Clean up E0040

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0040.md

@@ -1,8 +1,6 @@
-It is not allowed to manually call destructors in Rust. It is also not
-necessary to do this since `drop` is called automatically whenever a value goes
-out of scope.
+It is not allowed to manually call destructors in Rust.
 
-Here's an example of this error:
+Erroneous code example:
 
 ```compile_fail,E0040
 struct Foo {
@@ -20,3 +18,22 @@ fn main() {
     x.drop(); // error: explicit use of destructor method
 }
 ```
+
+It is unnecessary to do this since `drop` is called automatically whenever a
+value goes out of scope. However, if you really need to drop a value by hand,
+you can use the `std::mem::drop` function:
+
+```
+struct Foo {
+    x: i32,
+}
+impl Drop for Foo {
+    fn drop(&mut self) {
+        println!("kaboom");
+    }
+}
+fn main() {
+    let mut x = Foo { x: -7 };
+    drop(x); // ok!
+}
+```