Add proposal for useful String.Index descriptions

[lorentey]

This retroactively conforms String.Index to CustomStringConvertible.

rdar://22794467

[benrimmington]

You could mention that LLDB uses the same format (since Swift 5.8):

• SwiftFormatters.cpp

• SwiftStringIndex.h

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Was there any discussion about using debugDescription and CustomDebugStringConvertible instead? Or do you want to reserve those for a more verbose format?

[lorentey]

Was there any discussion about using debugDescription and CustomDebugStringConvertible instead? Or do you want to reserve those for a more verbose format?

Yes! After pitching this stuff, I investigated the role of the CustomDebugStringConvertible protocol, and I've had a long-standing reminder to start a forum discussion clarifying it. Surprisingly, it turns out it debugDescription isn't actually there to produce a more verbose display, or to give more information about the item. (For all I know, it may have been designed for that purpose, but that is certainly not how Swift is using it.)

CustomDebugStringConvertible is somewhat of a misnomer -- it actual de facto purpose is to produce a display that is functionally equivalent to description, but in a form that works well in structured, syntactic output: i.e., the string it returns needs to be free of unpaired parenthesis/brackets/quotes/etc, naked top-level commas/spaces, etc.

Collection types such as Array or Dictionary use debugDescription to print their contents, to avoid unintelligible output. Therefore, debugDescription must not be overly verbose -- it just needs to produce a syntactically well-formed format. (It will appear in array/dictionary/etc descriptions, where brevity is important.)