[swift-evolution] [Pitch] Improve String Literals

[Charlie Monroe]

AFAIK, there was an optimization for literal string concatenation that was broken in Xcode 8.3 (and the Swift version that comes with it), but is now fixed in master.

https://bugs.swift.org/browse/SR-4348 <https://bugs.swift.org/browse/SR-4348>

> On May 16, 2017, at 10:47 PM, Ben Rimmington via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On 16 May 2017, at 21:27, Tony Allevato <tony.allevato at gmail.com> wrote:
>> 
>>> On Tue, May 16, 2017 at 1:25 PM David Hart <david at hartbit.com> wrote:
>>> The problem I see is that + is an operator of the Standard Library and not part of the core language. I wouldn’t want + to sometimes to be a runtime operation and other times a compile-time operation. No, I really think we need strong language support here.
>> 
>> Why not? Compile-time concatenation of string literals delimited by `+` sounds like a perfectly reasonable compiler optimization.
>> 
>> Your argument would also state that the compiler should not do constant folding like turning `5 + 7` into `12`, would it not? What makes that situation different?
> 
> When does the compiler do constant folding?
> 
> 	Welcome to Apple Swift version 3.1 (swiftlang-802.0.53 clang-802.0.42). Type :help for assistance.
> 	  1> import Foundation
> 	  2> let x: NSNumber = 12
> 	x: __NSCFNumber = Int64(12)
> 	  3> let y: NSNumber = 5 + 7
> 	error: repl.swift:3:21: error: cannot convert value of type 'Int' to specified type 'NSNumber'
> 	let y: NSNumber = 5 + 7
> 	                  ~~^~~
> 	                  NSNumber( )
> 
> -- Ben
> 
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