Rollup merge of rust-lang#100576 - joboet:movable_const_remutex, r=Mark-Simulacrum

Make `ReentrantMutex` movable and `const`

As `MovableMutex` is now `const`, it can be used to simplify the implementation and interface of the internal reentrant mutex type. Consequently, the standard output and error streams do not need to be wrapped in `OnceLock` and `OnceLock::get_or_init_pin()` can be removed. Moving the initialization of the `Stdout` buffer to `StdoutLock` means that there is only one synchronization primitive required, at the cost of probably two instructions for every read/write.

Author: matthiaskrgr

library/std/src/io/stdio.rs

@@ -5,10 +5,9 @@ mod tests;
 
 use crate::io::prelude::*;
 
-use crate::cell::{Cell, RefCell};
+use crate::cell::{Cell, RefCell, RefMut};
 use crate::fmt;
 use crate::io::{self, BufReader, IoSlice, IoSliceMut, LineWriter, Lines};
-use crate::pin::Pin;
 use crate::sync::atomic::{AtomicBool, Ordering};
 use crate::sync::{Arc, Mutex, MutexGuard, OnceLock};
 use crate::sys::stdio;
@@ -526,7 +525,7 @@ pub struct Stdout {
     // FIXME: this should be LineWriter or BufWriter depending on the state of
     //        stdout (tty or not). Note that if this is not line buffered it
     //        should also flush-on-panic or some form of flush-on-abort.
-    inner: Pin<&'static ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>>,
+    inner: &'static ReentrantMutex<RefCell<Option<LineWriter<StdoutRaw>>>>,
 }
 
 /// A locked reference to the [`Stdout`] handle.
@@ -548,10 +547,11 @@ pub struct Stdout {
 #[must_use = "if unused stdout will immediately unlock"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct StdoutLock<'a> {
-    inner: ReentrantMutexGuard<'a, RefCell<LineWriter<StdoutRaw>>>,
+    inner: ReentrantMutexGuard<'a, RefCell<Option<LineWriter<StdoutRaw>>>>,
 }
 
-static STDOUT: OnceLock<ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>> = OnceLock::new();
+static STDOUT: ReentrantMutex<RefCell<Option<LineWriter<StdoutRaw>>>> =
+    ReentrantMutex::new(RefCell::new(None));
 
 /// Constructs a new handle to the standard output of the current process.
 ///
@@ -602,25 +602,18 @@ static STDOUT: OnceLock<ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>> = OnceLo
 #[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stdout() -> Stdout {
-    Stdout {
-        inner: Pin::static_ref(&STDOUT).get_or_init_pin(
-            || unsafe { ReentrantMutex::new(RefCell::new(LineWriter::new(stdout_raw()))) },
-            |mutex| unsafe { mutex.init() },
-        ),
-    }
+    Stdout { inner: &STDOUT }
 }
 
 pub fn cleanup() {
-    if let Some(instance) = STDOUT.get() {
-        // Flush the data and disable buffering during shutdown
-        // by replacing the line writer by one with zero
-        // buffering capacity.
-        // We use try_lock() instead of lock(), because someone
-        // might have leaked a StdoutLock, which would
-        // otherwise cause a deadlock here.
-        if let Some(lock) = Pin::static_ref(instance).try_lock() {
-            *lock.borrow_mut() = LineWriter::with_capacity(0, stdout_raw());
-        }
+    // Flush the data and disable buffering during shutdown
+    // by replacing the line writer by one with zero
+    // buffering capacity.
+    // We use try_lock() instead of lock(), because someone
+    // might have leaked a StdoutLock, which would
+    // otherwise cause a deadlock here.
+    if let Some(lock) = STDOUT.try_lock() {
+        *lock.borrow_mut() = Some(LineWriter::with_capacity(0, stdout_raw()));
     }
 }
 
