Expand macro expression

[edgarogh]

I'm making an expression/function macro that takes a string literal, transforms it, and returns a new string literal. The problem I'm having is that string-literal-returning macros like concat!, env!, stringify! (and I might forget some) cannot be given as a parameter to my macro due to the order in which macros are expanded (they're passed as macro expressions, not expanded). I spent quite a bit of time in the doc and didn't find any way to expand a ExprMacro into the expanded macro's TokenStream, so I can use this in place.

Is there such a feature ? If no, is it planned, or impossible to do ?

[dtolnay]

I wouldn't want this to be built into syn, which is only supposed to be a parser. However it would be reasonable to address in a different crate.

[malobre]

@edgarogh Did you find a crate that could do this ?

Edit: For anyone interested there's also the unstable proc_macro_expand feature.

[Nemo157]

You can proxy your proc-macro through a macro-rules-macro using https://docs.rs/with_builtin_macros to support a subset of the std macros. I have an example using it and also extended that with optional support for the proc-macro-expand feature too in Nullus157/stylish-rs#5.