A token tree is a recursive structure used by Rust macros. It can contain one identifier, a sequence of identifiers, or nested token trees, allowing a macro to match complete argument sequences. section 5.1.2
Matching variadic macro input
In `macro_rules! special_println`, `($($arg:tt)*)` names each matched token tree as `$arg`. Parentheses group the repeated rule, while `*` permits any number of matches. The expansion `$($arg)*` forwards the captured tokens to `println!`, preserving arguments without adding punctuation. section 5.1.2
Token-tree repetition uses familiar quantifiers:
- `*` matches zero or more occurrences, `+` matches one or more, and `?` matches zero or one. section 5.1.2
- Because a token tree may itself be a sequence, a matcher can pass the complete captured sequence through without inserting separators. section 5.1.2
macro_rules! special_println {
($($arg:tt)*) => { #1
println!($($arg)*) #2
};
}
Inspect the compact `special_println!` definition to see token-tree capture and forwarding together. section 5.1.2
