If you used a loop {} construct instead of the for loop, you wouldn't need the unreachable (0, 0, false) case, since a loop {} that never terminates has the never type (!) which Rust can automatically coerce to any other type.

I'm having the exact same problem, my solution does work with 1 tho, my solution fails for the second test case, it just says "Value is not what was expected".

From a Python interpreter session with my solution:

>>> grade(0)
'F'
>>> grade(1)
'A'
>>> grade(0.7)
'C'
>>> grade(0.6)
'D'
>>> grade(0.5)
'F'
>>> grade(0.8)
'B'
>>> grade(0.9)
'A'
>>> 

I don't see anything wrong with this.

On Python, you can check the function's code object: calculate.func_code.co_consts contains all constants (numbers) used in the code. A simple check would be if 2 not in calculate.func_code.co_consts: test.fail()