A token tree is a unit within Rust macro input: a TokenStream is a sequence of TokenTrees, and each tree can recursively contain further trees or tokens. Declarative macros capture this broad shape with the `tt` fragment specifier. section 3.6
Use `tt` when a matcher must preserve syntax that is too varied for a narrower fragment such as `ident`, `expr`, or `ty`. Its flexibility is deliberate, but it also accepts so much that mistakes become harder to reject; start with a concrete fragment and widen only when needed. section 2.1.4
Forwarding optional macro input
A `lazy_static!` matcher captures trailing optional input as `$($t:tt)*` and passes it to an internal macro. The token-tree repetition keeps that extra syntax intact without requiring the entry matcher to classify every possible token. section 2.3
Inspecting procedural-macro input
A procedural macro can iterate over its incoming TokenStream, take a TokenTree, and match it as an identifier. This works for a small derive macro, though manually extracting and generating syntax grows burdensome for richer input. section 3.6
Listing 2.20 lazy_static macro entry point, simplified slightly
#[macro_export]
macro_rules! lazy_static {
($(#[$attr:meta])* static ref $N:ident : $T:ty =
$e:expr; $($t:tt)*) => {
__lazy_static_internal!($(#[$attr])* () static ref $N :
$T = $e; $($t)*);
};
// other arms...
}
Inspect a practical matcher that combines typed fragments with a repeated `tt` tail for additional input. section 2.3
