Macros producing statements are evaluated out-of-order

[udoprog]

Macros in statement positions might produce expressions or statements. This causes issues if the macro produces an item, because items need to be indexed before macros using them in case they use the item produced.

As an example, println! can use a constant value as the first argument, but this would currently cause issues:

pub fn main() {
    println!(FORMAT, "World");
    declare_format!(); // produces: `const FORMAT = "Hello {}";
}

This could be solved in following ways:

• Identify with metadata that a macro needs to use constant evaluation, in which case it is not permitted to expand to an item.
• Not allowing constant evaluation in macros, which would be a shame.
• Live with the fact that combining const values and macros have an inherent evaluation order.

Related: #153