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/sys/unix/args.rs

@@ -80,13 +80,13 @@ mod imp {
     use crate::ptr;
     use crate::sync::atomic::{AtomicIsize, AtomicPtr, Ordering};
 
-    use crate::sys_common::mutex::Mutex;
+    use crate::sys_common::mutex::StaticMutex;
 
     static ARGC: AtomicIsize = AtomicIsize::new(0);
     static ARGV: AtomicPtr<*const u8> = AtomicPtr::new(ptr::null_mut());
     // We never call `ENV_LOCK.init()`, so it is UB to attempt to
     // acquire this mutex reentrantly!
-    static LOCK: Mutex = Mutex::new();
+    static LOCK: StaticMutex = StaticMutex::new();
 
     unsafe fn really_init(argc: isize, argv: *const *const u8) {
         let _guard = LOCK.lock();

library/std/src/sys/unix/os.rs

@@ -21,7 +21,7 @@ use crate::slice;
 use crate::str;
 use crate::sys::cvt;
 use crate::sys::fd;
-use crate::sys_common::mutex::{Mutex, MutexGuard};
+use crate::sys_common::mutex::{StaticMutex, StaticMutexGuard};
 use crate::vec;
 
 use libc::{c_char, c_int, c_void};
@@ -470,10 +470,9 @@ pub unsafe fn environ() -> *mut *const *const c_char {
     &mut environ
 }
 
-pub unsafe fn env_lock() -> MutexGuard<'static> {
-    // We never call `ENV_LOCK.init()`, so it is UB to attempt to
-    // acquire this mutex reentrantly!
-    static ENV_LOCK: Mutex = Mutex::new();
+pub unsafe fn env_lock() -> StaticMutexGuard<'static> {
+    // It is UB to attempt to acquire this mutex reentrantly!
+    static ENV_LOCK: StaticMutex = StaticMutex::new();
     ENV_LOCK.lock()
 }