Remove the Arc rt::init allocation for thread info

std/src/lib.rs

@@ -364,6 +364,7 @@
 #![feature(str_internals)]
 #![feature(strict_provenance)]
 #![feature(strict_provenance_atomic_ptr)]
+#![feature(sync_unsafe_cell)]
 #![feature(ub_checks)]
 // tidy-alphabetical-end
 //

std/src/rt.rs

@@ -110,7 +110,7 @@ unsafe fn init(argc: isize, argv: *const *const u8, sigpipe: u8) {
     // handle does not match the current ID, we should attempt to use the
     // current thread ID here instead of unconditionally creating a new
     // one. Also see #130210.
-    let thread = Thread::new_main(thread::current_id());
+    let thread = unsafe { Thread::new_main(thread::current_id()) };
     if let Err(_thread) = thread::set_current(thread) {
         // `thread::current` will create a new handle if none has been set yet.
         // Thus, if someone uses it before main, this call will fail. That's a

std/src/thread/mod.rs

@@ -158,9 +158,12 @@
 #[cfg(all(test, not(any(target_os = "emscripten", target_os = "wasi"))))]
 mod tests;
 
+use core::cell::SyncUnsafeCell;
+use core::ffi::CStr;
+use core::mem::MaybeUninit;
+
 use crate::any::Any;
 use crate::cell::UnsafeCell;
-use crate::ffi::CStr;
 use crate::marker::PhantomData;
 use crate::mem::{self, ManuallyDrop, forget};
 use crate::num::NonZero;
@@ -1125,7 +1128,7 @@ pub fn park_timeout(dur: Duration) {
     let guard = PanicGuard;
     // SAFETY: park_timeout is called on the parker owned by this thread.
     unsafe {
-        current().inner.as_ref().parker().park_timeout(dur);
+        current().0.parker().park_timeout(dur);
     }
     // No panic occurred, do not abort.
     forget(guard);
@@ -1232,65 +1235,114 @@ impl ThreadId {
 // Thread
 ////////////////////////////////////////////////////////////////////////////////
 
-/// The internal representation of a `Thread`'s name.
-enum ThreadName {
-    Main,
-    Other(ThreadNameString),
-    Unnamed,
-}
-
 // This module ensures private fields are kept private, which is necessary to enforce the safety requirements.
 mod thread_name_string {
     use core::str;
 
-    use super::ThreadName;
     use crate::ffi::{CStr, CString};
 
     /// Like a `String` it's guaranteed UTF-8 and like a `CString` it's null terminated.
     pub(crate) struct ThreadNameString {
         inner: CString,
     }
+
+    impl ThreadNameString {
+        pub fn as_str(&self) -> &str {
+            // SAFETY: `self.inner` is only initialised via `String`, which upholds the validity invariant of `str`.
+            unsafe { str::from_utf8_unchecked(self.inner.to_bytes()) }
+        }
+    }
+
     impl core::ops::Deref for ThreadNameString {
         type Target = CStr;
         fn deref(&self) -> &CStr {
             &self.inner
         }
     }
+
     impl From<String> for ThreadNameString {
         fn from(s: String) -> Self {
             Self {
                 inner: CString::new(s).expect("thread name may not contain interior null bytes"),
             }
         }
     }
-    impl ThreadName {
-        pub fn as_cstr(&self) -> Option<&CStr> {
-            match self {
-                ThreadName::Main => Some(c"main"),
-                ThreadName::Other(other) => Some(other),
-                ThreadName::Unnamed => None,
-            }
-        }
-
-        pub fn as_str(&self) -> Option<&str> {
-            // SAFETY: `as_cstr` can only return `Some` for a fixed CStr or a `ThreadNameString`,
-            // which is guaranteed to be UTF-8.
-            self.as_cstr().map(|s| unsafe { str::from_utf8_unchecked(s.to_bytes()) })
-        }
-    }
 }
 pub(crate) use thread_name_string::ThreadNameString;
 
