rust-macros

I have seen the @ symbol used in macros but I cannot find mention of it in the Rust Book or in any official documentation or blog posts. For example, in this Stack Overflow answer it is used like this: macro_rules! instructions { (enum $ename:ident { $($vname:ident ( $($vty: ty),* )),* }) => { enum $ename { $($vname ( $($vty),* )),* } impl $ename { fn len(&self) -> usize { match self { $($ename::$vname(..) => instructions!(@count ($($vty),*))),* } } } }; (@count ()) => (0); (@count ($a:ty)) => (1); (@count ($a:ty, $b:ty)) => (2); (@count ($a:ty, $b:ty, $c:ty)) => (3); } instructions! { enum

I'm reading a book about Rust, and start playing with Rust macros . All metavariable types are explained there and have examples, except the last one – tt . According to the book, it is a “a single token tree”. I'm curious, what is it and what is it used for? Can you please provide an example? That's a notion introduced to ensure that whatever is in a macro invocation correctly matches () , [] and {} pairs. tt will match any single token or any pair of parenthesis/brackets/braces with their content . For example, for the following program: fn main() { println!("Hello world!"); } The token

I have two modules in separate files within the same crate, where the crate has macro_rules enabled. I want to use the macros defined in one module in another module. // macros.rs #[macro_export] // or not? is ineffectual for this, afaik macro_rules! my_macro(...) // something.rs use macros; // use macros::my_macro; <-- unresolved import (for obvious reasons) my_macro!() // <-- how? I currently hit the compiler error " macro undefined: 'my_macro' "... which makes sense; the macro system runs before the module system. How do I work around that? Lukas Kalbertodt Macros within the same crate #