It's almost but not quite arbitrary. It still has to be a valid Rust token stream with matching braces/parens. Since it's whitespace insensitive with respect to tokens and "-" is its own token, "(foo-bar)" and "(foo - bar)" result in the same token stream minus the span information.

You can use proc_macro::Span.line()/column() [1] to track how much whitespace there is between tokens and merge them, but the macro will have to rename them to valid Rust identities ("foo-bar" to "foo_bar") and make sure there's no collisions.

[1] https://doc.rust-lang.org/proc_macro/struct.Span.html#method...