Auto merge of #42258 - Mark-Simulacrum:rollup, r=Mark-Simulacrum

Rollup of 10 pull requests

- Successful merges: #42103, #42137, #42162, #42167, #42175, #42207, #42217, #42246, #42249, #42251
- Failed merges:

src/doc/unstable-book/src/SUMMARY.md

@@ -25,7 +25,6 @@
     - [cfg_target_has_atomic](language-features/cfg-target-has-atomic.md)
     - [cfg_target_thread_local](language-features/cfg-target-thread-local.md)
     - [cfg_target_vendor](language-features/cfg-target-vendor.md)
-    - [closure_to_fn_coercion](language-features/closure-to-fn-coercion.md)
     - [compiler_builtins](language-features/compiler-builtins.md)
     - [concat_idents](language-features/concat-idents.md)
     - [conservative_impl_trait](language-features/conservative-impl-trait.md)
@@ -154,6 +153,7 @@
     - [io](library-features/io.md)
     - [ip](library-features/ip.md)
     - [iter_rfind](library-features/iter-rfind.md)
+    - [iterator_step_by](library-features/iterator-step-by.md)
     - [libstd_io_internals](library-features/libstd-io-internals.md)
     - [libstd_sys_internals](library-features/libstd-sys-internals.md)
     - [libstd_thread_internals](library-features/libstd-thread-internals.md)

src/liballoc/arc.rs

@@ -91,6 +91,24 @@ const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
 /// pointers from children back to their parents.
 ///
+/// # Cloning references
+///
+/// Creating a new reference from an existing reference counted pointer is done using the
+/// `Clone` trait implemented for [`Arc<T>`][`arc`] and [`Weak<T>`][`weak`].
+///
+/// ```
+/// use std::sync::Arc;
+/// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
+/// // The two syntaxes below are equivalent.
+/// let a = foo.clone();
+/// let b = Arc::clone(&foo);
+/// // a and b both point to the same memory location as foo.
+/// ```
+///
+/// The `Arc::clone(&from)` syntax is the most idiomatic because it conveys more explicitly
+/// the meaning of the code. In the example above, this syntax makes it easier to see that
+/// this code is creating a new reference rather than copying the whole content of foo.
+///
 /// ## `Deref` behavior
 ///
 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
@@ -138,7 +156,7 @@ const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 /// let five = Arc::new(5);
 ///
 /// for _ in 0..10 {
-///     let five = five.clone();
+///     let five = Arc::clone(&five);
 ///
 ///     thread::spawn(move || {
 ///         println!("{:?}", five);
@@ -158,7 +176,7 @@ const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 /// let val = Arc::new(AtomicUsize::new(5));
 ///
 /// for _ in 0..10 {
-///     let val = val.clone();
+///     let val = Arc::clone(&val);
 ///
 ///     thread::spawn(move || {
 ///         let v = val.fetch_add(1, Ordering::SeqCst);
@@ -282,7 +300,7 @@ impl<T> Arc<T> {
     /// assert_eq!(Arc::try_unwrap(x), Ok(3));
     ///
     /// let x = Arc::new(4);
-    /// let _y = x.clone();
+    /// let _y = Arc::clone(&x);
     /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
     /// ```
     #[inline]
@@ -451,7 +469,7 @@ impl<T: ?Sized> Arc<T> {
     /// use std::sync::Arc;
     ///
     /// let five = Arc::new(5);
-    /// let _also_five = five.clone();
+    /// let _also_five = Arc::clone(&five);
     ///
     /// // This assertion is deterministic because we haven't shared
     /// // the `Arc` between threads.
@@ -499,7 +517,7 @@ impl<T: ?Sized> Arc<T> {
     /// use std::sync::Arc;
     ///
     /// let five = Arc::new(5);
-    /// let same_five = five.clone();
+    /// let same_five = Arc::clone(&five);
     /// let other_five = Arc::new(5);
     ///
     /// assert!(Arc::ptr_eq(&five, &same_five));
@@ -524,7 +542,7 @@ impl<T: ?Sized> Clone for Arc<T> {
     ///
     /// let five = Arc::new(5);
     ///
-    /// five.clone();
+    /// Arc::clone(&five);
     /// ```
     #[inline]
     fn clone(&self) -> Arc<T> {
@@ -591,7 +609,7 @@ impl<T: Clone> Arc<T> {
     /// let mut data = Arc::new(5);
     ///
     /// *Arc::make_mut(&mut data) += 1;         // Won't clone anything
-    /// let mut other_data = data.clone();      // Won't clone inner data
+    /// let mut other_data = Arc::clone(&data); // Won't clone inner data
     /// *Arc::make_mut(&mut data) += 1;         // Clones inner data
     /// *Arc::make_mut(&mut data) += 1;         // Won't clone anything
     /// *Arc::make_mut(&mut other_data) *= 2;   // Won't clone anything
@@ -679,7 +697,7 @@ impl<T: ?Sized> Arc<T> {
     /// *Arc::get_mut(&mut x).unwrap() = 4;
     /// assert_eq!(*x, 4);
     ///
-    /// let _y = x.clone();
+    /// let _y = Arc::clone(&x);
     /// assert!(Arc::get_mut(&mut x).is_none());
     /// ```
     #[inline]
@@ -751,7 +769,7 @@ unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
     /// }
     ///
     /// let foo  = Arc::new(Foo);
-    /// let foo2 = foo.clone();
+    /// let foo2 = Arc::clone(&foo);
     ///
     /// drop(foo);    // Doesn't print anything
     /// drop(foo2);   // Prints "dropped!"
@@ -903,11 +921,11 @@ impl<T: ?Sized> Clone for Weak<T> {
     /// # Examples
     ///
     /// ```
-    /// use std::sync::Arc;
+    /// use std::sync::{Arc, Weak};
     ///
     /// let weak_five = Arc::downgrade(&Arc::new(5));
     ///
-    /// weak_five.clone();
+    /// Weak::clone(&weak_five);
     /// ```
     #[inline]
     fn clone(&self) -> Weak<T> {
@@ -956,7 +974,7 @@ impl<T: ?Sized> Drop for Weak<T> {
     /// # Examples
     ///
     /// ```
-    /// use std::sync::Arc;
+    /// use std::sync::{Arc, Weak};
     ///
     /// struct Foo;
     ///
@@ -968,7 +986,7 @@ impl<T: ?Sized> Drop for Weak<T> {
     ///
     /// let foo = Arc::new(Foo);
     /// let weak_foo = Arc::downgrade(&foo);
-    /// let other_weak_foo = weak_foo.clone();
+    /// let other_weak_foo = Weak::clone(&weak_foo);
     ///
     /// drop(weak_foo);   // Doesn't print anything
     /// drop(foo);        // Prints "dropped!"

