More safe handling of usize -> isize conversion with sane failure mode

src/mock.rs

@@ -6,6 +6,7 @@
 // option.  This file may not be copied, modified, or distributed
 // except according to those terms.
 use core::{I2CDevice, I2CMessage, I2CTransfer};
+use std::convert::TryFrom;
 use std::io;
 
 /// I2C mock result type
@@ -44,7 +45,12 @@ impl I2CRegisterMap {
     fn read(&mut self, data: &mut [u8]) -> I2CResult<()> {
         let len = data.len();
         data.clone_from_slice(&self.registers[self.offset..(self.offset + len)]);
-        println!("READ  | 0x{:X} : {:?}", self.offset as isize - data.len() as isize, data);
+        println!(
+            "READ  | 0x{:X} : {:?}",
+            isize::try_from(self.offset).unwrap_or(0xBAD)
+                - isize::try_from(data.len()).unwrap_or(0xBAD),
+            data
+        );
         Ok(())
     }