I for the love me can't understand how this one ain't the most upvoted both as clever and best practices

hats off for avoiding regex altogether, I tried the same and had 10x the numer of locs

Good point, I don't remember if I did this for a good reason, but given that I initialize bits to 0 I think that & may be superfluous.

Great point, I swing languages very often and some don't have that notation so that betrayed me here

For the C version:

• meaning of arguments should be properly documented
• return value should be properly documented (there is a typo which makes things worse)
• the test code should not be so generic, it is very cumbersome to read and understand
• does not specify what to return if there are only T or F values

It didn't but Rust is very picky about type conversions, u32 is what https://doc.rust-lang.org/std/char/fn.from_digit.html takes. You can cast with "as" but it is not ideal. If this causes problems to existing people who passed the test then I agree that we shouldn't change it.

In Rust (and any strongly typed language really) the digit should be unsigned