Auto merge of #91486 - matthiaskrgr:rollup-699fo18, r=matthiaskrgr

Rollup of 10 pull requests

Successful merges:

 - #88906 (Implement write() method for Box<MaybeUninit<T>>)
 - #90269 (Make `Option::expect` unstably const)
 - #90854 (Type can be unsized and uninhabited)
 - #91170 (rustdoc: preload fonts)
 - #91273 (Fix ICE #91268 by checking that the snippet ends with a `)`)
 - #91381 (Android: -ldl must appear after -lgcc when linking)
 - #91453 (Document Windows TLS drop behaviour)
 - #91462 (Use try_normalize_erasing_regions in needs_drop)
 - #91474 (suppress warning about set_errno being unused on DragonFly)
 - #91483 (Sync rustfmt subtree)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

alloc/src/boxed.rs

@@ -763,6 +763,42 @@ impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
         let (raw, alloc) = Box::into_raw_with_allocator(self);
         unsafe { Box::from_raw_in(raw as *mut T, alloc) }
     }
+
+    /// Writes the value and converts to `Box<T, A>`.
+    ///
+    /// This method converts the box similarly to [`Box::assume_init`] but
+    /// writes `value` into it before conversion thus guaranteeing safety.
+    /// In some scenarios use of this method may improve performance because
+    /// the compiler may be able to optimize copying from stack.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    ///
+    /// let big_box = Box::<[usize; 1024]>::new_uninit();
+    ///
+    /// let mut array = [0; 1024];
+    /// for (i, place) in array.iter_mut().enumerate() {
+    ///     *place = i;
+    /// }
+    ///
+    /// // The optimizer may be able to elide this copy, so previous code writes
+    /// // to heap directly.
+    /// let big_box = Box::write(big_box, array);
+    ///
+    /// for (i, x) in big_box.iter().enumerate() {
+    ///     assert_eq!(*x, i);
+    /// }
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    #[inline]
+    pub fn write(mut boxed: Self, value: T) -> Box<T, A> {
+        unsafe {
+            (*boxed).write(value);
+            boxed.assume_init()
+        }
+    }
 }
 
 impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {

core/src/option.rs

@@ -703,7 +703,8 @@ impl<T> Option<T> {
     #[inline]
     #[track_caller]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn expect(self, msg: &str) -> T {
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const fn expect(self, msg: &str) -> T {
         match self {
             Some(val) => val,
             None => expect_failed(msg),
@@ -1658,7 +1659,7 @@ impl<T, E> Option<Result<T, E>> {
 #[inline(never)]
 #[cold]
 #[track_caller]
-fn expect_failed(msg: &str) -> ! {
+const fn expect_failed(msg: &str) -> ! {
     panic!("{}", msg)
 }
 

std/src/sys/unix/os.rs

@@ -97,6 +97,7 @@ pub fn errno() -> i32 {
 }
 
 #[cfg(target_os = "dragonfly")]
+#[allow(dead_code)]
 pub fn set_errno(e: i32) {
     extern "C" {
         #[thread_local]

std/src/thread/local.rs

@@ -76,7 +76,21 @@ use crate::fmt;
 ///    destroyed, but not all platforms have this guard. Those platforms that do
 ///    not guard typically have a synthetic limit after which point no more
 ///    destructors are run.
+/// 3. When the process exits on Windows systems, TLS destructors may only be
+///    run on the thread that causes the process to exit. This is because the
+///    other threads may be forcibly terminated.
 ///
+/// ## Synchronization in thread-local destructors
+///
+/// On Windows, synchronization operations (such as [`JoinHandle::join`]) in
+/// thread local destructors are prone to deadlocks and so should be avoided.
+/// This is because the [loader lock] is held while a destructor is run. The
+/// lock is acquired whenever a thread starts or exits or when a DLL is loaded
+/// or unloaded. Therefore these events are blocked for as long as a thread
+/// local destructor is running.
+///
+/// [loader lock]: https://docs.microsoft.com/en-us/windows/win32/dlls/dynamic-link-library-best-practices
+/// [`JoinHandle::join`]: crate::thread::JoinHandle::join
 /// [`with`]: LocalKey::with
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct LocalKey<T: 'static> {

unwind/build.rs

@@ -17,6 +17,9 @@ fn main() {
         } else {
             println!("cargo:rustc-link-lib=gcc");
         }
+
+        // Android's unwinding library depends on dl_iterate_phdr in `libdl`.
+        println!("cargo:rustc-link-lib=dl");
     } else if target.contains("freebsd") {
         println!("cargo:rustc-link-lib=gcc_s");
     } else if target.contains("netbsd") {