When reporting errors in macros like println! Rust compiler shows the precise location within the format string where something has gone wrong, e.g.:
11 | println!("I'm going to substitute {foo}... or not");
| ^^^^^ not found in this scope
How I do the same thing for my format!-like macro? Both ways of reporting errors in procedural macro (quote_spanned! + compile_error! or the experimental Diagnostic API) rely on proc_macro::Span and I don't see a way to construct a Span for a location inside a token, even via experimental APIs.