But... the ability to use macros to create forms that look like "real" syntax is sort of the point. I don't think you can fix this with a language feature or style convention.
Metaprogramming is "dangerous" because the meta constructs don't (can't!) act like what they look liks. Metaprogramming is "great" because the meta constructs look like simple programming but have surprisingly deep behavior.
As I see it all you did was add a "!" to the confusion, honestly. People likely to get tripped up by e.g. the "address of macro" problem above are going to be no less confused by the extra syntax IMHO.
When you see an exclamation point, you know that compile-time fishiness is happening. If you don't see it, you know that there's nothing fishy going on. You might not think that's useful, but I hardly see how it can add to the confusion.
For clarity I just want to note here that Rust macros are structural (like Scheme) rather than textual (like C).
People don't expect an ordinary-looking identifier like "linux" to macro-expand to something else, like the number 1. That was the source of the confusion in the OP. Syntactically distinguishing identifiers that will be macro-expanded from identifiers that won't does solve this problem.
But again, that's situational. This particular macro is confusing because it (1) looks like an identifier and (2) doesn't act like one, instead being a 1970's style platform identifier that (3) no one actually uses in favor of constructs like __linux__ or __GNU_SOURCE__ or whatever.
But the more abstract point is that macros allow you to provide abstractions that do look just like an identifier (or function call, etc...) and do act like identifiers in the context where they're used. And that this is a good thing, because that kind of syntactic flexibility is useful in defining domain-specific languages that make the expression of hard problems clearer.
Adding a bunch of "!" to those DSLs isn't helping anything.
Basically, we have a macro that was abused here. Your solutions it to make it harder to abuse macros, mine is to accept the occasional abuse (with a "don't do that" in most cases, those here it's a backwards-compatibility problem) in exchange for more clarity on the hard stuff.
As I understand it, it is one of Rust's goals to be predictable in the code it runs/emits. This is fitting in a language designed for performance-critical systems. I see what you mean about the point of macros being that they can transparently add compile-time behavior. That's just a different, inconsistent philosophy from that of Rust.
Sure. Lots of successful languages exist without macro facilities. Lots exist with it. Frankly I don't see much correlation with "performance-critical systems" given that the overwhelmingly dominant languages in that field both have metaprogramming facilities (C++ has two!).
I was just quibbling with the idea that putting a magic "!" on a macro somehow makes it a better design choice. I don't think it does, either as a macro or as a safety mechanism.
Metaprogramming is "dangerous" because the meta constructs don't (can't!) act like what they look liks. Metaprogramming is "great" because the meta constructs look like simple programming but have surprisingly deep behavior.
As I see it all you did was add a "!" to the confusion, honestly. People likely to get tripped up by e.g. the "address of macro" problem above are going to be no less confused by the extra syntax IMHO.