compile error in expression position

[ModProg]

A function macro in expression position requires further consideration in regard to syntax.

A solution would be to expand to {compile_error!{ "error"} dummy + expr}

this is not valid in item/stmt position though, as it creates a new scope

Solution idea

Add expr(ssion) as a flag in the manyhow attribute, this will produce the extra wrapping in {}

[ModProg]

We should investigate what proc-macro-error does, as it also supports dummy code output.