Reason about nested free variables that appear in a function

signature.  In a nutshell, the idea is to (1) report an error if, for
a region pointer `'a T`, the lifetime `'a` is longer than any
lifetimes that appear in `T` (in other words, if a borrowed pointer
outlives any portion of its contents) and then (2) use this to assume
that in a function like `fn(self: &'a &'b T)`, the relationship `'a <=
'b` holds. This is needed for #5656.  Fixes #5728.

src/libcore/str.rs

@@ -301,7 +301,11 @@ pub fn slice_shift_char<'a>(s: &'a str) -> (char, &'a str) {
 
 /// Prepend a char to a string
 pub fn unshift_char(s: &mut ~str, ch: char) {
-    *s = from_char(ch) + *s;
+    // This could be more efficient.
+    let mut new_str = ~"";
+    new_str.push_char(ch);
+    new_str.push_str(*s);
+    *s = new_str;
 }
 
 /**