-/// The internal representation of a `Thread` handle
-struct Inner {
-    name: ThreadName, // Guaranteed to be UTF-8
+static MAIN_THREAD_INFO: SyncUnsafeCell<(MaybeUninit<ThreadId>, MaybeUninit<Parker>)> =
+    SyncUnsafeCell::new((MaybeUninit::uninit(), MaybeUninit::uninit()));
+
+/// The internal representation of a `Thread` that is not the main thread.
+struct OtherInner {
+    name: Option<ThreadNameString>,
     id: ThreadId,
     parker: Parker,
 }
 
+/// The internal representation of a `Thread` handle.
+#[derive(Clone)]
+enum Inner {
+    /// Represents the main thread. May only be constructed by Thread::new_main.
+    Main(&'static (ThreadId, Parker)),
+    /// Represents any other thread.
+    Other(Pin<Arc<OtherInner>>),
+}
+
 impl Inner {
-    fn parker(self: Pin<&Self>) -> Pin<&Parker> {
-        unsafe { Pin::map_unchecked(self, |inner| &inner.parker) }
+    fn id(&self) -> ThreadId {
+        match self {
+            Self::Main((thread_id, _)) => *thread_id,
+            Self::Other(other) => other.id,
+        }
+    }
+
+    fn cname(&self) -> Option<&CStr> {
+        match self {
+            Self::Main(_) => Some(c"main"),
+            Self::Other(other) => other.name.as_deref(),
+        }
+    }
+
+    fn name(&self) -> Option<&str> {
+        match self {
+            Self::Main(_) => Some("main"),
+            Self::Other(other) => other.name.as_ref().map(ThreadNameString::as_str),
+        }
+    }
+
+    fn into_raw(self) -> *const () {
+        match self {
+            // Just return the pointer to `MAIN_THREAD_INFO`.
+            Self::Main(ptr) => crate::ptr::from_ref(ptr).cast(),
+            Self::Other(arc) => {
+                // Safety: We only expose an opaque pointer, which maintains the `Pin` invariant.
+                let inner = unsafe { Pin::into_inner_unchecked(arc) };
+                Arc::into_raw(inner) as *const ()
+            }
+        }
+    }
+
+    /// # Safety
+    ///
+    /// See [`Thread::from_raw`].
+    unsafe fn from_raw(ptr: *const ()) -> Self {
+        // If the pointer is to `MAIN_THREAD_INFO`, we know it is the `Main` variant.
+        if crate::ptr::eq(ptr.cast(), &MAIN_THREAD_INFO) {
+            Self::Main(unsafe { &*ptr.cast() })
+        } else {
+            // Safety: Upheld by caller
+            Self::Other(unsafe { Pin::new_unchecked(Arc::from_raw(ptr as *const OtherInner)) })
+        }
+    }
+
+    fn parker(&self) -> Pin<&Parker> {
+        match self {
+            Self::Main((_, parker_ref)) => Pin::static_ref(parker_ref),
+            Self::Other(inner) => unsafe {
+                Pin::map_unchecked(inner.as_ref(), |inner| &inner.parker)
+            },
+        }
     }
 }
 
@@ -1314,41 +1366,55 @@ impl Inner {
 /// docs of [`Builder`] and [`spawn`] for more details.
 ///
 /// [`thread::current`]: current::current
-pub struct Thread {
-    inner: Pin<Arc<Inner>>,
-}
+pub struct Thread(Inner);
 
 impl Thread {
     /// Used only internally to construct a thread object without spawning.
     pub(crate) fn new(id: ThreadId, name: String) -> Thread {
-        Self::new_inner(id, ThreadName::Other(name.into()))
+        Self::new_inner(id, Some(ThreadNameString::from(name)))
     }
 
     pub(crate) fn new_unnamed(id: ThreadId) -> Thread {
-        Self::new_inner(id, ThreadName::Unnamed)
+        Self::new_inner(id, None)
     }
 
