[swift-evolution] Prohibit invisible characters in identifier names

[Chris Lattner]

> On Jun 23, 2016, at 9:17 AM, João Pinheiro via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On 21 Jun 2016, at 20:15, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> On Tue, Jun 21, 2016 at 1:16 PM, Joe Groff <jgroff at apple.com <mailto:jgroff at apple.com>> wrote:
>> Any discussion about this ought to start from UAX #31, the Unicode consortium's recommendations on identifiers in programming languages:
>> 
>> http://unicode.org/reports/tr31/ <http://unicode.org/reports/tr31/>
>> 
>> Section 2.3 specifically calls out the situations in which ZWJ and ZWNJ need to be allowed. The document also describes a stability policy for handling new Unicode versions, other confusability issues, and many of the other problems with adopting Unicode in a programming language's syntax.
>> 
>> That's a fantastic document--a very edifying read. Given Swift's robust support for Unicode in its core libraries, it's kind of surprising to me that identifiers aren't canonicalized at compile time. From a quick first read, faithful adoption of UAX #31 recommendations would address most if not all of the confusability and zero-width security issues raised in this conversation.
> 
> From what I've read of UAX #31 <http://unicode.org/reports/tr31/> it does seem to address all of the invisible character issues raised in the discussion. Given their unicode status of of Default_Ignorable_Code_Points, I believe the best course of action would be to canonicalise identifiers by allowing invisible characters only where appropriate and ignoring them everywhere else.
> 
> The alternative to ignoring them would be to not canonicalise identifiers and treat invisible characters as an error instead.
> 
> This doesn't address the issue of unicode confusable characters, but solving that has additional problems of its own and would probably be better addressed in a different proposal entirely.
> 
> I'd like to start writing the proposal if there is agreement that this would be the best course of action.

Sounds great, please do.  Thanks!

-Chris

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