Auto merge of #71992 - Dylan-DPC:rollup-29qjvpe, r=Dylan-DPC

Rollup of 5 pull requests

Successful merges:

 - #70733 (Add Arc::{incr,decr}_strong_count)
 - #71598 (improve Drop documentation)
 - #71783 (Detect errors caused by `async` block in 2015 edition)
 - #71903 (reword "possible candidate" import suggestion)
 - #71960 (Fix E0284 to not use incorrect wording)

Failed merges:

r? @ghost

Author: bors

src/liballoc/sync.rs

@@ -776,6 +776,79 @@ impl<T: ?Sized> Arc<T> {
         this.inner().strong.load(SeqCst)
     }
 
+    /// Increments the strong reference count on the `Arc<T>` associated with the
+    /// provided pointer by one.
+    ///
+    /// # Safety
+    ///
+    /// The pointer must have been obtained through `Arc::into_raw`, and the
+    /// associated `Arc` instance must be valid (i.e. the strong count must be at
+    /// least 1) for the duration of this method.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(arc_mutate_strong_count)]
+    ///
+    /// use std::sync::Arc;
+    ///
+    /// let five = Arc::new(5);
+    ///
+    /// unsafe {
+    ///     let ptr = Arc::into_raw(five);
+    ///     Arc::incr_strong_count(ptr);
+    ///
+    ///     // This assertion is deterministic because we haven't shared
+    ///     // the `Arc` between threads.
+    ///     let five = Arc::from_raw(ptr);
+    ///     assert_eq!(2, Arc::strong_count(&five));
+    /// }
+    /// ```
+    #[inline]
+    #[unstable(feature = "arc_mutate_strong_count", issue = "71983")]
+    pub unsafe fn incr_strong_count(ptr: *const T) {
+        // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
+        let arc = mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr));
+        // Now increase refcount, but don't drop new refcount either
+        let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
+    }
+
+    /// Decrements the strong reference count on the `Arc<T>` associated with the
+    /// provided pointer by one.
+    ///
+    /// # Safety
+    ///
+    /// The pointer must have been obtained through `Arc::into_raw`, and the
+    /// associated `Arc` instance must be valid (i.e. the strong count must be at
+    /// least 1) when invoking this method. This method can be used to release the final
+    /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
+    /// released.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(arc_mutate_strong_count)]
+    ///
+    /// use std::sync::Arc;
+    ///
+    /// let five = Arc::new(5);
+    ///
+    /// unsafe {
+    ///     let ptr = Arc::into_raw(five);
+    ///     Arc::decr_strong_count(ptr);
+    ///
+    ///     // This assertion is deterministic because we haven't shared
+    ///     // the `Arc` between threads.
+    ///     let five = Arc::from_raw(ptr);
+    ///     assert_eq!(0, Arc::strong_count(&five));
+    /// }
+    /// ```
+    #[inline]
+    #[unstable(feature = "arc_mutate_strong_count", issue = "71983")]
+    pub unsafe fn decr_strong_count(ptr: *const T) {
+        mem::drop(Arc::from_raw(ptr));
+    }
+
     #[inline]
     fn inner(&self) -> &ArcInner<T> {
         // This unsafety is ok because while this arc is alive we're guaranteed

src/liballoc/task.rs

@@ -1,6 +1,6 @@
 #![unstable(feature = "wake_trait", issue = "69912")]
 //! Types and Traits for working with asynchronous tasks.
-use core::mem::{self, ManuallyDrop};
+use core::mem::ManuallyDrop;
 use core::task::{RawWaker, RawWakerVTable, Waker};
 
 use crate::sync::Arc;
@@ -60,9 +60,11 @@ impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for RawWaker {
 fn raw_waker<W: Wake + Send + Sync + 'static>(waker: Arc<W>) -> RawWaker {
     // Increment the reference count of the arc to clone it.
     unsafe fn clone_waker<W: Wake + Send + Sync + 'static>(waker: *const ()) -> RawWaker {
-        let waker: Arc<W> = Arc::from_raw(waker as *const W);
-        mem::forget(Arc::clone(&waker));
-        raw_waker(waker)
+        Arc::incr_strong_count(waker as *const W);
+        RawWaker::new(
+            waker as *const (),
+            &RawWakerVTable::new(clone_waker::<W>, wake::<W>, wake_by_ref::<W>, drop_waker::<W>),
+        )
     }
 
