+            unsafe { mutex.unlock() };
+            drop(mutex);
+        } else {
+            // The mutex is locked. This happens if a MutexGuard is leaked.
+            // In this case, we just leak the Mutex too.
+            forget(mutex);
+        }
+    }
+
+    fn cancel_init(_: Box<Self>) {
+        // In this case, we can just drop it without any checks,
+        // since it cannot have been locked yet.
+    }
 }
 
 impl Mutex {
diff --git a/library/std/src/sys/unix/locks/pthread_rwlock.rs b/library/std/src/sys/unix/locks/pthread_rwlock.rs
index 75e5759c7879d..f5a6d232ae492 100644
--- a/library/std/src/sys/unix/locks/pthread_rwlock.rs
+++ b/library/std/src/sys/unix/locks/pthread_rwlock.rs
@@ -17,6 +17,21 @@ impl LazyInit for RwLock {
     fn init() -> Box<Self> {
         Box::new(Self::new())
     }
+
+    fn destroy(mut rwlock: Box<Self>) {
+        // We're not allowed to pthread_rwlock_destroy a locked rwlock,
+        // so check first if it's unlocked.
+        if *rwlock.write_locked.get_mut() || *rwlock.num_readers.get_mut() != 0 {
+            // The rwlock is locked. This happens if a RwLock{Read,Write}Guard is leaked.
+            // In this case, we just leak the RwLock too.
+            forget(rwlock);
+        }
+    }
+
+    fn cancel_init(_: Box<Self>) {
+        // In this case, we can just drop it without any checks,
+        // since it cannot have been locked yet.
+    }
 }
 
 impl RwLock {
diff --git a/library/std/src/sys_common/lazy_box.rs b/library/std/src/sys_common/lazy_box.rs
index 647c13d243724..63c3316bdeb28 100644
--- a/library/std/src/sys_common/lazy_box.rs
+++ b/library/std/src/sys_common/lazy_box.rs
@@ -21,8 +21,21 @@ pub(crate) trait LazyInit {
     ///
     /// 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.)
+    /// their own box. All but one of them will be passed to `cancel_init` right after.
     fn init() -> Box<Self>;
+
+    /// Any surplus boxes from `init()` that lost the initialization race
+    /// are passed to this function for disposal.
+    ///
+    /// The default implementation calls destroy().
+    fn cancel_init(x: Box<Self>) {
+        Self::destroy(x);
+    }
+
+    /// This is called to destroy a used box.
+    ///
+    /// The default implementation just drops it.
+    fn destroy(_: Box<Self>) {}
 }
 
 impl<T: LazyInit> LazyBox<T> {
@@ -45,7 +58,7 @@ impl<T: LazyInit> LazyBox<T> {
             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) });
+                T::cancel_init(unsafe { Box::from_raw(new_ptr) });
                 ptr
             }
         }
@@ -71,7 +84,7 @@ 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) });
+            T::destroy(unsafe { Box::from_raw(ptr) });
         }
     }
 }