Is it possible to write something as complex as `print!` in a pure Rust macro?

Question

I am starting out learning Rust macros, but the documentation is somewhat limited. Which is fine — they're an expert feature, I guess. While I can do basic code generation, implementation of traits, and so on, some of the built-in macros seem well beyond that, such as the various print macros, which examine a string literal and use that for code expansion.

I looked at the source for print! and it calls another macro called format_args. Unfortunately this doesn't seem to be built in "pure Rust" the comment just says "compiler built-in."

Is it possible to write something as complex as print! in a pure Rust macro? If so, how would it be done?

I'm actually interested in building a "compile time trie" -- basically recognizing certain fixed strings as "keywords" fixed at compile time. This would be performant (probably) but mostly I'm just interested in code generation.

Answer 1

format_args is implemented in the compiler itself, in the libsyntax_ext crate. The name is registered in the register_builtins function, and the code to process it has its entry point in the expand_format_args function.

Macros that do such detailed syntax processing cannot be defined using the macro_rules! construct. They can be defined with a procedural macro; however, this feature is currently unstable (can only be used with the nightly compiler and is subject to sudden and unannounced changes) and rather sparsely documented.

Answer 2

Rust macros cannot parse string literals, so it's not possible to create a direct Rust equivalent of format_args!.

What you could do is to use a macro to transform the function-call-like syntax into something that represents the variadic argument list in the Rust type system in some way (say, as a heterogeneous single-linked list, or a builder type). This can then be passed to a regular Rust function, along with the format string. But you will not be able to implement compile-time type checking of the format string this way.