@@ -712,26 +705,38 @@ impl Write for &Stdout {
     }
 }
 
+impl StdoutLock<'_> {
+    #[inline]
+    fn inner(&self) -> RefMut<'_, LineWriter<StdoutRaw>> {
+        #[cold]
+        fn init() -> LineWriter<StdoutRaw> {
+            LineWriter::new(stdout_raw())
+        }
+
+        RefMut::map(self.inner.borrow_mut(), |w| w.get_or_insert_with(init))
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Write for StdoutLock<'_> {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-        self.inner.borrow_mut().write(buf)
+        self.inner().write(buf)
     }
     fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
-        self.inner.borrow_mut().write_vectored(bufs)
+        self.inner().write_vectored(bufs)
     }
     #[inline]
     fn is_write_vectored(&self) -> bool {
-        self.inner.borrow_mut().is_write_vectored()
+        self.inner().is_write_vectored()
     }
     fn flush(&mut self) -> io::Result<()> {
-        self.inner.borrow_mut().flush()
+        self.inner().flush()
     }
     fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
-        self.inner.borrow_mut().write_all(buf)
+        self.inner().write_all(buf)
     }
     fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
-        self.inner.borrow_mut().write_all_vectored(bufs)
+        self.inner().write_all_vectored(bufs)
     }
 }
 
@@ -761,7 +766,7 @@ impl fmt::Debug for StdoutLock<'_> {
 /// standard library or via raw Windows API calls, will fail.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Stderr {
-    inner: Pin<&'static ReentrantMutex<RefCell<StderrRaw>>>,
+    inner: &'static ReentrantMutex<RefCell<StderrRaw>>,
 }
 
 /// A locked reference to the [`Stderr`] handle.
@@ -834,16 +839,12 @@ pub struct StderrLock<'a> {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn stderr() -> Stderr {
     // Note that unlike `stdout()` we don't use `at_exit` here to register a
-    // destructor. Stderr is not buffered , so there's no need to run a
+    // destructor. Stderr is not buffered, so there's no need to run a
     // destructor for flushing the buffer
-    static INSTANCE: OnceLock<ReentrantMutex<RefCell<StderrRaw>>> = OnceLock::new();
+    static INSTANCE: ReentrantMutex<RefCell<StderrRaw>> =
+        ReentrantMutex::new(RefCell::new(stderr_raw()));
 
-    Stderr {
-        inner: Pin::static_ref(&INSTANCE).get_or_init_pin(
-            || unsafe { ReentrantMutex::new(RefCell::new(stderr_raw())) },
-            |mutex| unsafe { mutex.init() },
-        ),
-    }
+    Stderr { inner: &INSTANCE }
 }
 
 impl Stderr {

library/std/src/sync/once_lock.rs

@@ -3,7 +3,6 @@ use crate::fmt;
 use crate::marker::PhantomData;
 use crate::mem::MaybeUninit;
 use crate::panic::{RefUnwindSafe, UnwindSafe};
-use crate::pin::Pin;
 use crate::sync::Once;
 
 /// A synchronization primitive which can be written to only once.
@@ -223,60 +222,6 @@ impl<T> OnceLock<T> {
         Ok(unsafe { self.get_unchecked() })
     }
 
