add macros examples

macros/macros3.rs

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+// Make me compile, without taking the macro out of the module! Scroll down for hints :)
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+mod macros {
+    macro_rules! my_macro {
+        () => {
+            println!("Check out my macro!");
+        };
+    }
+}
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+fn main() {
+    my_macro!();
+}
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+// In order to use a macro outside of its module, you need to do something special to the module to
+// lift the macro out into its parent.
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+// The same trick also works on "extern crate" statements for crates that have exported macros, if
+// you've seen any of those around.