Add std::os::darwin::thread::MainThreadMarker

Author: madsmtm

library/std/src/os/darwin/mod.rs

@@ -21,3 +21,5 @@ pub mod fs;
 // well as `std::os::macos`/`std::os::ios`, because those modules precede the
 // decision to remove these.
 pub(super) mod raw;
+
+pub mod thread;

library/std/src/os/darwin/thread.rs

@@ -0,0 +1,217 @@
+//! Darwin-specific extensions to threads.
+#![unstable(feature = "darwin_mtm", issue = "none")]
+
+use crate::fmt;
+use crate::marker::PhantomData;
+
+/// A marker type for functionality only available on the main thread.
+///
+/// The main thread is a system-level property on Darwin platforms, and has extra capabilities not
+/// available on other threads. This is usually relevant when using native GUI frameworks, where
+/// most operations must be done on the main thread.
+///
+/// This type enables you to manage that capability. By design, it is neither [`Send`] nor [`Sync`],
+/// and can only be created on the main thread, meaning that if you have an instance of this, you
+/// are guaranteed to be on the main thread / have the "main-thread capability".
+///
+/// [The `main` function][main-functions] will run on the main thread. This type can also be used
+/// with `#![no_main]` or other such cases where Rust is not defining the binary entry point.
+///
+/// See the following links for more information on main-thread-only APIs:
+/// - [Are the Cocoa Frameworks Thread Safe?](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/CocoaFundamentals/AddingBehaviortoaCocoaProgram/AddingBehaviorCocoa.html#//apple_ref/doc/uid/TP40002974-CH5-SW47)
+/// - [About Threaded Programming](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Multithreading/AboutThreads/AboutThreads.html)
+/// - [Thread Safety Summary](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Multithreading/ThreadSafetySummary/ThreadSafetySummary.html#//apple_ref/doc/uid/10000057i-CH12-SW1)
+/// - [Technical Note TN2028 - Threading Architectures](https://developer.apple.com/library/archive/technotes/tn/tn2028.html#//apple_ref/doc/uid/DTS10003065)
+/// - [Thread Management](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Multithreading/CreatingThreads/CreatingThreads.html)
+/// - [Swift's `@MainActor`](https://developer.apple.com/documentation/swift/mainactor)
+/// - [Main Thread Only APIs on OS X](https://www.dribin.org/dave/blog/archives/2009/02/01/main_thread_apis/)
+/// - [Mike Ash' article on thread safety](https://www.mikeash.com/pyblog/friday-qa-2009-01-09.html)
+///
+/// [main-functions]: https://doc.rust-lang.org/reference/crates-and-source-files.html#main-functions
+///
+///
+/// # Main Thread Checker
+///
+/// Xcode provides a tool called the ["Main Thread Checker"][mtc] which verifies that UI APIs are
+/// being used from the correct thread. This is not as principled as `MainThreadMarker`, but is
+/// helpful for catching mistakes.
+///
+/// You can use this tool on macOS by loading `libMainThreadChecker.dylib` into your process using
+/// `DYLD_INSERT_LIBRARIES`:
+///
+/// ```console
+/// DYLD_INSERT_LIBRARIES=/Applications/Xcode.app/Contents/Developer/usr/lib/libMainThreadChecker.dylib MTC_RESET_INSERT_LIBRARIES=0 cargo run
+/// ```
+///
+/// If you're not running your binary through Cargo, you can omit
+/// [`MTC_RESET_INSERT_LIBRARIES`][mtc-reset].
+///
+/// ```console
+/// DYLD_INSERT_LIBRARIES=/Applications/Xcode.app/Contents/Developer/usr/lib/libMainThreadChecker.dylib target/debug/myapp
+/// ```
+///
+/// If you're developing for iOS, you probably better off enabling the tool in Xcode's own UI.
+///
+/// See [this excellent blog post][mtc-cfg] for details on further configuration options.
+///
+/// [mtc]: https://developer.apple.com/documentation/xcode/diagnosing-memory-thread-and-crash-issues-early#Detect-improper-UI-updates-on-background-threads
+/// [mtc-reset]: https://bryce.co/main-thread-checker-configuration/#mtc_reset_insert_libraries
+/// [mtc-cfg]: https://bryce.co/main-thread-checker-configuration/
+///
+///
+/// # Examples
+///
+/// Retrieve the main thread marker in different situations.
+///
+/// ```
+/// #![feature(darwin_mtm)]
+/// use std::os::darwin::thread::MainThreadMarker;
+///
+/// # // doc test explicitly uses `fn main` to show that that's where it counts.
+/// fn main() {
+///     // The thread that `fn main` runs on is the main thread.
+///     assert!(MainThreadMarker::new().is_some());
+///
+///     // Subsequently spawned threads are not the main thread.
+///     std::thread::spawn(|| {
+///         assert!(MainThreadMarker::new().is_none());
+///     }).join().unwrap();
+/// }
+/// ```
+///
+/// Create a static that is only usable on the main thread. This is similar to a thread-local, but
+/// can be more efficient because it doesn't handle multiple threads.
+///
+/// ```
+/// #![feature(sync_unsafe_cell)]
+/// #![feature(darwin_mtm)]
+/// use std::os::darwin::thread::MainThreadMarker;
+/// use std::sync::SyncUnsafeCell;
+///
+/// static MAIN_THREAD_ONLY_VALUE: SyncUnsafeCell<i32> = SyncUnsafeCell::new(0);
