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Chapter 2. Variables and assignment

In most programming languages, variable declarations and assignments barely warrant a dedicated chapter: you declare a variable, put a value in it with =, and read or replace it whenever you like. When learning Rust, however, many people write their very first lines of let and = and immediately run into compiler errors about moved values, Copy versus Move, or not being allowed to call a method without mut.

Chapter 1 showed why: in garbage-collected languages, = merely copies a value or an object reference while the runtime handles reclamation behind the scenes; in Rust, every let and = hands off or triggers the cleanup responsibility we derived in Chapter 1 at compile time.

This chapter leaves pseudocode behind. We open the compiler and trace our memory model through the most fundamental Rust syntax—let and =—in three steps:

  • Step 1: Initial binding (let x = ...)—creating a new variable to take on an initial value. Starting from our first runnable program, we verify why the same = splits into three paths—Copy, Move, and borrowing—across integers, String, and references (&T).
  • Step 2: Reassignment (x = ...)—updating an existing variable. We unpack the two permissions unlocked by let mut and follow the compiler’s state tracking to answer where the old value goes when a new value takes its place.
  • Step 3: Shadowing (writing let x = ... again)—creating a new variable with the same name. We contrast reassignment with shadowing and verify through an experiment whether a shadowed value is dropped on the spot.

By the end of this chapter, when looking at a dozen lines of Rust code—without memorizing compiler error codes—you should be able to answer three questions line by line:

  1. Which action is this: is this line creating a new variable (initial let), updating an existing variable (reassignment), or shadowing an old variable with a new let of the same name?
  2. How is the right-hand side handed off: is the value on the right copied, moved, or borrowed by address?
  3. When is the old value freed: which variables are still valid right now, which variable owns each heap buffer, and is that buffer freed right at the assignment line or at the end of the scope (})?

Table of Contents

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