> Interesting, so to make sure I understand, if I were writing an SQL library I could require (at the type check level) that a query I was passed was constructed with a specific formatter, and that formatter would encode any substrings that I templated into it in a way appropriate for SQL?

Yes. That's what Java's upcoming templated strings feature does. The library provides a templating policy as the way to create the appropriate type it requires.

> I wonder how this would handle something like HTML injection, where the desired encoding can change within the same string depending on whether you’re in an attribute or a text block?

The client library decides how to interpret and format the string in its policy.