std and extra: use as_c_str instead of as_buf in a couple places

These uses are assuming the strings are null terminated, so it
should be using `as_c_str` instead of `as_buf`

src/libstd/rt/borrowck.rs

@@ -76,7 +76,7 @@ unsafe fn fail_borrowed(box: *mut BoxRepr, file: *c_char, line: size_t) {
     match try_take_task_borrow_list() {
         None => { // not recording borrows
             let msg = "borrowed";
-            do msg.as_buf |msg_p, _| {
+            do msg.as_c_str |msg_p| {
                 sys::begin_unwind_(msg_p as *c_char, file, line);
             }
         }
@@ -92,7 +92,7 @@ unsafe fn fail_borrowed(box: *mut BoxRepr, file: *c_char, line: size_t) {
                     sep = " and at ";
                 }
             }
-            do msg.as_buf |msg_p, _| {
+            do msg.as_c_str |msg_p| {
                 sys::begin_unwind_(msg_p as *c_char, file, line)
             }
         }
@@ -231,7 +231,7 @@ pub unsafe fn unrecord_borrow(a: *u8, old_ref_count: uint,
             let br = borrow_list.pop();
             if br.box != a || br.file != file || br.line != line {
                 let err = fmt!("wrong borrow found, br=%?", br);
-                do err.as_buf |msg_p, _| {
+                do err.as_c_str |msg_p| {
                     sys::begin_unwind_(msg_p as *c_char, file, line)
                 }
             }

src/libstd/sys.rs

@@ -123,13 +123,9 @@ pub trait FailWithCause {
 
 impl FailWithCause for ~str {
     fn fail_with(cause: ~str, file: &'static str, line: uint) -> ! {
-        do cause.as_buf |msg_buf, _msg_len| {
-            do file.as_buf |file_buf, _file_len| {
-                unsafe {
-                    let msg_buf = cast::transmute(msg_buf);
-                    let file_buf = cast::transmute(file_buf);
-                    begin_unwind_(msg_buf, file_buf, line as libc::size_t)
-                }
+        do cause.as_c_str |msg_buf| {
+            do file.as_c_str |file_buf| {
+                begin_unwind_(msg_buf, file_buf, line as libc::size_t)
             }
         }
     }
@@ -137,13 +133,9 @@ fn fail_with(cause: ~str, file: &'static str, line: uint) -> ! {
 
 impl FailWithCause for &'static str {
     fn fail_with(cause: &'static str, file: &'static str, line: uint) -> ! {
-        do cause.as_buf |msg_buf, _msg_len| {
-            do file.as_buf |file_buf, _file_len| {
-                unsafe {
-                    let msg_buf = cast::transmute(msg_buf);
-                    let file_buf = cast::transmute(file_buf);
-                    begin_unwind_(msg_buf, file_buf, line as libc::size_t)
-                }
+        do cause.as_c_str |msg_buf| {
+            do file.as_c_str |file_buf| {
+                begin_unwind_(msg_buf, file_buf, line as libc::size_t)
             }
         }
     }

src/libstd/unstable/lang.rs

@@ -56,8 +56,8 @@ 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_buf |p, _len| {
-        fail_(p as *c_char, file, line);
+    do msg.as_c_str |buf| {
+        fail_(buf, file, line);
     }
 }