auto merge of #19134 : sinistersnare/rust/patch-2, r=alexcrichton

Vec<T> can index now so its a useless conversion.

src/libcore/str.rs

@@ -1858,7 +1858,6 @@ pub trait StrPrelude for Sized? {
     /// ```rust
     /// let string = "a\nb\nc";
     /// let lines: Vec<&str> = string.lines().collect();
-    /// let lines = lines.as_slice();
     ///
     /// assert!(string.subslice_offset(lines[0]) == 0); // &"a"
     /// assert!(string.subslice_offset(lines[1]) == 2); // &"b"