auto decoding / [OT] swift 5 defaults to utf-8, gets rid of auto decoding, sees 20% improvement serving web page

[Nicholas Wilson]

https://swift.org/blog/utf8-string/

Choice paragraph:

>For example, source code (like most content) is encoded as 
>UTF-8, so SourceKit represents and communicates positions in 
>source code as offsets into a UTF-8 buffer. In Swift 4.2, 
>writing an efficient client of a UTF-8 based service required 
>maintaining a bidirectional index-mapping table from UTF-8 
>offsets to UTF-16 indices. Even forming a Swift 4.2 string from 
>UTF-8 content involves transcoding the content to UTF-16, which 
>can expensive. For example, SwiftNIO saw a 20% speed improvement 
>when serving up the homepage of swift.org by just upgrading to 
>Swift 5, due to skipping this transcoding.

A plan of action on what to do about auto decoding is on the 
agenda for the dconf AGM, but I figured there is no harm starting 
the discussion now.

My current vague idea is to add a version(Phobos_NoAutodecode) 
that changes the default behaviour, as opt in and then deprecate 
the use of the decoding .front/.popFront and require .by[W|D]char 
for when decoding is actually desired.

Anyway more/better/less vaguely defined ideas welcome.