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src/unsafe/unions.md

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+# Unions
+
+Unions are like enums, but you need to track the active field yourself:
+
+```rust,editable
+#[repr(C)]
+union MyUnion {
+    i: u8,
+    b: bool,
+}
+
+fn main() {
+    let u = MyUnion { i: 42 };
+    println!("int: {}", unsafe { u.i });
+    println!("bool: {}", unsafe { u.b });  // Undefined behavior!
+}
+```