[swift-evolution] Prohibit invisible characters in identifier names

[Xiaodi Wu]

On Thu, Jun 23, 2016 at 2:54 PM, João Pinheiro <joao at joaopinheiro.org>
wrote:

>
> > On 23 Jun 2016, at 20:43, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> > That's cool, although my preferred solution would be more closely
> aligned with UAX #31: overtly disallow the glyphs in Table 4 (instead of
> ignoring them) except in the specific scenarios for ZWJ and ZWNJ identified
> in UAX #31, then afterwards internally represent the identifier as its
> NFC-normalized string.
>
> Explicitly disallowing them was my initial idea, but I think it would end
> up being a confusing error for users to encounter. Ignoring the invisible
> characters and leaving it up to a linter to remove them is less likely to
> cause confusion for users.
>
> I'll be sure to describe the alternative of explicitly prohibiting them in
> the proposal though.
>

I would strongly urge you to propose explicitly prohibiting them just as
UAX #31 recommends. Their reasoning is that these characters, which include
those that reverse text direction or control joining, can cause one
identifier to be maliciously changed to look like another. If you ignore
these characters instead of prohibiting them, an identifier that visually
appears as one string could in fact be a different one to the compiler.

Moreover, a compiler error can be made helpful by saying that the offending
character is potentially invisible and it can come with a fix-it to remove
the offending character. I don't think that would confuse the user at all.
It would be more confusing if invisible characters that caused one thing to
look identical to another were silently permitted.


> Sincerely,
> João Pinheiro
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