What gets me is that this affects later languages too. For example, Swift's internal String storage is either ASCII or UTF-16, precisely because it needs to mai

What gets me is that this affects later languages too. For example, Swift's internal String storage is either ASCII or UTF-16, precisely because it needs to maintain NSString's performance guarantees when bridging to Obj-C. That said, it seems the plan for Swift 5 is to change String's internal storage to be UTF-8, and then when bridging to NSString it will provide amortized constant-time UTF-16 codepoint access by calculating and caching "breadcrumbs" for fast lookup of UTF-16 offsets.