Is it possible to declare variables procedurally using Rust macros?

Question

Basically, there are two parts to this question:

1. Can you pass an unknown identifier to a macro in Rust?

2. Can you combine strings to generate

Answer 1

In cases where concat_idents doesn't work (which is most cases I'd like to use it) changing the problem from concatenated identifiers to using namespaces does work.

That is, instead of the non-working code:

macro_rules! test {
    ($x:ident) => ({
        struct concat_idents!(hello_, $x) {}
        enum contact_idents!(hello_, $x) {}
    })
}

The user can name the namespace, and then have preset names as shown below:

macro_rules! test {
    ($x:ident) => ({
        mod $x {
            struct HelloStruct {}
            enum HelloEnum {}
        }
    })
}

Now you have a name based on the macro's argument. This technique is only helpful in specific cases.