In terms of readability, I could probably change u64::extended_parse("-9223372036854775808") to u64::extended_parse(&format("{}", i64::MIN)), but I'm not sure what to do with u64::extended_parse("-9223372036854775809"), maybe something like &format("{}", i64::MIN as i128 - 1)?

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)

In terms of readability, I could probably change u64::extended_parse("-9223372036854775808") to u64::extended_parse(&format("{}", i64::MIN)), but I'm not sure what to do with u64::extended_parse("-9223372036854775809"), maybe something like &format("{}", i64::MIN as i128 - 1)?

I think it's ok to hardcode values in tests (worth adding a comment though).

Added some more tests cases, checking the overflow value as well.
Changed test name to unsigned_hex_negative_wraparound and moved it above test_overflow.
Changed the overflow logic to use two's complement so now we are able to parse down to -u64::MAX, like GNU.

There's a specific CI which is failing because it thinks that fffffffffffffffc is a word

@eduardorittner you can suppress the error by adding the following to the top of the file:

// spell-checker:ignore fffffffffffffffc

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

@eduardorittner you can suppress the error by adding the following to the top of the file:

// spell-checker:ignore fffffffffffffffc

Thanks!

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)