src/liballoc/rc.rs

@@ -55,6 +55,24 @@
 //! [`Weak<T>`][`Weak`] does not auto-dereference to `T`, because the value may have
 //! already been destroyed.
 //!
+//! # Cloning references
+//!
+//! Creating a new reference from an existing reference counted pointer is done using the
+//! `Clone` trait implemented for [`Rc<T>`][`Rc`] and [`Weak<T>`][`Weak`].
+//!
+//! ```
+//! use std::rc::Rc;
+//! let foo = Rc::new(vec![1.0, 2.0, 3.0]);
+//! // The two syntaxes below are equivalent.
+//! let a = foo.clone();
+//! let b = Rc::clone(&foo);
+//! // a and b both point to the same memory location as foo.
+//! ```
+//!
+//! The `Rc::clone(&from)` syntax is the most idiomatic because it conveys more explicitly
+//! the meaning of the code. In the example above, this syntax makes it easier to see that
+//! this code is creating a new reference rather than copying the whole content of foo.
+//!
 //! # Examples
 //!
 //! Consider a scenario where a set of `Gadget`s are owned by a given `Owner`.
@@ -90,11 +108,11 @@
 //!     // the reference count in the process.
 //!     let gadget1 = Gadget {
 //!         id: 1,
-//!         owner: gadget_owner.clone(),
+//!         owner: Rc::clone(&gadget_owner),
 //!     };
 //!     let gadget2 = Gadget {
 //!         id: 2,
-//!         owner: gadget_owner.clone(),
+//!         owner: Rc::clone(&gadget_owner),
 //!     };
 //!
 //!     // Dispose of our local variable `gadget_owner`.
@@ -163,13 +181,13 @@
 //!     let gadget1 = Rc::new(
 //!         Gadget {
 //!             id: 1,
-//!             owner: gadget_owner.clone(),
+//!             owner: Rc::clone(&gadget_owner),
 //!         }
 //!     );
 //!     let gadget2 = Rc::new(
 //!         Gadget {
 //!             id: 2,
-//!             owner: gadget_owner.clone(),
+//!             owner: Rc::clone(&gadget_owner),
 //!         }
 //!     );
 //!
@@ -316,7 +334,7 @@ impl<T> Rc<T> {
     /// assert_eq!(Rc::try_unwrap(x), Ok(3));
     ///
     /// let x = Rc::new(4);
-    /// let _y = x.clone();
+    /// let _y = Rc::clone(&x);
     /// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
     /// ```
     #[inline]
@@ -508,7 +526,7 @@ impl<T: ?Sized> Rc<T> {
     /// use std::rc::Rc;
     ///
     /// let five = Rc::new(5);
-    /// let _also_five = five.clone();
+    /// let _also_five = Rc::clone(&five);
     ///
     /// assert_eq!(2, Rc::strong_count(&five));
     /// ```
@@ -550,7 +568,7 @@ impl<T: ?Sized> Rc<T> {
     /// *Rc::get_mut(&mut x).unwrap() = 4;
     /// assert_eq!(*x, 4);
     ///
-    /// let _y = x.clone();
+    /// let _y = Rc::clone(&x);
     /// assert!(Rc::get_mut(&mut x).is_none());
     /// ```
     #[inline]
@@ -576,7 +594,7 @@ impl<T: ?Sized> Rc<T> {
     /// use std::rc::Rc;
     ///
     /// let five = Rc::new(5);
-    /// let same_five = five.clone();
+    /// let same_five = Rc::clone(&five);
     /// let other_five = Rc::new(5);
     ///
     /// assert!(Rc::ptr_eq(&five, &same_five));
@@ -608,7 +626,7 @@ impl<T: Clone> Rc<T> {
     /// let mut data = Rc::new(5);
     ///
     /// *Rc::make_mut(&mut data) += 1;        // Won't clone anything
