Add as_str method for split whitespace str iterators

This commit adds `as_str` methods to `SplitWhitespace` and `SplitAsciiWhitespace`
str iterators. The methods return the remainder, similar to `as_str` methods on
`Chars` and other split iterators.

This commit also makes fields of some iterators `pub(crate)`.

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

@@ -13,7 +13,8 @@ use crate::ops::Try;
 #[stable(feature = "rust1", since = "1.0.0")]
 #[derive(Clone)]
 pub struct Filter<I, P> {
-    iter: I,
+    // Used for `SplitWhitespace` and `SplitAsciiWhitespace` `as_str` methods
+    pub(crate) iter: I,
     predicate: P,
 }
 impl<I, P> Filter<I, P> {

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

@@ -57,7 +57,8 @@ use crate::ops::Try;
 #[stable(feature = "rust1", since = "1.0.0")]
 #[derive(Clone)]
 pub struct Map<I, F> {
-    iter: I,
+    // Used for `SplitWhitespace` and `SplitAsciiWhitespace` `as_str` methods
+    pub(crate) iter: I,
     f: F,
 }
 

library/core/src/slice/iter.rs

@@ -335,9 +335,11 @@ pub struct Split<'a, T: 'a, P>
 where
     P: FnMut(&T) -> bool,
 {
-    v: &'a [T],
+    // Used for `SplitWhitespace` and `SplitAsciiWhitespace` `as_str` methods
+    pub(crate) v: &'a [T],
     pred: P,
-    finished: bool,
+    // Used for `SplitAsciiWhitespace` `as_str` method
+    pub(crate) finished: bool,
 }
 
 impl<'a, T: 'a, P: FnMut(&T) -> bool> Split<'a, T, P> {

library/core/src/str/iter.rs

@@ -1203,6 +1203,30 @@ impl<'a> DoubleEndedIterator for SplitWhitespace<'a> {
 #[stable(feature = "fused", since = "1.26.0")]
 impl FusedIterator for SplitWhitespace<'_> {}
 
+impl<'a> SplitWhitespace<'a> {
+    /// Returns remainder of the splitted string
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(str_split_whitespace_as_str)]
+    ///
+    /// let mut split = "Mary had a little lamb".split_whitespace();
+    /// assert_eq!(split.as_str(), "Mary had a little lamb");
+    ///
+    /// split.next();
+    /// assert_eq!(split.as_str(), "had a little lamb");
+    ///
+    /// split.by_ref().for_each(drop);
+    /// assert_eq!(split.as_str(), "");
+    /// ```
+    #[inline]
+    #[unstable(feature = "str_split_whitespace_as_str", issue = "77998")]
+    pub fn as_str(&self) -> &'a str {
+        self.inner.iter.as_str()
+    }
+}
+
 #[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
 impl<'a> Iterator for SplitAsciiWhitespace<'a> {
     type Item = &'a str;
@@ -1234,6 +1258,37 @@ impl<'a> DoubleEndedIterator for SplitAsciiWhitespace<'a> {
 #[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
 impl FusedIterator for SplitAsciiWhitespace<'_> {}
 
+impl<'a> SplitAsciiWhitespace<'a> {
+    /// Returns remainder of the splitted string
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(str_split_whitespace_as_str)]
+    ///
+    /// let mut split = "Mary had a little lamb".split_ascii_whitespace();
+    /// assert_eq!(split.as_str(), "Mary had a little lamb");
+    ///
+    /// split.next();
+    /// assert_eq!(split.as_str(), "had a little lamb");
+    ///
+    /// split.by_ref().for_each(drop);
+    /// assert_eq!(split.as_str(), "");
+    /// ```
+    #[inline]
+    #[unstable(feature = "str_split_whitespace_as_str", issue = "77998")]
+    pub fn as_str(&self) -> &'a str {
+        if self.inner.iter.iter.finished {
+            return "";
+        }
+
+        // Safety:
+        //
+        // Slice is created from str.
+        unsafe { crate::str::from_utf8_unchecked(&self.inner.iter.iter.v) }
+    }
+}
+
 #[stable(feature = "split_inclusive", since = "1.51.0")]
 impl<'a, P: Pattern<'a>> Iterator for SplitInclusive<'a, P> {
     type Item = &'a str;