Improve test coverage

All reachable branches in the encoding/decoding functions are now
checked.

Author: nicholasbishop

src/lib.rs

@@ -202,23 +202,40 @@ mod tests {
         let input = "őэ╋";
         let mut buffer = [0u16; 3];
 
-        let result = encode(input, &mut buffer);
-        assert_eq!(result.unwrap(), 3);
-
+        assert_eq!(encode(input, &mut buffer), Ok(3));
         assert_eq!(buffer[..], [0x0151, 0x044D, 0x254B]);
+
+        let mut buffer = [0u16; 2];
+        assert_eq!(encode(input, &mut buffer), Err(Error::BufferOverflow));
+
+        let input = "😎";
+        assert_eq!(encode(input, &mut buffer), Err(Error::MultiByte));
     }
 
     #[test]
     fn decoding() {
         let input = "$¢ह한";
         let mut u16_buffer = [0u16; 4];
-        let result = encode(input, &mut u16_buffer);
-        assert_eq!(result.unwrap(), 4);
+        assert_eq!(encode(input, &mut u16_buffer), Ok(4));
 
         let mut u8_buffer = [0u8; 9];
-        let result = decode(&u16_buffer, &mut u8_buffer);
-        assert_eq!(result.unwrap(), 9);
+        assert_eq!(decode(&u16_buffer, &mut u8_buffer), Ok(9));
         assert_eq!(core::str::from_utf8(&u8_buffer[0..9]), Ok("$¢ह한"));
+
+        // `decode` has three branches that can return `BufferOverflow`,
+        // check each of them.
+        assert_eq!(
+            decode(&u16_buffer, &mut u8_buffer[..0]),
+            Err(Error::BufferOverflow)
+        );
+        assert_eq!(
+            decode(&u16_buffer, &mut u8_buffer[..1]),
+            Err(Error::BufferOverflow)
+        );
+        assert_eq!(
+            decode(&u16_buffer, &mut u8_buffer[..3]),
+            Err(Error::BufferOverflow)
+        );
     }
 
     #[test]