Fix subtle error in caching during kind computation that could cause linear

values to be copied.  Rewrite kind computation so that instead of directly
computing the kind it computes what kinds of values are present in the type,
and then derive kinds based on that. I find this easier to think about.

Fixes #4821.

src/libcore/str.rs

@@ -220,6 +220,16 @@ pub pure fn connect(v: &[~str], sep: &str) -> ~str {
     s
 }
 
+/// Concatenate a vector of strings, placing a given separator between each
+pub pure fn connect_slices(v: &[&str], sep: &str) -> ~str {
+    let mut s = ~"", first = true;
+    for vec::each(v) |ss| {
+        if first { first = false; } else { unsafe { push_str(&mut s, sep); } }
+        unsafe { push_str(&mut s, *ss) };
+    }
+    s
+}
+
 /// Given a string, make a new string with repeated copies of it
 pub pure fn repeat(ss: &str, nn: uint) -> ~str {
     let mut acc = ~"";
@@ -2667,6 +2677,17 @@ mod tests {
         t(~[~"hi"], ~" ", ~"hi");
     }
 
+    #[test]
+    fn test_connect_slices() {
+        fn t(v: &[&str], sep: &str, s: &str) {
+            assert connect_slices(v, sep) == s.to_str();
+        }
+        t(["you", "know", "I'm", "no", "good"],
+          " ", "you know I'm no good");
+        t([], " ", "");
+        t(["hi"], " ", "hi");
+    }
+
     #[test]
     fn test_repeat() {
         assert repeat(~"x", 4) == ~"xxxx";