What does the tt metavariable type mean in Rust macros?

Question

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?

Answer 1

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 trees would be:

• fn
• main
• ()
  • ∅
• { println!("Hello world!"); }
  • println
  • !
  • ("Hello world!")
    • "Hello world!"
  • ;

Each one forms a tree where simple tokens (fn, main etc.) are leaves, and anything surrounded by (), [] or {} has a subtree. Note that ( does not appear alone in the token tree: it's not possible to match ( without matching the corresponding ).

For example:

macro_rules! {     (fn $name:ident $params:tt $body:tt) => { /* … */ } } 

would match the above function with $name → main, $params → (), $body → { println!("Hello world!"); }.

Token tree is the least demanding metavariable type: it matches anything. It's often used in macros which have a “don't really care” part, and especially in macros which have a “head” and a “tail” part. For example, the println! macros have a branch matching ($fmt:expr, $($arg:tt)*) where $fmt is the format string, and $($arg:tt)* means “all the rest” and is just forwarded to format_args!. Which means that println! does not need to know the actual format and do complicated matching with it.