auto merge of #8296 : erickt/rust/remove-str-trailing-nulls, r=erickt

This PR fixes #7235 and #3371, which removes trailing nulls from `str` types. Instead, it replaces the creation of c strings with a new type, `std::c_str::CString`, which wraps a malloced byte array, and respects:

*  No interior nulls
* Ends with a trailing null

src/libstd/unstable/dynamic_lib.rs

@@ -15,6 +15,7 @@ Dynamic library facilities.
 A simple wrapper over the platforms dynamic library facilities
 
 */
+use c_str::ToCStr;
 use cast;
 use path;
 use libc;
@@ -65,7 +66,7 @@ impl DynamicLibrary {
         // T but that feature is still unimplemented
 
         let maybe_symbol_value = do dl::check_for_errors_in {
-            do symbol.as_c_str |raw_string| {
+            do symbol.to_c_str().with_ref |raw_string| {
                 dl::symbol(self.handle, raw_string)
             }
         };
@@ -135,6 +136,7 @@ mod test {
 #[cfg(target_os = "macos")]
 #[cfg(target_os = "freebsd")]
 mod dl {
+    use c_str::ToCStr;
     use libc;
     use path;
     use ptr;
@@ -143,7 +145,7 @@ mod dl {
     use result::*;
 
     pub unsafe fn open_external(filename: &path::Path) -> *libc::c_void {
-        do filename.to_str().as_c_str |raw_name| {
+        do filename.to_c_str().with_ref |raw_name| {
             dlopen(raw_name, Lazy as libc::c_int)
         }
     }

src/libstd/unstable/extfmt.rs

@@ -549,12 +549,14 @@ pub mod rt {
         // For strings, precision is the maximum characters
         // displayed
         let unpadded = match cv.precision {
-          CountImplied => s,
-          CountIs(max) => if (max as uint) < s.char_len() {
-            s.slice(0, max as uint)
-          } else {
-            s
-          }
+            CountImplied => s,
+            CountIs(max) => {
+                if (max as uint) < s.char_len() {
+                    s.slice(0, max as uint)
+                } else {
+                    s
+                }
+            }
         };
         pad(cv, unpadded, None, PadNozero, buf);
     }

src/libstd/unstable/lang.rs

@@ -10,6 +10,7 @@
 
 //! Runtime calls emitted by the compiler.
 
+use c_str::ToCStr;
 use cast::transmute;
 use libc::{c_char, c_uchar, c_void, size_t, uintptr_t};
 use str;
@@ -28,7 +29,7 @@ pub fn fail_bounds_check(file: *c_char, line: size_t,
                          index: size_t, len: size_t) {
     let msg = fmt!("index out of bounds: the len is %d but the index is %d",
                     len as int, index as int);
-    do msg.as_c_str |buf| {
+    do msg.to_c_str().with_ref |buf| {
         fail_(buf, file, line);
     }
 }