-    /// Internal-only API that gets the contents of the cell, initializing it
-    /// in two steps with `f` and `g` if the cell was empty.
-    ///
-    /// `f` is called to construct the value, which is then moved into the cell
-    /// and given as a (pinned) mutable reference to `g` to finish
-    /// initialization.
-    ///
-    /// This allows `g` to inspect an manipulate the value after it has been
-    /// moved into its final place in the cell, but before the cell is
-    /// considered initialized.
-    ///
-    /// # Panics
-    ///
-    /// If `f` or `g` panics, the panic is propagated to the caller, and the
-    /// cell remains uninitialized.
-    ///
-    /// With the current implementation, if `g` panics, the value from `f` will
-    /// not be dropped. This should probably be fixed if this is ever used for
-    /// a type where this matters.
-    ///
-    /// It is an error to reentrantly initialize the cell from `f`. The exact
-    /// outcome is unspecified. Current implementation deadlocks, but this may
-    /// be changed to a panic in the future.
-    pub(crate) fn get_or_init_pin<F, G>(self: Pin<&Self>, f: F, g: G) -> Pin<&T>
-    where
-        F: FnOnce() -> T,
-        G: FnOnce(Pin<&mut T>),
-    {
-        if let Some(value) = self.get_ref().get() {
-            // SAFETY: The inner value was already initialized, and will not be
-            // moved anymore.
-            return unsafe { Pin::new_unchecked(value) };
-        }
-
-        let slot = &self.value;
-
-        // Ignore poisoning from other threads
-        // If another thread panics, then we'll be able to run our closure
-        self.once.call_once_force(|_| {
-            let value = f();
-            // SAFETY: We use the Once (self.once) to guarantee unique access
-            // to the UnsafeCell (slot).
-            let value: &mut T = unsafe { (&mut *slot.get()).write(value) };
-            // SAFETY: The value has been written to its final place in
-            // self.value. We do not to move it anymore, which we promise here
-            // with a Pin<&mut T>.
-            g(unsafe { Pin::new_unchecked(value) });
-        });
-
-        // SAFETY: The inner value has been initialized, and will not be moved
-        // anymore.
-        unsafe { Pin::new_unchecked(self.get_ref().get_unchecked()) }
-    }
-
     /// Consumes the `OnceLock`, returning the wrapped value. Returns
     /// `None` if the cell was empty.
     ///

library/std/src/sys/hermit/mutex.rs

@@ -174,9 +174,6 @@ impl Mutex {
         Mutex { inner: Spinlock::new(MutexInner::new()) }
     }
 
-    #[inline]
-    pub unsafe fn init(&mut self) {}
-
     #[inline]
     pub unsafe fn lock(&self) {
         loop {

library/std/src/sys/itron/mutex.rs

@@ -31,12 +31,6 @@ impl Mutex {
         Mutex { mtx: SpinIdOnceCell::new() }
     }
 
-    pub unsafe fn init(&mut self) {
-        // Initialize `self.mtx` eagerly
-        let id = new_mtx().unwrap_or_else(|e| fail(e, &"acre_mtx"));
-        unsafe { self.mtx.set_unchecked((id, ())) };
-    }
-
     /// Get the inner mutex's ID, which is lazily created.
     fn raw(&self) -> abi::ID {
         match self.mtx.get_or_try_init(|| new_mtx().map(|id| (id, ()))) {

library/std/src/sys/sgx/mutex.rs

@@ -20,9 +20,6 @@ impl Mutex {
         Mutex { inner: SpinMutex::new(WaitVariable::new(false)) }
     }
 
-    #[inline]
-    pub unsafe fn init(&mut self) {}
-
     #[inline]
     pub unsafe fn lock(&self) {
         let mut guard = self.inner.lock();

library/std/src/sys/unix/locks/futex_mutex.rs

@@ -19,9 +19,6 @@ impl Mutex {
         Self { futex: AtomicU32::new(0) }
     }
 
-    #[inline]
-    pub unsafe fn init(&mut self) {}
-
     #[inline]
     pub unsafe fn try_lock(&self) -> bool {
         self.futex.compare_exchange(0, 1, Acquire, Relaxed).is_ok()

library/std/src/sys/unix/locks/pthread_mutex.rs

@@ -52,7 +52,7 @@ impl Mutex {
         Mutex { inner: UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER) }
     }
     #[inline]
-    pub unsafe fn init(&mut self) {
+    unsafe fn init(&mut self) {
         // Issue #33770
         //
         // A pthread mutex initialized with PTHREAD_MUTEX_INITIALIZER will have

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

@@ -16,9 +16,6 @@ impl Mutex {
         Mutex { locked: Cell::new(false) }
     }
 
-    #[inline]
-    pub unsafe fn init(&mut self) {}
-
     #[inline]
     pub unsafe fn lock(&self) {
         assert_eq!(self.locked.replace(true), false, "cannot recursively acquire mutex");

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

@@ -37,8 +37,6 @@ impl Mutex {
     pub const fn new() -> Mutex {
         Mutex { srwlock: UnsafeCell::new(c::SRWLOCK_INIT) }
     }
-    #[inline]
-    pub unsafe fn init(&mut self) {}
 
     #[inline]
     pub unsafe fn lock(&self) {