diff --git a/library/std/src/sys/unix/locks/futex.rs b/library/std/src/sys/unix/locks/futex.rs
index 630351d0dc278..d97777e4da29d 100644
--- a/library/std/src/sys/unix/locks/futex.rs
+++ b/library/std/src/sys/unix/locks/futex.rs
@@ -1,5 +1,6 @@
+use crate::cell::UnsafeCell;
 use crate::sync::atomic::{
-    AtomicI32,
+    AtomicI32, AtomicUsize,
     Ordering::{Acquire, Relaxed, Release},
 };
 use crate::sys::futex::{futex_wait, futex_wake, futex_wake_all};
@@ -162,3 +163,98 @@ impl Condvar {
         r
     }
 }
+
+/// A reentrant mutex. Used by stdout().lock() and friends.
+///
+/// The 'owner' field tracks which thread has locked the mutex.
+///
+/// We use current_thread_unique_ptr() as the thread identifier,
+/// which is just the address of a thread local variable.
+///
+/// If `owner` is set to the identifier of the current thread,
+/// we assume the mutex is already locked and instead of locking it again,
+/// we increment `lock_count`.
+///
+/// When unlocking, we decrement `lock_count`, and only unlock the mutex when
+/// it reaches zero.
+///
+/// `lock_count` is protected by the mutex and only accessed by the thread that has
+/// locked the mutex, so needs no synchronization.
+///
+/// `owner` can be checked by other threads that want to see if they already
+/// hold the lock, so needs to be atomic. If it compares equal, we're on the
+/// same thread that holds the mutex and memory access can use relaxed ordering
+/// since we're not dealing with multiple threads. If it compares unequal,
+/// synchronization is left to the mutex, making relaxed memory ordering for
+/// the `owner` field fine in all cases.
+pub struct ReentrantMutex {
+    mutex: Mutex,
+    owner: AtomicUsize,
+    lock_count: UnsafeCell<u32>,
+}
+
+unsafe impl Send for ReentrantMutex {}
+unsafe impl Sync for ReentrantMutex {}
+
+impl ReentrantMutex {
+    #[inline]
+    pub const unsafe fn uninitialized() -> Self {
+        Self { mutex: Mutex::new(), owner: AtomicUsize::new(0), lock_count: UnsafeCell::new(0) }
+    }
+
+    #[inline]
+    pub unsafe fn init(&self) {}
+
+    #[inline]
+    pub unsafe fn destroy(&self) {}
+
+    pub unsafe fn try_lock(&self) -> bool {
+        let this_thread = current_thread_unique_ptr();
+        if self.owner.load(Relaxed) == this_thread {
+            self.increment_lock_count();
+            true
+        } else if self.mutex.try_lock() {
+            self.owner.store(this_thread, Relaxed);
+            debug_assert_eq!(*self.lock_count.get(), 0);
+            *self.lock_count.get() = 1;
+            true
+        } else {
+            false
+        }
+    }
+
+    pub unsafe fn lock(&self) {
+        let this_thread = current_thread_unique_ptr();
+        if self.owner.load(Relaxed) == this_thread {
+            self.increment_lock_count();
+        } else {
+            self.mutex.lock();
+            self.owner.store(this_thread, Relaxed);
+            debug_assert_eq!(*self.lock_count.get(), 0);
+            *self.lock_count.get() = 1;
+        }
+    }
+
+    unsafe fn increment_lock_count(&self) {
+        *self.lock_count.get() = (*self.lock_count.get())
+            .checked_add(1)
+            .expect("lock count overflow in reentrant mutex");
+    }
+
+    pub unsafe fn unlock(&self) {
+        *self.lock_count.get() -= 1;
+        if *self.lock_count.get() == 0 {
+            self.owner.store(0, Relaxed);
+            self.mutex.unlock();
+        }
+    }
+}
+
+/// Get an address that is unique per running thread.
+///
+/// This can be used as a non-null usize-sized ID.
+pub fn current_thread_unique_ptr() -> usize {
+    // Use a non-drop type to make sure it's still available during thread destruction.
+    thread_local! { static X: u8 = const { 0 } }
+    X.with(|x| <*const _>::addr(x))
+}
diff --git a/library/std/src/sys/unix/locks/mod.rs b/library/std/src/sys/unix/locks/mod.rs
index 85afc939d2e89..e0404f40c69bf 100644
--- a/library/std/src/sys/unix/locks/mod.rs
+++ b/library/std/src/sys/unix/locks/mod.rs
@@ -5,11 +5,7 @@ cfg_if::cfg_if! {
     ))] {
         mod futex;
         mod futex_rwlock;
-        #[allow(dead_code)]
-        mod pthread_mutex; // Only used for PthreadMutexAttr, needed by pthread_remutex.
-        mod pthread_remutex; // FIXME: Implement this using a futex
-        pub use futex::{Mutex, MovableMutex, Condvar, MovableCondvar};
-        pub use pthread_remutex::ReentrantMutex;
+        pub use futex::{Mutex, MovableMutex, Condvar, MovableCondvar, ReentrantMutex};
         pub use futex_rwlock::{RwLock, MovableRwLock};
     } else {
         mod pthread_mutex;
diff --git a/library/std/src/sys/unix/mod.rs b/library/std/src/sys/unix/mod.rs
index e65c11b6d09fa..3ad03a8868199 100644
--- a/library/std/src/sys/unix/mod.rs
+++ b/library/std/src/sys/unix/mod.rs
@@ -215,6 +215,7 @@ where
     }
 }
 
+#[allow(dead_code)] // Not used on all platforms.
 pub fn cvt_nz(error: libc::c_int) -> crate::io::Result<()> {
     if error == 0 { Ok(()) } else { Err(crate::io::Error::from_raw_os_error(error)) }
 }
diff --git a/library/std/src/thread/local.rs b/library/std/src/thread/local.rs
index fc307c5666d63..047d07ad57df1 100644
--- a/library/std/src/thread/local.rs
+++ b/library/std/src/thread/local.rs
@@ -179,6 +179,7 @@ macro_rules! __thread_local_inner {
     // used to generate the `LocalKey` value for const-initialized thread locals
     (@key $t:ty, const $init:expr) => {{
         #[cfg_attr(not(windows), inline(always))] // see comments below
+        #[deny(unsafe_op_in_unsafe_fn)]
         unsafe fn __getit(
             _init: $crate::option::Option<&mut $crate::option::Option<$t>>,
         ) -> $crate::option::Option<&'static $t> {
@@ -193,7 +194,7 @@ macro_rules! __thread_local_inner {
             #[cfg(all(target_family = "wasm", not(target_feature = "atomics")))]
             {
                 static mut VAL: $t = INIT_EXPR;
-                $crate::option::Option::Some(&VAL)
+                unsafe { $crate::option::Option::Some(&VAL) }
             }
 
             // If the platform has support for `#[thread_local]`, use it.