-    /// let mut other_data = data.clone();    // Won't clone inner data
+    /// let mut other_data = Rc::clone(&data);    // Won't clone inner data
     /// *Rc::make_mut(&mut data) += 1;        // Clones inner data
     /// *Rc::make_mut(&mut data) += 1;        // Won't clone anything
     /// *Rc::make_mut(&mut other_data) *= 2;  // Won't clone anything
@@ -680,7 +698,7 @@ unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
     /// }
     ///
     /// let foo  = Rc::new(Foo);
-    /// let foo2 = foo.clone();
+    /// let foo2 = Rc::clone(&foo);
     ///
     /// drop(foo);    // Doesn't print anything
     /// drop(foo2);   // Prints "dropped!"
@@ -720,7 +738,7 @@ impl<T: ?Sized> Clone for Rc<T> {
     ///
     /// let five = Rc::new(5);
     ///
-    /// five.clone();
+    /// Rc::clone(&five);
     /// ```
     #[inline]
     fn clone(&self) -> Rc<T> {
@@ -1050,7 +1068,7 @@ impl<T: ?Sized> Drop for Weak<T> {
     /// # Examples
     ///
     /// ```
-    /// use std::rc::Rc;
+    /// use std::rc::{Rc, Weak};
     ///
     /// struct Foo;
     ///
@@ -1062,7 +1080,7 @@ impl<T: ?Sized> Drop for Weak<T> {
     ///
     /// let foo = Rc::new(Foo);
     /// let weak_foo = Rc::downgrade(&foo);
-    /// let other_weak_foo = weak_foo.clone();
+    /// let other_weak_foo = Weak::clone(&weak_foo);
     ///
     /// drop(weak_foo);   // Doesn't print anything
     /// drop(foo);        // Prints "dropped!"
@@ -1090,11 +1108,11 @@ impl<T: ?Sized> Clone for Weak<T> {
     /// # Examples
     ///
     /// ```
-    /// use std::rc::Rc;
+    /// use std::rc::{Rc, Weak};
     ///
     /// let weak_five = Rc::downgrade(&Rc::new(5));
     ///
-    /// weak_five.clone();
+    /// Weak::clone(&weak_five);
     /// ```
     #[inline]
     fn clone(&self) -> Weak<T> {

src/libcore/iter/mod.rs

@@ -520,7 +520,7 @@ impl<I> Iterator for Cycle<I> where I: Clone + Iterator {
 #[unstable(feature = "fused", issue = "35602")]
 impl<I> FusedIterator for Cycle<I> where I: Clone + Iterator {}
 
-/// An iterator that steps by n elements every iteration.
+/// An adapter for stepping iterators by a custom amount.
 ///
 /// This `struct` is created by the [`step_by`] method on [`Iterator`]. See
 /// its documentation for more.
@@ -553,8 +553,27 @@ impl<I> Iterator for StepBy<I> where I: Iterator {
             self.iter.nth(self.step)
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let inner_hint = self.iter.size_hint();
+
+        if self.first_take {
+            let f = |n| if n == 0 { 0 } else { 1 + (n-1)/(self.step+1) };
+            (f(inner_hint.0), inner_hint.1.map(f))
+        } else {
+            let f = |n| n / (self.step+1);
+            (f(inner_hint.0), inner_hint.1.map(f))
+        }
+    }
 }
 
+// StepBy can only make the iterator shorter, so the len will still fit.
+#[unstable(feature = "iterator_step_by",
+           reason = "unstable replacement of Range::step_by",
+           issue = "27741")]
+impl<I> ExactSizeIterator for StepBy<I> where I: ExactSizeIterator {}
+
 /// An iterator that strings two iterators together.
 ///
 /// This `struct` is created by the [`chain`] method on [`Iterator`]. See its