-    /// Constructs the thread handle for the main thread.
-    pub(crate) fn new_main(id: ThreadId) -> Thread {
-        Self::new_inner(id, ThreadName::Main)
+    /// Used in runtime to construct main thread
+    ///
+    /// # Safety
+    ///
+    /// This must only ever be called once, and must be called on the main thread.
+    pub(crate) unsafe fn new_main(thread_id: ThreadId) -> Thread {
+        // Safety: As this is only called once and on the main thread, nothing else is accessing MAIN_THREAD_INFO
+        // as the only other read occurs in `main_thread_info` *after* the main thread has been constructed,
+        // and this function is the only one that constructs the main thread.
+        //
+        // Pre-main thread spawning cannot hit this either, as the caller promises that this is only called on the main thread.
+        let main_thread_info = unsafe { &mut *MAIN_THREAD_INFO.get() };
+
+        unsafe { Parker::new_in_place((&raw mut main_thread_info.1).cast()) };
+        main_thread_info.0.write(thread_id);
+
+        // Store a `'static` ref to the initialised ThreadId and Parker,
+        // to avoid having to repeatedly prove initialisation.
+        Self(Inner::Main(unsafe { &*MAIN_THREAD_INFO.get().cast() }))
     }
 
-    fn new_inner(id: ThreadId, name: ThreadName) -> Thread {
+    fn new_inner(id: ThreadId, name: Option<ThreadNameString>) -> Thread {
         // We have to use `unsafe` here to construct the `Parker` in-place,
         // which is required for the UNIX implementation.
         //
         // SAFETY: We pin the Arc immediately after creation, so its address never
         // changes.
         let inner = unsafe {
-            let mut arc = Arc::<Inner>::new_uninit();
+            let mut arc = Arc::<OtherInner>::new_uninit();
             let ptr = Arc::get_mut_unchecked(&mut arc).as_mut_ptr();
             (&raw mut (*ptr).name).write(name);
             (&raw mut (*ptr).id).write(id);
             Parker::new_in_place(&raw mut (*ptr).parker);
             Pin::new_unchecked(arc.assume_init())
         };
 
-        Thread { inner }
+        Self(Inner::Other(inner))
     }
 
     /// Like the public [`park`], but callable on any handle. This is used to
@@ -1357,7 +1423,7 @@ impl Thread {
     /// # Safety
     /// May only be called from the thread to which this handle belongs.
     pub(crate) unsafe fn park(&self) {
-        unsafe { self.inner.as_ref().parker().park() }
+        unsafe { self.0.parker().park() }
     }
 
     /// Atomically makes the handle's token available if it is not already.
@@ -1393,7 +1459,7 @@ impl Thread {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn unpark(&self) {
-        self.inner.as_ref().parker().unpark();
+        self.0.parker().unpark();
     }
 
     /// Gets the thread's unique identifier.
@@ -1413,7 +1479,7 @@ impl Thread {
     #[stable(feature = "thread_id", since = "1.19.0")]
     #[must_use]
     pub fn id(&self) -> ThreadId {
-        self.inner.id
+        self.0.id()
     }
 
     /// Gets the thread's name.
@@ -1456,7 +1522,11 @@ impl Thread {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
     pub fn name(&self) -> Option<&str> {
-        self.inner.name.as_str()
+        self.0.name()
+    }
+
+    fn cname(&self) -> Option<&CStr> {
+        self.0.cname()
     }
 
     /// Consumes the `Thread`, returning a raw pointer.
@@ -1480,9 +1550,7 @@ impl Thread {
     /// ```
     #[unstable(feature = "thread_raw", issue = "97523")]
     pub fn into_raw(self) -> *const () {
-        // Safety: We only expose an opaque pointer, which maintains the `Pin` invariant.
-        let inner = unsafe { Pin::into_inner_unchecked(self.inner) };
-        Arc::into_raw(inner) as *const ()
+        self.0.into_raw()
     }
 
     /// Constructs a `Thread` from a raw pointer.
@@ -1504,11 +1572,7 @@ impl Thread {
     #[unstable(feature = "thread_raw", issue = "97523")]
     pub unsafe fn from_raw(ptr: *const ()) -> Thread {
         // Safety: Upheld by caller.
-        unsafe { Thread { inner: Pin::new_unchecked(Arc::from_raw(ptr as *const Inner)) } }
-    }
-
-    fn cname(&self) -> Option<&CStr> {
-        self.inner.name.as_cstr()
+        unsafe { Thread(Inner::from_raw(ptr)) }
     }
 }