"Allows more operations" seems like a straightforward meaning for "different semantics" to me. In Safe Rust, you can't access union members. In Unsafe Rust you can.

I agree that it's a little fanciful, but I also don't think it's really wrong.

> To suggest that this means that the language itself has infinite sublanguages obscures its actual sublanguages, such as the built-in macro-rules DSL.

In many languages, the macro language is either homoiconic or nearly homoiconic. In Crystal, for example, the macro DSL looks like normal Crystal, but with a few extra sigils. You can't invent new syntax in it.

When we talk about understanding C we generally take that to include the C preprocessor, despite the latter being a conceptually separate macro language. I think the same standard (except much, much better) applies to procedural macros in Rust: you need to understand a separate set of token production and consumption rules that interact but do not align with Rust's core syntax.

> This is a misunderstanding of the reference manual

I wasn't referring to the production rules themselves; I'm aware they're on most pages. I was referring to the fact that the attribute language is explicitly taken directly from a separate language (C#).