auto merge of #20744 : huonw/rust/fix-string-slicing, r=pnkfelix

Author: bors

src/libcollections/string.rs

@@ -932,7 +932,7 @@ pub trait ToString {
     fn to_string(&self) -> String;
 }
 
-impl<T: fmt::String> ToString for T {
+impl<T: fmt::String + ?Sized> ToString for T {
     fn to_string(&self) -> String {
         use core::fmt::Writer;
         let mut buf = String::new();
@@ -994,6 +994,12 @@ mod tests {
       assert_eq!(owned.as_ref().map(|s| s.as_slice()), Some("string"));
     }
 
+    #[test]
+    fn test_unsized_to_string() {
+        let s: &str = "abc";
+        let _: String = (*s).to_string();
+    }
+
     #[test]
     fn test_from_utf8() {
         let xs = b"hello".to_vec();