std: Changing the meaning of the count to splitn

This commit is an implementation of [RFC 979][rfc] which changes the meaning of
the count parameter to the `splitn` function on strings and slices. The
parameter now means the number of items that are returned from the iterator, not
the number of splits that are made.

[rfc]: https://github.com/rust-lang/rfcs/pull/979

Closes #23911
[breaking-change]

src/libcollections/slice.rs

@@ -328,9 +328,12 @@ impl<T> [T] {
     }
 
     /// Returns an iterator over subslices separated by elements that match
-    /// `pred`, limited to splitting at most `n` times.  The matched element is
+    /// `pred`, limited to returning at most `n` items.  The matched element is
     /// not contained in the subslices.
     ///
+    /// The last element returned, if any, will contain the remainder of the
+    /// slice.
+    ///
     /// # Examples
     ///
     /// Print the slice split once by numbers divisible by 3 (i.e. `[10, 40]`,
@@ -338,7 +341,7 @@ impl<T> [T] {
     ///
     /// ```
     /// let v = [10, 40, 30, 20, 60, 50];
-    /// for group in v.splitn(1, |num| *num % 3 == 0) {
+    /// for group in v.splitn(2, |num| *num % 3 == 0) {
     ///     println!("{:?}", group);
     /// }
     /// ```
@@ -349,10 +352,13 @@ impl<T> [T] {
     }
 
     /// Returns an iterator over subslices separated by elements that match
-    /// `pred` limited to splitting at most `n` times. This starts at the end of
+    /// `pred` limited to returning at most `n` items. This starts at the end of
     /// the slice and works backwards.  The matched element is not contained in
     /// the subslices.
     ///
+    /// The last element returned, if any, will contain the remainder of the
+    /// slice.
+    ///
     /// # Examples
     ///
     /// Print the slice split once, starting from the end, by numbers divisible
@@ -360,7 +366,7 @@ impl<T> [T] {
     ///
     /// ```
     /// let v = [10, 40, 30, 20, 60, 50];
-    /// for group in v.rsplitn(1, |num| *num % 3 == 0) {
+    /// for group in v.rsplitn(2, |num| *num % 3 == 0) {
     ///     println!("{:?}", group);
     /// }
     /// ```
@@ -626,8 +632,11 @@ impl<T> [T] {
     }
 
     /// Returns an iterator over subslices separated by elements that match
-    /// `pred`, limited to splitting at most `n` times.  The matched element is
+    /// `pred`, limited to returning at most `n` items.  The matched element is
     /// not contained in the subslices.
+    ///
+    /// The last element returned, if any, will contain the remainder of the
+    /// slice.
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn splitn_mut<F>(&mut self, n: usize, pred: F) -> SplitNMut<T, F>
@@ -636,9 +645,12 @@ impl<T> [T] {
     }
 
     /// Returns an iterator over subslices separated by elements that match
-    /// `pred` limited to splitting at most `n` times. This starts at the end of
+    /// `pred` limited to returning at most `n` items. This starts at the end of
     /// the slice and works backwards.  The matched element is not contained in
     /// the subslices.
+    ///
+    /// The last element returned, if any, will contain the remainder of the
+    /// slice.
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn rsplitn_mut<F>(&mut self,  n: usize, pred: F) -> RSplitNMut<T, F>

src/libcollections/str.rs

@@ -610,24 +610,27 @@ impl str {
         core_str::StrExt::split(&self[..], pat)
     }
 
-    /// An iterator over substrings of `self`, separated by characters matched by a pattern,
-    /// restricted to splitting at most `count` times.
+    /// An iterator over substrings of `self`, separated by characters matched
+    /// by a pattern, returning most `count` items.
     ///
     /// The pattern can be a simple `&str`, or a closure that determines
     /// the split.
     ///
+    /// The last element returned, if any, will contain the remainder of the
+    /// string.
+    ///
     /// # Examples
     ///
     /// Simple `&str` patterns:
     ///
     /// ```
     /// let v: Vec<&str> = "Mary had a little lambda".splitn(2, ' ').collect();
-    /// assert_eq!(v, ["Mary", "had", "a little lambda"]);
+    /// assert_eq!(v, ["Mary", "had a little lambda"]);
     ///
     /// let v: Vec<&str> = "lionXXtigerXleopard".splitn(2, 'X').collect();
-    /// assert_eq!(v, ["lion", "", "tigerXleopard"]);
+    /// assert_eq!(v, ["lion", "XtigerXleopard"]);
     ///
-    /// let v: Vec<&str> = "abcXdef".splitn(0, 'X').collect();
+    /// let v: Vec<&str> = "abcXdef".splitn(1, 'X').collect();
     /// assert_eq!(v, ["abcXdef"]);
     ///
     /// let v: Vec<&str> = "".splitn(1, 'X').collect();
@@ -637,7 +640,7 @@ impl str {
     /// More complex patterns with a lambda:
     ///
     /// ```
-    /// let v: Vec<&str> = "abc1def2ghi".splitn(1, |c: char| c.is_numeric()).collect();
+    /// let v: Vec<&str> = "abc1def2ghi".splitn(2, |c: char| c.is_numeric()).collect();
     /// assert_eq!(v, ["abc", "def2ghi"]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -705,25 +708,28 @@ impl str {
     }
 
     /// An iterator over substrings of `self`, separated by a pattern,
-    /// starting from the end of the string, restricted to splitting
-    /// at most `count` times.
+    /// starting from the end of the string, restricted to returning
+    /// at most `count` items.
+    ///
+    /// The last element returned, if any, will contain the remainder of the
+    /// string.
     ///
     /// # Examples
     ///
     /// Simple patterns:
     ///
     /// ```
-    /// let v: Vec<&str> = "Mary had a little lamb".rsplitn(2, ' ').collect();
+    /// let v: Vec<&str> = "Mary had a little lamb".rsplitn(3, ' ').collect();
     /// assert_eq!(v, ["lamb", "little", "Mary had a"]);
     ///
-    /// let v: Vec<&str> = "lion::tiger::leopard".rsplitn(1, "::").collect();
+    /// let v: Vec<&str> = "lion::tiger::leopard".rsplitn(2, "::").collect();
     /// assert_eq!(v, ["leopard", "lion::tiger"]);
     /// ```
     ///
     /// More complex patterns with a lambda:
     ///
     /// ```
-    /// let v: Vec<&str> = "abc1def2ghi".rsplitn(1, |c: char| c.is_numeric()).collect();
+    /// let v: Vec<&str> = "abc1def2ghi".rsplitn(2, |c: char| c.is_numeric()).collect();
     /// assert_eq!(v, ["ghi", "abc1def"]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]