Add a project for demonstrating tests

adder/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "adder"
+version = "0.1.0"

adder/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "adder"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

adder/src/lib.rs

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+pub fn add(left: u64, right: u64) -> u64 {
+    left + right
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        let result = add(2, 2);
+        assert_eq!(result, 4);
+    }
+}