Fix Use After Free

The String Pool now uses just a HashSet, that stores the actual Interned
Strings. The old code also stored the str slices that we where looking
for as keys, but there were never interned properly, so they were super
likely to get freed at some point and cause a Use after Free.

Fixes #47

src/string_pool.rs

@@ -6,7 +6,7 @@ use std::borrow::Borrow;
 use std::cell::{Cell,RefCell};
 use std::cmp::max;
 use std::collections::LinkedList;
-use std::collections::hash_map::HashMap;
+use std::collections::hash_set::HashSet;
 use std::default::Default;
 use std::ops::Deref;
 use std::slice;
@@ -131,7 +131,7 @@ pub struct StringPool {
     start: Cell<*mut u8>,
     end: Cell<*const u8>,
     chunks: RefCell<LinkedList<Chunk>>,
-    index: RefCell<HashMap<InternedString, InternedString>>,
+    index: RefCell<HashSet<InternedString>>,
 }
 
 static CAPACITY: usize = 10240;
@@ -147,12 +147,17 @@ impl StringPool {
     }
 
     pub fn intern<'s>(&'s self, s: &str) -> &'s str {
-        if s == "" { return ""; }
-
-        let search_string = InternedString::from_str(s);
+        if s == "" {
+            return "";
+        }
 
         let mut index = self.index.borrow_mut();
-        let interned_str = *index.entry(search_string).or_insert_with(|| self.do_intern(s));
+        if let Some(interned) = index.get(s) {
+            return unsafe { mem::transmute(interned as &str) };
+        }
+
+        index.insert(self.do_intern(s));
+        let interned_str: &str = index.get(s).unwrap();
 
         // The lifetime is really matched to us
         unsafe { mem::transmute(interned_str) }