Rollup merge of #77147 - fusion-engineering-forks:static-mutex, r=dtolnay

Split sys_common::Mutex in StaticMutex and MovableMutex.

The (unsafe) `Mutex` from `sys_common` had a rather complicated interface. You were supposed to call `init()` manually, unless you could guarantee it was neither moved nor used reentrantly.

Calling `destroy()` was also optional, although it was unclear if 1) resources might be leaked or not, and 2) if `destroy()` should only be called when `init()` was called.

This allowed for a number of interesting (confusing?) different ways to use this `Mutex`, all captured in a single type.

In practice, this type was only ever used in two ways:

1. As a static variable. In this case, neither `init()` nor `destroy()` are called. The variable is never moved, and it is never used reentrantly. It is only ever locked using the `LockGuard`, never with `raw_lock`.

2. As a `Box`ed variable. In this case, both `init()` and `destroy()` are called, it will be moved and possibly used reentrantly.

No other combinations are used anywhere in `std`.

This change simplifies things by splitting this `Mutex` type into two types matching the two use cases: `StaticMutex` and `MovableMutex`.

The interface of both new types is now both safer and simpler. The first one does not call nor expose `init`/`destroy`, and the second one calls those automatically in its `new()` and `Drop` functions. Also, the locking functions of `MovableMutex` are no longer unsafe.

---

This will also make it easier to conditionally box mutexes later, by moving that decision into sys/sys_common. Some of the mutex implementations (at least those of Wasm and 'sys/unsupported') are safe to move, so wouldn't need a box. ~~(But that's blocked on  #76932 for now.)~~ (See #77380.)

library/std/src/thread/mod.rs

@@ -972,9 +972,8 @@ pub struct ThreadId(NonZeroU64);
 impl ThreadId {
     // Generate a new unique thread ID.
     fn new() -> ThreadId {
-        // We never call `GUARD.init()`, so it is UB to attempt to
-        // acquire this mutex reentrantly!
-        static GUARD: mutex::Mutex = mutex::Mutex::new();
+        // It is UB to attempt to acquire this mutex reentrantly!
+        static GUARD: mutex::StaticMutex = mutex::StaticMutex::new();
         static mut COUNTER: u64 = 1;
 
         unsafe {