core: Make str::as_bytes handle failure. Closes #2156

Author: brson

src/libcore/str.rs

@@ -1509,10 +1509,8 @@ let i = str::as_bytes(\"Hello World\") { |bytes| vec::len(bytes) };
 ~~~
 "]
 fn as_bytes<T>(s: str, f: fn([u8]) -> T) -> T unsafe {
-    let mut v: [u8] = ::unsafe::reinterpret_cast(s);
-    let r = f(v);
-    ::unsafe::forget(v);
-    r
+    let v: *[u8] = ::unsafe::reinterpret_cast(ptr::addr_of(s));
+    f(*v)
 }
 
 #[doc = "
@@ -2449,6 +2447,14 @@ mod tests {
         assert (c == "AAA");
     }
 
+    #[test]
+    #[ignore(cfg(target_os = "win32"))]
+    #[should_fail]
+    fn test_as_bytes_fail() {
+        // Don't double free
+        as_bytes("") {|_bytes| fail }
+    }
+
     #[test]
     fn test_as_buf() unsafe {
         let a = "Abcdefg";

src/test/run-fail/issue-2156.rs

@@ -0,0 +1,10 @@
+// error-pattern:explicit failure
+// Don't double free the string
+use std;
+import io::{reader, reader_util};
+
+fn main() {
+    io::with_str_reader("") { |rdr|
+        alt rdr.read_char() { '=' { } _ { fail } }
+    }
+}