In this recipe, we create a macro named Check_Val!, which basically plays the role as a match expression arm, but the matching happens through the Rust syntax trees during compilation time. The common syntax of a pattern is as follows:

( $( $x:expr ),* ) => { ... };

Here, the term pattern refers to the left-hand side of =>, which is known as matcher in Rust.

The $x:expr matcher will match any Rust expression and will bind that to syntax tree to the metavariable $x, any Rust tokens that appear in a matcher must match exactly.

We have two pattern matching cases here: x => $e:expr and y => $e:expr. The metavariable is $e, which is used in macro definitions for operations to be done following the successful pattern matching of a macro rule. When we call Check_Val!(y => 3); in the main function, the output is mode Y:3. Here the second case is passed and the value of $e is the same as that of the arguments passed to the Check_Val! macro in the main function.