Rollup merge of #123879 - beetrees:missing-unsafe, r=Mark-Simulacrum

Add missing `unsafe` to some internal `std` functions

Adds `unsafe` to a few internal functions that have safety requirements but were previously not marked as `unsafe`. Specifically:

- `std::sys::pal::unix:🧵:min_stack_size` needs to be `unsafe` as `__pthread_get_minstack` might dereference the passed pointer. All callers currently pass a valid initialised `libc::pthread_attr_t`.
- `std:🧵:Thread::new` (and `new_inner`) need to be `unsafe` as it requires the passed thread name to be valid UTF-8, otherwise `Thread::name` will trigger undefined behaviour. I've taken the opportunity to split out the unnamed thread case into a separate `new_unnamed` function to make the safety requirement clearer. All callers meet the safety requirement now that #123505 has been merged.

library/std/src/sys/pal/unix/thread.rs

@@ -709,7 +709,7 @@ fn find_mountpoint(group_path: &Path) -> Option<(Cow<'static, str>, &Path)> {
 // is created in an application with big thread-local storage requirements.
 // See #6233 for rationale and details.
 #[cfg(all(target_os = "linux", target_env = "gnu"))]
-fn min_stack_size(attr: *const libc::pthread_attr_t) -> usize {
+unsafe fn min_stack_size(attr: *const libc::pthread_attr_t) -> usize {
     // We use dlsym to avoid an ELF version dependency on GLIBC_PRIVATE. (#23628)
     // We shouldn't really be using such an internal symbol, but there's currently
     // no other way to account for the TLS size.
@@ -723,11 +723,11 @@ fn min_stack_size(attr: *const libc::pthread_attr_t) -> usize {
 
 // No point in looking up __pthread_get_minstack() on non-glibc platforms.
 #[cfg(all(not(all(target_os = "linux", target_env = "gnu")), not(target_os = "netbsd")))]
-fn min_stack_size(_: *const libc::pthread_attr_t) -> usize {
+unsafe fn min_stack_size(_: *const libc::pthread_attr_t) -> usize {
     libc::PTHREAD_STACK_MIN
 }
 
 #[cfg(target_os = "netbsd")]
-fn min_stack_size(_: *const libc::pthread_attr_t) -> usize {
+unsafe fn min_stack_size(_: *const libc::pthread_attr_t) -> usize {
     2048 // just a guess
 }

library/std/src/sys/sync/rwlock/queue.rs

@@ -202,7 +202,7 @@ fn new(write: bool) -> Node {
     fn prepare(&mut self) {
         // Fall back to creating an unnamed `Thread` handle to allow locking in
         // TLS destructors.
-        self.thread.get_or_init(|| thread::try_current().unwrap_or_else(|| Thread::new(None)));
+        self.thread.get_or_init(|| thread::try_current().unwrap_or_else(Thread::new_unnamed));
         self.completed = AtomicBool::new(false);
     }
 

library/std/src/thread/mod.rs

@@ -487,9 +487,11 @@ unsafe fn spawn_unchecked_<'a, 'scope, F, T>(
             amt
         });
 
-        let my_thread = Thread::new(name.map(|name| {
-            CString::new(name).expect("thread name may not contain interior null bytes")
-        }));
+        let my_thread = name.map_or_else(Thread::new_unnamed, |name| unsafe {
+            Thread::new(
+                CString::new(name).expect("thread name may not contain interior null bytes"),
+            )
+        });
         let their_thread = my_thread.clone();
 
         let my_packet: Arc<Packet<'scope, T>> = Arc::new(Packet {
@@ -711,7 +713,7 @@ pub(crate) fn set_current(thread: Thread) {
 /// In contrast to the public `current` function, this will not panic if called
 /// from inside a TLS destructor.
 pub(crate) fn try_current() -> Option<Thread> {
-    CURRENT.try_with(|current| current.get_or_init(|| Thread::new(None)).clone()).ok()
+    CURRENT.try_with(|current| current.get_or_init(|| Thread::new_unnamed()).clone()).ok()
 }
 
 /// Gets a handle to the thread that invokes it.
@@ -1307,21 +1309,26 @@ pub struct Thread {
 }
 
 impl Thread {
-    // Used only internally to construct a thread object without spawning
-    pub(crate) fn new(name: Option<CString>) -> Thread {
-        if let Some(name) = name {
-            Self::new_inner(ThreadName::Other(name))
-        } else {
-            Self::new_inner(ThreadName::Unnamed)
-        }
+    /// Used only internally to construct a thread object without spawning.
+    ///
+    /// # Safety
+    /// `name` must be valid UTF-8.
+    pub(crate) unsafe fn new(name: CString) -> Thread {
+        unsafe { Self::new_inner(ThreadName::Other(name)) }
+    }
+
+    pub(crate) fn new_unnamed() -> Thread {
+        unsafe { Self::new_inner(ThreadName::Unnamed) }
     }
 
     // Used in runtime to construct main thread
     pub(crate) fn new_main() -> Thread {
-        Self::new_inner(ThreadName::Main)
+        unsafe { Self::new_inner(ThreadName::Main) }
     }
 
-    fn new_inner(name: ThreadName) -> Thread {
+    /// # Safety
+    /// If `name` is `ThreadName::Other(_)`, the contained string must be valid UTF-8.
+    unsafe fn new_inner(name: ThreadName) -> Thread {
         // We have to use `unsafe` here to construct the `Parker` in-place,
         // which is required for the UNIX implementation.
         //