avoid using the word 'initialized' to talk about that non-reentrant-capable state of the mutex

src/libstd/io/lazy.rs

@@ -15,6 +15,7 @@ use sys_common;
 use sys_common::mutex::Mutex;
 
 pub struct Lazy<T> {
+    // We never call `lock.init()`, so it is UB to attempt to acquire this mutex reentrantly!
     lock: Mutex,
     ptr: Cell<*mut Arc<T>>,
     init: fn() -> Arc<T>,
@@ -29,8 +30,6 @@ impl<T: Send + Sync + 'static> Lazy<T> {
     /// Safety: `init` must not call `get` on the variable that is being
     /// initialized.
     pub const unsafe fn new(init: fn() -> Arc<T>) -> Lazy<T> {
-        // `lock` is never initialized fully, so it is UB to attempt to
-        // acquire this mutex reentrantly!
         Lazy {
             lock: Mutex::new(),
             ptr: Cell::new(ptr::null_mut()),

src/libstd/sys/unix/args.rs

@@ -80,7 +80,7 @@ mod imp {
 
     static mut ARGC: isize = 0;
     static mut ARGV: *const *const u8 = ptr::null();
-    // `ENV_LOCK` is never initialized fully, so it is UB to attempt to
+    // We never call `ENV_LOCK.init()`, so it is UB to attempt to
     // acquire this mutex reentrantly!
     static LOCK: Mutex = Mutex::new();
 

src/libstd/sys/unix/os.rs

@@ -33,7 +33,7 @@ use sys::fd;
 use vec;
 
 const TMPBUF_SZ: usize = 128;
-// `ENV_LOCK` is never initialized fully, so it is UB to attempt to
+// We never call `ENV_LOCK.init()`, so it is UB to attempt to
 // acquire this mutex reentrantly!
 static ENV_LOCK: Mutex = Mutex::new();
 

src/libstd/sys_common/at_exit_imp.rs

@@ -23,7 +23,7 @@ type Queue = Vec<Box<dyn FnBox()>>;
 // on poisoning and this module needs to operate at a lower level than requiring
 // the thread infrastructure to be in place (useful on the borders of
 // initialization/destruction).
-// `LOCK` is never initialized fully, so it is UB to attempt to
+// We never call `LOCK.init()`, so it is UB to attempt to
 // acquire this mutex reentrantly!
 static LOCK: Mutex = Mutex::new();
 static mut QUEUE: *mut Queue = ptr::null_mut();

src/libstd/sys_common/mutex.rs

@@ -32,7 +32,9 @@ impl Mutex {
     /// Prepare the mutex for use.
     ///
     /// This should be called once the mutex is at a stable memory address.
-    /// Behavior is undefined unless this is called before any other operation.
+    /// If called, this must be the very first thing that happens to the mutex.
+    /// Calling it in parallel with or after any operation (including another
+    /// `init()`) is undefined behavior.
     #[inline]
     pub unsafe fn init(&mut self) { self.0.init() }
 

src/libstd/sys_common/thread_local.rs

@@ -161,7 +161,7 @@ impl StaticKey {
         // Additionally a 0-index of a tls key hasn't been seen on windows, so
         // we just simplify the whole branch.
         if imp::requires_synchronized_create() {
-            // `INIT_LOCK` is never initialized fully, so it is UB to attempt to
+            // We never call `INIT_LOCK.init()`, so it is UB to attempt to
             // acquire this mutex reentrantly!
             static INIT_LOCK: Mutex = Mutex::new();
             let _guard = INIT_LOCK.lock();

src/libstd/thread/mod.rs

@@ -940,7 +940,7 @@ pub struct ThreadId(u64);
 impl ThreadId {
     // Generate a new unique thread ID.
     fn new() -> ThreadId {
-        // `GUARD` is never initialized fully, so it is UB to attempt to
+        // We never call `GUARD.init()`, so it is UB to attempt to
         // acquire this mutex reentrantly!
         static GUARD: mutex::Mutex = mutex::Mutex::new();
         static mut COUNTER: u64 = 0;