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In Rust, you provide a string — that is injected to be invoked internally. In C++, we’d just provide a callable.
This is because Rust’s attribute grammar can’t support a callable here.
I don't do C++ as a life choice, and thus not 100% sure what the author means here. But I have the feeling that he is wrong, on multiple levels even 😉
Correct - Rust's attribute grammar allows any parseable sequence of tokens enclosed in #[attr ...] basically. Serde specifically requires things to be in strings, but this is not a requirement of modern Rust or modern versions of syn (if you're comfortable writing your own parser for the meta).
The author is not a Rust expert though, so I'm not surprised to see this assumption. It doesn't take away from the article though.
Edit: for fun, syn has an example parsing an attribute in an attribute
Not only that. We don't just "inject" raw strings with the syn/quote duality. Stringified or not, the token tree will be parse-checked into the expected syn type before being used in generated code.
So the distinction is both wrong and irrelevant. This is what I meant by wrong on multiple levels/layers 😉