fragment specifier

The part of a macro pattern that says what kind of code a variable matches, like expr or ident.

A fragment specifier ( $x:expr,$ name:ident, $t:ty,$ ($rest:tt)*) constrains a macro variable to a syntactic category.

Examples

$value:expr matches any expression; the pattern ($a:expr $b:expr) is rejected because an expr may only be followed by =>, , or ;.