Disallow additional invisible characters in Julia source

[c42f]

There's many invisible unicode characters.

https://invisible-characters.com/

We currently disallow several of them

https://github.com/JuliaLang/julia/blob/c24517918dd7ea33df4eb0c965dd7d45d530d7b0/src/julia-parser.scm#L596

Namely

'\u00ad' # soft hyphen
'\u200b' # zero width space
'\u200c' # zero width non-joiner
'\u200d' # zero width joiner
'\u200e' # left-to-right mark
'\u200f' # right-to-left mark
'\u2060' # word joiner
'\u2061' # function application

It appears the reference parser also attempts to disallow '\u115f' # Hangul Choseong filler , but this doesn't work due to Base.is_id_char returning true for that character. I'm not sure this is a problem, I didn't find obvious information about what this filler character is for and whether it might be required for writing identifiers in Korean.

Anyway, should we disallow more of the list from https://invisible-characters.com/ ?

[inkydragon]

Maybe we could just disallow all those invisible characters. Wait until we find a use for certain characters, or when someone complains that certain characters are not available, and then we could relax those restrictions.

We could also introduce a new function to detect whether a particular character is visible or not. Perhaps called isInvisibleCharacter.

And a longer list in VSCode: https://github.com/microsoft/vscode/blob/44a490b514be5972328549ea4a634d4d1587ae2c/src/vs/base/common/strings.ts#L1192-L1193 ref: https://github.com/flopp/invisible-characters/issues/1

[c42f]

Awesome, thanks for the pointer to the list from VSCode. I definitely didn't want to trawl through the unicode standard myself looking for those!

It seems reasonable to me to just add the big list and wait to see whether it's any problem in practice. I'm ... almost ... willing to bet there's no code in General which uses these (except for the ascii whitespace)

[Seelengrab]

I think lots of these fillers are a remnant of converting KS X 1001 to Unicode. As far as I can tell, it shouldn't be produced in a unicode environment at all, according to this unicode document (at least the Hangul Filler). They're in principle allowed because they're in the Letter, Other category.

As mentioned when this kind of thing came up in the discussion surrounding bidi confusion here, I don't think disallowing these on the language level is the right place - it seems more appropriate for a linter.