Impl From<Peekable> for Iterator

library/core/src/iter/adapters/peekable.rs

@@ -7,6 +7,29 @@ use crate::ops::{ControlFlow, Try};
 ///
 /// This `struct` is created by the [`peekable`] method on [`Iterator`]. See its
 /// documentation for more.
+/// 
+/// `Peakable` has an implementation: `impl<I: Interator> From<Peekable<I>> for I`, use `I::from()` can get back to [`Iterator`].
+/// ```rust
+/// struct Counter {
+///     v: usize,
+/// }
+///
+/// impl Iterator for Counter {
+///     type Item = usize;
+///
+///     fn next(&mut self) -> Option<Self::Item> {
+///         let v = self.v;
+///         self.v += 1;
+///         Some(v)
+///     }
+/// }
+///
+/// let counter = Counter { v: 0 };
+/// let mut peekable = counter.peekable();
+/// assert_eq!(peekable.peek(), Some(&0));
+/// let counter = Counter::from(peekable);
+/// assert_eq!(counter.next(), Some(0));
+/// ```
 ///
 /// [`peekable`]: Iterator::peekable
 /// [`Iterator`]: trait.Iterator.html
@@ -26,6 +49,12 @@ impl<I: Iterator> Peekable<I> {
     }
 }
 
+impl<I: Iterator> From<Peekable<I>> for I {
+    fn from(p: Peekable<I>) -> Self {
+        p.iter
+    }
+}
+
 // Peekable must remember if a None has been seen in the `.peek()` method.
 // It ensures that `.peek(); .peek();` or `.peek(); .next();` only advances the
 // underlying iterator at most once. This does not by itself make the iterator