Rollup merge of #76932 - fusion-engineering-forks:condvar-promise, r=sfackler

Relax promises about condition variable.

For quite a while now, there have been plans to at some point use parking_lot or some other more efficient implementation of mutexes and condition variables. Right now, Mutex and CondVar both Box the 'real' mutex/condvar inside, to give it a stable address. This was done because implementations like pthread and Windows critical sections may not be moved. More efficient implementations based on futexes, WaitOnAddress, Windows SRW locks, parking_lot, etc. may be moved (while not borrowed), so wouldn't need boxing.

However, not boxing them (which would be great goal to achieve), breaks a promise std currently makes about CondVar. CondVar promises to panic when used with different mutexes, to ensure consistent behaviour on all platforms. To this check, a mutex is considered 'the same' if the address of the 'real mutex' in the Box is the same. This address doesn't change when moving a `std::mutex::Mutex` object, effectively giving it an identity that survives moves of the Mutex object. If we ever switch to a non-boxed version, they no longer carry such an identity, and this check can no longer be made.

Four options:
1. Always box mutexes.
2. Add a `MutexId` similar to `ThreadId`. Making mutexes bigger, and making it hard to ever have a `const fn new` for them.
3. Making the requirement of CondVar stricter: panic if the Mutex object itself moved.
4. Making the promise of CondVar weaker: don't promise to panic.

1, 2, and 3 seem like bad options. This PR updates the documentation for 4.

Author: jonas-schievink

library/std/src/sync/condvar.rs

@@ -78,13 +78,9 @@ impl WaitTimeoutResult {
 /// and a mutex. The predicate is always verified inside of the mutex before
 /// determining that a thread must block.
 ///
-/// Functions in this module will block the current **thread** of execution and
-/// are bindings to system-provided condition variables where possible. Note
-/// that this module places one additional restriction over the system condition
-/// variables: each condvar can be used with precisely one mutex at runtime. Any
-/// attempt to use multiple mutexes on the same condition variable will result
-/// in a runtime panic. If this is not desired, then the unsafe primitives in
-/// `sys` do not have this restriction but may result in undefined behavior.
+/// Functions in this module will block the current **thread** of execution.
+/// Note that any attempt to use multiple mutexes on the same condition
+/// variable may result in a runtime panic.
 ///
 /// # Examples
 ///
@@ -159,10 +155,8 @@ impl Condvar {
     ///
     /// # Panics
     ///
-    /// This function will [`panic!`] if it is used with more than one mutex
-    /// over time. Each condition variable is dynamically bound to exactly one
-    /// mutex to ensure defined behavior across platforms. If this functionality
-    /// is not desired, then unsafe primitives in `sys` are provided.
+    /// This function may [`panic!`] if it is used with more than one mutex
+    /// over time.
     ///
     /// [`notify_one`]: Self::notify_one
     /// [`notify_all`]: Self::notify_all