Articulating the ineffable. Programming language theory 
cognitive psychology. PhD @Stanford
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But doesn't the rust built-in have access to type information that rust users do not have? With static intro any user of the language can run the type checker on an arbitrary expression at compile time.
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.
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