[chrismorgan]

In Rust, an invalid Unicode string simply cannot exist (* unless you use unsafe, but all bets are off then). An important part of this is that the code unit, the scalar value and the string are three different types (u8, char, str). Iteration must decide if it wants to go by code unit or by scalar value (… or by extended grapheme cluster, but that’s not provided in std).

JavaScript’s problems start with not having separate code unit or scalar value types. Sequences of UTF-16 code units, individual UTF-16 code units and scalar values all use the type string. (Code unit and scalar value also both use number in some contexts.)

The first step to fixing JavaScript’s bad semantics would be separating the code unit and scalar value types. If you did that… the changes required to support strict strings are perhaps surprisingly small. Even migrating to UTF-8 semantics is not very hard then.

Unfortunately, JavaScript seems very determined to do stupid things and allow stupid things and then do more stupid things with the stupid things it foolishly allowed.