[swift-evolution] [Proposal][Discussion] Deprecate Tuple Shuffles

[Robert Widmann]

- Parse: Has to account for the inclusion of tuple shuffles whenever it parses patterns (see the switch-statement example in the proposal)
- Sema: Has to perform argument matching by computing these tuple shuffle mappings thereby complicating the solver and the parts of solution application.  Really, the only place this has a valid use is in the error handling path where we can use the tuple shuffle to emit a fixit that properly reorders arguments - something we should be doing even today <https://bugs.swift.org/browse/SR-4715>.  Tuple shuffles are also allowed to reorder around variadic arguments which makes matching that much more difficult.
- SIL: Has to account for tuple shuffles in multiple places.  One notable one is that SILGen has to support two different paths when lowering tuple shuffles - one for variadic shuffles and the other for “normal” shuffles.  Each path supports a different subset of the features necessary to implement the full feature that is tuple shuffles, neither can really be simplified down to a common core in their current iteration.

If you want some numbers, I spent the evening removing them from the codebase and came up with a win of about 1500 LoC.  Each line of code supporting a feature that people aren’t actually using.

~Robert Widmann


> On May 4, 2017, at 10:35 PM, Tony Arnold via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On 5 May 2017, at 12:27, Félix Cloutier via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Why?
> 
> Not trying to be smart, but the reasoning is in Robert’s proposal:
> 
>>> Their inclusion in the language complicates every part of the compiler stack, uses a syntax that can be confused for type annotations (https://twitter.com/CodaFi_/status/860246169854894081), contradicts the goals of earlier SE's (see SE-0060), and makes non-sensical patterns possible in surprising places.
> 
> 
> Robert, maybe you could include some detail about how this feature is complicating the compiler stack, and what will be improved by it’s removal?
> 
> ====
> 
> That being said, I’m all for you guys making your lives easier at the cost of something we shouldn’t be using in the first place…
> 
> 
> Tony
> 
> 
> ----------
> Tony Arnold
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> 
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