The original would first check that there is right amount of numeric
characters and then parsed them using the std::str::parse, which
internally checks the characters again and also checks for -/+ prefix,
which is not necessary in this case.
Since we are already going over the characters, we may as well do the
parsing ourselves. The length of the function is roughly the same and
it is faster:
name simplify-from-str ns/iter reimplement-number ns/iter diff ns/iter diff % speedup
datetime::tests::bench_datetime_from_str 448 365 -83 -18.53% x 1.23
datetime::tests::bench_datetime_parse_from_rfc2822 242 195 -47 -19.42% x 1.24
datetime::tests::bench_datetime_parse_from_rfc3339 234 166 -68 -29.06% x 1.41
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