Add code

example_7/.gitignore

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+/target

example_7/Cargo.toml

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+[package]
+name = "example_7"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

example_7/src/main.rs

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+fn double_free() {
+    // every value has an owner, responsible for destructor (RAII).
+    // compiler checks:
+
+    // only ever one owner:
+    // no double-free
+
+    let x: Vec<u32> = Vec::new();
+    let y = x;
+    drop(x); // illegal, y is now owner
+}
+
+fn main() {
+    double_free();
+    immutable();
+}
+
+// Functions and structs can be declared any order (looking at you c++ and python!)
+fn immutable() {
+    let v = Vec::new();
+
+    // this compiles just fine:
+    println!("len: {}", v.len());
+
+    // this will not compile; would need mutable access
+    v.push(42);
+}