+///
+/// fn set(value: i32, _mtm: MainThreadMarker) {
+///     // SAFETY: We have an instance of `MainThreadMarker`, so we know that
+///     // we're running on the main thread (and thus do not need any
+///     // synchronization, since the only accesses to this value is from the
+///     // main thread).
+///     unsafe { *MAIN_THREAD_ONLY_VALUE.get() = value };
+/// }
+///
+/// fn get(_mtm: MainThreadMarker) -> &'static u32 {
+///     // SAFETY: Same as above.
+///     unsafe { *MAIN_THREAD_ONLY_VALUE.get() }
+/// }
+///
+/// // Usage
+/// fn main() {
+///     let mtm = MainThreadMarker::new().expect("must be on the main thread");
+///     set(42, mtm);
+///     assert_eq!(get(mtm), 42);
+/// }
+/// ```
+#[derive(Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
+//              ^^^^ this is valid because it's still `!Send` and `!Sync`.
+#[unstable(feature = "darwin_mtm", issue = "none")]
+pub struct MainThreadMarker {
+    // No lifetime information needed; the main thread is static and available throughout the entire
+    // program!
+
+    // Ensure `!Send` and `!Sync`.
+    _priv: PhantomData<*mut ()>,
+}
+
+// Manually implementing these results in slightly better error messages.
+#[unstable(feature = "darwin_mtm", issue = "none")]
+impl !Send for MainThreadMarker {}
+#[unstable(feature = "darwin_mtm", issue = "none")]
+impl !Sync for MainThreadMarker {}
+
+impl MainThreadMarker {
+    /// Construct a new `MainThreadMarker`.
+    ///
+    /// Returns [`None`] if the current thread was not the main thread.
+    ///
+    ///
+    /// # Example
+    ///
+    /// Check whether the current thread is the main thread.
+    ///
+    /// ```
+    /// use std::os::darwin::thread::MainThreadMarker;
+    ///
+    /// if MainThreadMarker::new().is_some() {
+    ///     // Is the main thread
+    /// } else {
+    ///     // Not the main thread
+    /// }
+    /// ```
+    #[inline]
+    #[doc(alias = "is_main_thread")]
+    #[doc(alias = "pthread_main_np")]
+    #[doc(alias = "isMainThread")]
+    #[unstable(feature = "darwin_mtm", issue = "none")]
+    pub fn new() -> Option<Self> {
+        if is_main_thread() {
+            // SAFETY: We just checked that we are running on the main thread.
+            Some(unsafe { Self::new_unchecked() })
+        } else {
+            None
+        }
+    }
+
+    /// Construct a new `MainThreadMarker` without first checking whether the current thread is
+    /// the main one.
+    ///
+    ///
+    /// # Safety
+    ///
+    /// The current thread must be the main thread.
+    #[inline]
+    #[unstable(feature = "darwin_mtm", issue = "none")]
+    #[rustc_const_unstable(feature = "darwin_mtm", issue = "none")]
+    pub const unsafe fn new_unchecked() -> Self {
+        // SAFETY: Upheld by caller.
+        //
+        // We can't debug_assert that this actually is the main thread, both because this is
+        // `const` (to allow usage in `static`s), and because users may sometimes want to create
+        // this briefly, e.g. to access an API that in most cases requires the marker, but is safe
+        // to use without in specific cases.
+        Self { _priv: PhantomData }
+    }
+}
+
+#[unstable(feature = "darwin_mtm", issue = "none")]
+impl fmt::Debug for MainThreadMarker {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("MainThreadMarker").finish()
+    }
+}
+
+// Implementation:
+
+/// Whether the current thread is the main thread.
+#[inline]
+fn is_main_thread() -> bool {
+    // In Objective-C you would use `+[NSThread isMainThread]`, but benchmarks have shown that
+    // calling the underlying `pthread_main_np` directly is up to four times faster, so we use that
+    // instead.
+    //
+    // `pthread_main_np` is also included via. libSystem, so that avoids linking Foundation.
+
+    // SAFETY: Can be called from any thread.
+    //
+    // Apple's man page says:
+    // > The pthread_main_np() function returns 1 if the calling thread is the initial thread, 0 if
+    // > the calling thread is not the initial thread, and -1 if the thread's initialization has not
+    // > yet completed.
+    //
+    // However, Apple's header says:
+    // > Returns non-zero if the current thread is the main thread.
+    //
+    // So unclear if we should be doing a comparison against 1, or a negative comparison against 0?
+    // To be safe, we compare against 1, though in reality, the current implementation can only ever
+    // return 0 or 1:
+    // https://github.com/apple-oss-distributions/libpthread/blob/libpthread-535/src/pthread.c#L1084-L1089
+    unsafe { libc::pthread_main_np() == 1 }
+}

tests/ui/threads-sendsync/darwin_mtm_not_send_sync.rs

@@ -0,0 +1,15 @@
+// Test that `MainThreadMarker` is neither `Send` nor `Sync`.
+
+//@ only-apple
+
+use std::os::darwin::thread::MainThreadMarker;
+
+fn needs_sync<T: Sync>() {}
+fn needs_send<T: Send>() {}
+
+fn main() {
+    needs_sync::<MainThreadMarker>();
+    //~^ ERROR is not Sync
+    needs_send::<MainThreadMarker>();
+    //~^ ERROR is not Send
+}