rollup merge of #23951: alexcrichton/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/libcore/slice.rs

@@ -1126,18 +1126,20 @@ impl<T, I: SplitIter<Item=T>> Iterator for GenericSplitN<I> {
 
     #[inline]
     fn next(&mut self) -> Option<T> {
-        if self.count == 0 {
-            self.iter.finish()
-        } else {
-            self.count -= 1;
-            if self.invert { self.iter.next_back() } else { self.iter.next() }
+        match self.count {
+            0 => None,
+            1 => { self.count -= 1; self.iter.finish() }
+            _ => {
+                self.count -= 1;
+                if self.invert {self.iter.next_back()} else {self.iter.next()}
+            }
         }
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         let (lower, upper_opt) = self.iter.size_hint();
-        (lower, upper_opt.map(|upper| cmp::min(self.count + 1, upper)))
+        (lower, upper_opt.map(|upper| cmp::min(self.count, upper)))
     }
 }
 

src/libcore/str/mod.rs

@@ -489,7 +489,7 @@ struct CharSplits<'a, P: Pattern<'a>> {
 /// splitting at most `count` times.
 struct CharSplitsN<'a, P: Pattern<'a>> {
     iter: CharSplits<'a, P>,
-    /// The number of splits remaining
+    /// The number of items remaining
     count: usize,
 }
 
@@ -596,11 +596,10 @@ impl<'a, P: Pattern<'a>> Iterator for CharSplitsN<'a, P> {
 
     #[inline]
     fn next(&mut self) -> Option<&'a str> {
-        if self.count != 0 {
-            self.count -= 1;
-            self.iter.next()
-        } else {
-            self.iter.get_end()
+        match self.count {
+            0 => None,
+            1 => { self.count = 0; self.iter.get_end() }
+            _ => { self.count -= 1; self.iter.next() }
         }
     }
 }
@@ -650,11 +649,10 @@ impl<'a, P: Pattern<'a>> Iterator for RCharSplitsN<'a, P>
 
     #[inline]
     fn next(&mut self) -> Option<&'a str> {
-        if self.count != 0 {
-            self.count -= 1;
-            self.iter.next()
-        } else {
-            self.iter.get_remainder()
+        match self.count {
+            0 => None,
+            1 => { self.count -= 1; self.iter.get_remainder() }
+            _ => { self.count -= 1; self.iter.next() }
         }
     }
 }