Auto merge of #27900 - SimonSapin:Chars_as_str, r=alexcrichton

See #27775.

r? @alexcrichton

src/libcore/str/mod.rs

@@ -292,6 +292,18 @@ impl<'a> DoubleEndedIterator for Chars<'a> {
     }
 }
 
+impl<'a> Chars<'a> {
+    /// View the underlying data as a subslice of the original data.
+    ///
+    /// This has the same lifetime as the original slice, and so the
+    /// iterator can continue to be used while this exists.
+    #[unstable(feature = "iter_to_slice", issue = "27775")]
+    #[inline]
+    pub fn as_str(&self) -> &'a str {
+        unsafe { from_utf8_unchecked(self.iter.as_slice()) }
+    }
+}
+
 /// Iterator for a string's characters and their byte offsets.
 #[derive(Clone)]
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -339,6 +351,18 @@ impl<'a> DoubleEndedIterator for CharIndices<'a> {
     }
 }
 
+impl<'a> CharIndices<'a> {
+    /// View the underlying data as a subslice of the original data.
+    ///
+    /// This has the same lifetime as the original slice, and so the
+    /// iterator can continue to be used while this exists.
+    #[unstable(feature = "iter_to_slice", issue = "27775")]
+    #[inline]
+    pub fn as_str(&self) -> &'a str {
+        self.iter.as_str()
+    }
+}
+
 /// External iterator for a string's bytes.
 /// Use with the `std::iter` module.
 ///