Short answer: it cannot be done.
Long answer: the format! macro (and its derivatives) requires a string literal, that is a string known at compilation-time. In exchange for this requirement, if the arguments provided do not match the format, a compilation error is raised.
What you are looking for is known as a template engine. A non-exhaustive list of Rust template engines in no particular order:
- Handlebars
- Rustache
- Maud
- Horrorshow
- fomat-macros
- …
Template engines have different characteristics, and notably differ by the degree of validation occurring at compile-time or run-time and their flexibility (I seem to recall that Maud was very HTML-centric, for example). It’s up to you to find the one most fitting for your use case.