update string.md

solutions/collections/String.md

@@ -136,4 +136,32 @@ fn main() {
 
     println!("Success!")
 }
+```
+
+7.
+```rust
+use std::mem;
+
+fn main() {
+    let story = String::from("Rust By Practice");
+
+    // Prevent automatically dropping the String's data
+    let mut story = mem::ManuallyDrop::new(story);
+
+    let ptr = story.as_mut_ptr();
+    let len = story.len();
+    let capacity = story.capacity();
+
+    // story has nineteen bytes
+    assert_eq!(16, len);
+
+    // We can re-build a String out of ptr, len, and capacity. This is all
+    // unsafe because we are responsible for making sure the components are
+    // valid:
+    let s = unsafe { String::from_raw_parts(ptr, len, capacity) };
+
+    assert_eq!(*story, s);
+
+    println!("Success!")
+}
 ```