     // Wake by value, moving the Arc into the Wake::wake function
@@ -79,7 +81,7 @@ fn raw_waker<W: Wake + Send + Sync + 'static>(waker: Arc<W>) -> RawWaker {
 
     // Decrement the reference count of the Arc on drop
     unsafe fn drop_waker<W: Wake + Send + Sync + 'static>(waker: *const ()) {
-        mem::drop(Arc::from_raw(waker as *const W));
+        Arc::decr_strong_count(waker as *const W);
     }
 
     RawWaker::new(

src/libcore/ops/drop.rs

@@ -1,85 +1,139 @@
-/// Used to run some code when a value goes out of scope.
-/// This is sometimes called a 'destructor'.
+/// Custom code within the destructor.
 ///
-/// When a value goes out of scope, it will have its `drop` method called if
-/// its type implements `Drop`. Then, any fields the value contains will also
-/// be dropped recursively.
+/// When a value is no longer needed, Rust will run a "destructor" on that value.
+/// The most common way that a value is no longer needed is when it goes out of
+/// scope. Destructors may still run in other circumstances, but we're going to
+/// focus on scope for the examples here. To learn about some of those other cases,
+/// please see [the reference] section on destructors.
 ///
-/// Because of this recursive dropping, you do not need to implement this trait
-/// unless your type needs its own destructor logic.
+/// [the reference]: https://doc.rust-lang.org/reference/destructors.html
 ///
-/// Refer to [the chapter on `Drop` in *The Rust Programming Language*][book]
-/// for some more elaboration.
+/// This destructor consists of two components:
+/// - A call to `Drop::drop` for that value, if this special `Drop` trait is implemented for its type.
+/// - The automatically generated "drop glue" which recursively calls the destructors
+///     of the all fields of this value.
 ///
-/// [book]: ../../book/ch15-03-drop.html
+/// As Rust automatically calls the destructors of all contained fields,
+/// you don't have to implement `Drop` in most cases. But there are some cases where
+/// it is useful, for example for types which directly manage a resource.
+/// That resource may be memory, it may be a file descriptor, it may be a network socket.
+/// Once a value of that type is no longer going to be used, it should "clean up" its
+/// resource by freeing the memory or closing the file or socket. This is
+/// the job of a destructor, and therefore the job of `Drop::drop`.
 ///
-/// # Examples
+/// ## Examples
 ///
-/// ## Implementing `Drop`
+/// To see destructors in action, let's take a look at the following program:
 ///
-/// The `drop` method is called when `_x` goes out of scope, and therefore
-/// `main` prints `Dropping!`.
-///
-/// ```
+/// ```rust
 /// struct HasDrop;
 ///
 /// impl Drop for HasDrop {
 ///     fn drop(&mut self) {
-///         println!("Dropping!");
+///         println!("Dropping HasDrop!");
+///     }
+/// }
+///
+/// struct HasTwoDrops {
+///     one: HasDrop,
+///     two: HasDrop,
+/// }
+///
+/// impl Drop for HasTwoDrops {
+///     fn drop(&mut self) {
+///         println!("Dropping HasTwoDrops!");
 ///     }
 /// }
 ///
 /// fn main() {
-///     let _x = HasDrop;
+///     let _x = HasTwoDrops { one: HasDrop, two: HasDrop };
+///     println!("Running!");
 /// }
 /// ```
 ///
-/// ## Dropping is done recursively
+/// Rust will first call `Drop::drop` for `_x` and then for both `_x.one` and `_x.two`,
