Auto merge of #97791 - m-ou-se:const-locks, r=m-ou-se

Make {Mutex, Condvar, RwLock}::new() const.

This makes it possible to have `static M: Mutex<_> = Mutex::new(..);` 🎉

Our implementations [on Linux](https://github.com/rust-lang/rust/pull/95035), [on Windows](https://github.com/rust-lang/rust/pull/77380), and various BSDs and some tier 3 platforms have already been using a non-allocating const-constructible implementation. As of https://github.com/rust-lang/rust/pull/97647, the remaining platforms (most notably macOS) now have a const-constructible implementation as well. This means we can finally make these functions publicly const.

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

library/std/src/sync/condvar.rs

@@ -122,8 +122,10 @@ impl Condvar {
     /// let condvar = Condvar::new();
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
     #[must_use]
-    pub fn new() -> Condvar {
+    #[inline]
+    pub const fn new() -> Condvar {
         Condvar { inner: sys::Condvar::new() }
     }
 

library/std/src/sync/mutex.rs

@@ -213,7 +213,9 @@ impl<T> Mutex<T> {
     /// let mutex = Mutex::new(0);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn new(t: T) -> Mutex<T> {
+    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
+    #[inline]
+    pub const fn new(t: T) -> Mutex<T> {
         Mutex {
             inner: sys::MovableMutex::new(),
             poison: poison::Flag::new(),

library/std/src/sync/poison.rs

@@ -19,6 +19,7 @@ pub struct Flag {
 // all cases.
 
 impl Flag {
+    #[inline]
     pub const fn new() -> Flag {
         Flag { failed: AtomicBool::new(false) }
     }

library/std/src/sync/rwlock.rs

@@ -146,7 +146,9 @@ impl<T> RwLock<T> {
     /// let lock = RwLock::new(5);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn new(t: T) -> RwLock<T> {
+    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
+    #[inline]
+    pub const fn new(t: T) -> RwLock<T> {
         RwLock {
             inner: sys::MovableRwLock::new(),
             poison: poison::Flag::new(),