Loosen lifetime bounds on accessing named captures via Index.

The current definition equates the lifetime of the strings being
matched/captured and the string used as the capture name.  The latter
is usually `'static` (string literals), which leads to confusing
[borrow-check errors][err] on reasonable-looking code.

This change declares the two lifetimes as separate; this should be a
strict increase in the set of `impl` instances, so it's a minor version
bump.

[err][]: https://gist.github.com/jld/476a13a00ad05bd99a63

Author: jld

src/re.rs

@@ -992,11 +992,11 @@ impl<'t> Index<usize> for Captures<'t> {
 ///
 /// # Panics
 /// If there is no group named by the given value.
-impl<'t> Index<&'t str> for Captures<'t> {
+impl<'t, 'i> Index<&'i str> for Captures<'t> {
 
     type Output = str;
 
-    fn index<'a>(&'a self, name: &str) -> &'a str {
+    fn index<'a>(&'a self, name: &'i str) -> &'a str {
         match self.name(name) {
             None => panic!("no group named '{}'", name),
             Some(ref s) => s,
@@ -1295,4 +1295,16 @@ mod test {
         let cap = re.captures("abc").unwrap();
         let _ = cap["bad name"];
     }
+
+    #[test]
+    fn test_cap_index_lifetime() {
+        // This is a test of whether the types on `caps["..."]` are general
+        // enough.  If not, this will fail to typecheck.
+        fn inner(s: &str) -> usize {
+            let re = Regex::new(r"(?P<number>\d+)").unwrap();
+            let caps = re.captures(s).unwrap();
+            caps["number"].len()
+        }
+        assert_eq!(inner("123"), 3);
+    }
 }