from_str rounding issues when overflow from too many digits

[chris-cantor]

I am trying to add tests for the overflow behaviour of from_str and discovered a problem:

assert_eq!(
    Decimal::from_str("99999_99999_99999_99999_99999_99999.99999"),
    // it returns  Ok(100000_00000_00000_00000_00000_000) now
    Err(Error::from("Invalid decimal: overflow from too many digits"))
);

https://github.com/cantortrading/rust-decimal/runs/4657432688?check_suite_focus=true#step:9:87

It seems that the function is trying to round the input, such that 999,000 could be rounded to 1000,000 which is off by a lot.

What is the intended behaviour here? May be we should not attempt rounding before the decimal point?

[chris-cantor]

I discovered another edge case: the decimal parsing lost precision when there are too many leading 0s (i.e. ended up having > 28 coefficients)

assert_eq!(
    Decimal::from_str("00000_00000_00000_00000_00001.00001").unwrap(),
    Decimal::from_i128_with_scale(100001, 5) // it is now returning '1.0000'
);

https://github.com/cantortrading/rust-decimal/runs/4658904929?check_suite_focus=true#step:9:92

[paupino]

Fixed in #456