Only mention overflow behaviour in core::iter, don't change it

Author: tbu-

src/libcore/iter.rs

@@ -106,9 +106,15 @@ pub trait Iterator {
 
     /// Counts the number of elements in this iterator.
     ///
+    /// # Undefined overflow
+    ///
+    /// The method does no guarding against overflows, so counting elements of
+    /// an iterator with more than `usize::MAX` elements is undefined.
+    ///
     /// # Panics
     ///
-    /// Panics if the number of elements overflows a `usize`.
+    /// This functions might panic if the iterator has more than `usize::MAX`
+    /// elements.
     ///
     /// # Examples
     ///
@@ -119,10 +125,8 @@ pub trait Iterator {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn count(self) -> usize where Self: Sized {
-        self.fold(0, |cnt, _| match cnt.checked_add(1) {
-            Some(c) => c,
-            None => panic!("overflow while counting the elements of an iterator"),
-        })
+        // Might overflow.
+        self.fold(0, |cnt, _| cnt + 1)
     }
 
     /// Loops through the entire iterator, returning the last element.
@@ -156,10 +160,8 @@ pub trait Iterator {
     #[stable(feature = "rust1", since = "1.0.0")]
     fn nth(&mut self, mut n: usize) -> Option<Self::Item> where Self: Sized {
         for x in self.by_ref() {
-            n = match n.checked_sub(1) {
-                Some(nn) => nn,
-                None => return Some(x),
-            }
+            if n == 0 { return Some(x) }
+            n -= 1;
         }
         None
     }
@@ -693,10 +695,16 @@ pub trait Iterator {
     ///
     /// Does not consume the iterator past the first found element.
     ///
+    /// # Undefined overflow
+    ///
+    /// The method does no guarding against overflows, so when there are more
+    /// than `usize::MAX` non-matching elements, the overflow behaviour is
+    /// undefined.
+    ///
     /// # Panics
     ///
-    /// Panics if the number of elements overflows a `usize` and no matching
-    /// element was found until that point.
+    /// This functions might panic if the iterator has more than `usize::MAX`
+    /// non-matching elements.
     ///
     /// # Examples
     ///
@@ -712,15 +720,11 @@ pub trait Iterator {
         Self: Sized,
         P: FnMut(Self::Item) -> bool,
     {
-        let mut i = 0;
-        for x in self.by_ref() {
+        // `enumerate` might overflow.
+        for (i, x) in self.by_ref().enumerate() {
             if predicate(x) {
                 return Some(i);
             }
-            i = match i.checked_add(1) {
-                Some(ii) => ii,
-                None => panic!("overflow while getting the position of an element in an iterator"),
-            }
         }
         None
     }
@@ -1797,7 +1801,6 @@ pub struct Enumerate<I> {
 impl<I> Iterator for Enumerate<I> where I: Iterator {
     type Item = (usize, <I as Iterator>::Item);
 
-
     /// # Undefined overflow
     ///
     /// The method does no guarding against overflows, so enumerating more than