+/// meaning that running this will print
 ///
-/// When `outer` goes out of scope, the `drop` method will be called first for
-/// `Outer`, then for `Inner`. Therefore, `main` prints `Dropping Outer!` and
-/// then `Dropping Inner!`.
+/// ```text
+/// Running!
+/// Dropping HasTwoDrops!
+/// Dropping HasDrop!
+/// Dropping HasDrop!
+/// ```
+///
+/// Even if we remove the implementation of `Drop` for `HasTwoDrop`, the destructors of its fields are still called.
+/// This would result in
 ///
+/// ```test
+/// Running!
+/// Dropping HasDrop!
+/// Dropping HasDrop!
 /// ```
-/// struct Inner;
-/// struct Outer(Inner);
 ///
-/// impl Drop for Inner {
+/// ## You cannot call `Drop::drop` yourself
+///
+/// Because `Drop::drop` is used to clean up a value, it may be dangerous to use this value after
+/// the method has been called. As `Drop::drop` does not take ownership of its input,
+/// Rust prevents misuse by not allowing you to call `Drop::drop` directly.
+///
+/// In other words, if you tried to explicitly call `Drop::drop` in the above example, you'd get a compiler error.
+///
+/// If you'd like explicitly call the destructor of a value, [`std::mem::drop`] can be used instead.
+///
+/// [`std::mem::drop`]: ../../std/mem/fn.drop.html
+///
+/// ## Drop order
+///
+/// Which of our two `HasDrop` drops first, though? For structs, it's the same
+/// order that they're declared: first `one`, then `two`. If you'd like to try
+/// this yourself, you can modify `HasDrop` above to contain some data, like an
+/// integer, and then use it in the `println!` inside of `Drop`. This behavior is
+/// guaranteed by the language.
+///
+/// Unlike for structs, local variables are dropped in reverse order:
+///
+/// ```rust
+/// struct Foo;
+///
+/// impl Drop for Foo {
 ///     fn drop(&mut self) {
-///         println!("Dropping Inner!");
+///         println!("Dropping Foo!")
 ///     }
 /// }
 ///
-/// impl Drop for Outer {
+/// struct Bar;
+///
+/// impl Drop for Bar {
 ///     fn drop(&mut self) {
-///         println!("Dropping Outer!");
+///         println!("Dropping Bar!")
 ///     }
 /// }
 ///
 /// fn main() {
-///     let _x = Outer(Inner);
+///     let _foo = Foo;
+///     let _bar = Bar;
 /// }
 /// ```
 ///
-/// ## Variables are dropped in reverse order of declaration
-///
-/// `_first` is declared first and `_second` is declared second, so `main` will
-/// print `Declared second!` and then `Declared first!`.
+/// This will print
 ///
+/// ```text
+/// Dropping Bar!
+/// Dropping Foo!
 /// ```
-/// struct PrintOnDrop(&'static str);
 ///
-/// impl Drop for PrintOnDrop {
-///     fn drop(&mut self) {
-///         println!("{}", self.0);
-///     }
-/// }
+/// Please see [the reference] for the full rules.
 ///
-/// fn main() {
-///     let _first = PrintOnDrop("Declared first!");
-///     let _second = PrintOnDrop("Declared second!");
-/// }
-/// ```
+/// [the reference]: https://doc.rust-lang.org/reference/destructors.html
+///
+/// ## `Copy` and `Drop` are exclusive
+///
+/// You cannot implement both [`Copy`] and `Drop` on the same type. Types that
+/// are `Copy` get implicitly duplicated by the compiler, making it very
+/// hard to predict when, and how often destructors will be executed. As such,
+/// these types cannot have destructors.
+///
+/// [`Copy`]: ../../std/marker/trait.Copy.html
 #[lang = "drop"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Drop {