Articulating the ineffable. Programming language theory 
cognitive psychology. PhD @Stanford
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Oh I see -- you're right, Rust doesn't give user-space macros access to type information. I've wanted this for a while though (see: https://willcrichton.net/notes/type-directed-metaprogramming-in-rust/ …).
However, with traits you can get a lot of mileage out of syntax-directed transformations. Rust's println impl doesn't introspect types. format!("{:?}", 3) expands to Arg(3, fmt::Debug) which then only type-checks if 3 implements Debug.
That's why Rust doesn't have %d or %s, just {} and {:?}. The Debug/Format traits take care of checking whether an object is convertible to a string.
To see an example, go to this playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=07ad6d44dd855455c1e444d21357df18 … Then click "Tools > Expand Macros".
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