remove unnecessary cast

Author: RalfJung

src/librustc_mir/interpret/operand.rs

@@ -341,7 +341,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     // Turn the wide MPlace into a string (must already be dereferenced!)
     pub fn read_str(&self, mplace: MPlaceTy<'tcx, M::PointerTag>) -> InterpResult<'tcx, &str> {
         let len = mplace.len(self)?;
-        let bytes = self.memory.read_bytes(mplace.ptr, Size::from_bytes(u64::from(len)))?;
+        let bytes = self.memory.read_bytes(mplace.ptr, Size::from_bytes(len))?;
         let str = ::std::str::from_utf8(bytes)
             .map_err(|err| err_ub_format!("this string is not valid UTF-8: {}", err))?;
         Ok(str)