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add BigUint subtraction underflow error message #13801

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add BigUint subtraction underflow error message #13801

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10 changes: 9 additions & 1 deletion src/libnum/bigint.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -230,7 +230,8 @@ impl Sub<BigUint, BigUint> for BigUint {
lo
}).collect();

assert_eq!(borrow, 0); // <=> assert!((self >= other));
assert!(borrow == 0,
"Cannot subtract other from self because other is larger than self.");
return BigUint::new(diff);
}
}
Expand Down Expand Up @@ -1755,6 +1756,13 @@ mod biguint_tests {
}
}

#[test]
#[should_fail]
fn test_sub_fail_on_underflow() {
let (a, b) : (BigUint, BigUint) = (Zero::zero(), One::one());
a - b;
}

static mul_triples: &'static [(&'static [BigDigit],
&'static [BigDigit],
&'static [BigDigit])] = &[
Expand Down