Recipe

Find the line that frees each value

Use when: a program makes heap values such as Strings, and you need to say on which line each buffer goes back

Goal: for each owner, the closing brace that drops its value, and the order of the drops at a shared brace

Taught in The ownership rules · a later section extends this recipe with more cases

1. 1

   Write down each variable, made by a let, that holds a heap value. A tuple is one owner for everything inside it.

   Example

   In the city and band program, the owners are city and band. Each one holds one String.

   Why: rule 3 is about owners, so the owners are what you track.

2. 2

   For each owner, find its let. Then find the } that closes the block that let sits in.

   Example

   band is made inside the inner block, so its brace is the } on line 6. city is made in main's body, so its brace is on line 8.

   • the let is inside an inner block, such as an if body or a bare { } — the brace is that block's }.
   • the let is right in the function's body — the brace is the function's last }.
   • Not yet: the value is handed to another variable or into a function — covered in Predicting moves

   Why: the owner goes out of scope at that brace, and rule 3 drops its value there.

3. 3

   When two or more owners end at the same }, they are dropped in reverse order of their lets. The last one made goes first.

   Example

   In a main with let a = ...; and, further down, let c = ...;, both end at main's }. c is dropped first, then a.

   Why: Rust cleans up a block the way the stack (Chapter 6) works: newest first.

4. 4

   When an owner is dropped, every value it holds is dropped with it.

   Example

   let menu = (String::from("tea"), String::from("cake")); has one owner and two buffers. Both buffers are freed at main's }, when menu is dropped.

   Why: ownership forms a tree, and dropping the top of it drops everything underneath.