Rollup merge of rust-lang#25317 - steveklabnik:trpl_drop, r=alexcrichton

r? @alexcrichton

src/doc/trpl/drop.md

@@ -1,3 +1,67 @@
-% `Drop`
+% Drop
 
-Coming soon!
+Now that we’ve discussed traits, let’s talk about a particular trait provided
+by the Rust standard library, [`Drop`][drop]. The `Drop` trait provides a way
+to run some code when a value goes out of scope. For example:
+
+[drop]: ../std/ops/trait.Drop.html
+
+```rust
+struct HasDrop;
+
+impl Drop for HasDrop {
+    fn drop(&mut self) {
+        println!("Dropping!");
+    }
+}
+
+fn main() {
+    let x = HasDrop;
+
+    // do stuff
+
+} // x goes out of scope here
+```
+
+When `x` goes out of scope at the end of `main()`, the code for `Drop` will
+run. `Drop` has one method, which is also called `drop()`. It takes a mutable
+reference to `self`.
+
+That’s it! The mechanics of `Drop` are very simple, but there are some
+subtleties. For example, values are dropped in the opposite order they are
+declared. Here’s another example:
+
+```rust
+struct Firework {
+    strength: i32,
+}
+
+impl Drop for Firework {
+    fn drop(&mut self) {
+        println!("BOOM times {}!!!", self.strength);
+    }
+}
+
+fn main() {
+    let firecracker = Firework { strength: 1 };
+    let tnt = Firework { strength: 100 };
+}
+```
+
+This will output:
+
+```text
+BOOM times 100!!!
+BOOM times 1!!!
+```
+
+The TNT goes off before the firecracker does, because it was declared
+afterwards. Last in, first out.
+
+So what is `Drop` good for? Generally, `Drop` is used to clean up any resources
+associated with a `struct`. For example, the [`Arc<T>` type][arc] is a
+reference-counted type. When `Drop` is called, it will decrement the reference
+count, and if the total number of references is zero, will clean up the
+underlying value.
+
+[arc]: ../std/sync/struct.Arc.html