I was trying to improve tr, gave up, and I want to write down some findings, so I'm abusing the issue track to track the following issues

Sorry for creating yet another meta-issue, i've split it into a few manageable regular issues.

Should not accept unaligned [:upper:]

Moved to #6341

Should fix whitespace definition

Fixed by #6141

Fun fact: I was actually trying to detect a different bug (unicode whitespace), but that's apparently alright.

Does this ordering issue affect other character classes, too? No, apparently not.

Should not allow special classes in set2

Moved to #6342

Should handle high input bytes in command-line

Moved to #6343

Should handle high input bytes in input during translate complement

$ echo -en '\001amp\0376\0377' | tr -c 'abc' '0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789ABCDEF'     
1aadBC
$ echo -en '\001amp\0376\0377' | cargo run tr -c 'abc' '0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789ABCDEF'     
thread 'main' panicked at 'attempt to add with overflow', /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/iter/range.rs:391:1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
[$? = 101]

Fixed by #6340

Correctly "fill" set2 in case of complement

Moved to #6344

Replicate weird behavior around 256-limit

Moved to #6345

Feel free to grab this issue (or parts of it).

I gave up for two reasons:

• Deduplicating set1 and fixing a proper ordering for set2 completely goes against how the current tr implementation works, so I feel a total rewrite of solve_set_characters and TranslateOperationComplement is necessary. (EDIT: Fixed by tr: calculate complement set early #6340!)
• However, I can't build a reasonably-complete mental model of how exactly that needs to work. I hope the above examples provide a good starting point for whoever comes after me.