> the entire mechanism was designed for security ...

I don't have the same definition of security (i've read the JEP). Unlike TypeScript, you can not type the template values.

> built-in template processors, STR and FMT, don't do boxing ...

but all the others user-defined template processors (think a logger) are second class citizens and will do boxing.

Efficient String interpolation is hard without macros, that's why it's a macro in Rust.

This JEP tries to solve a harder problem, user defined template processors but do not provide the tool to make them efficient and never will. Are you suggesting that there is a JEP about macros in the making ?