Immutable variables in Rust - Step-by-Step Execution

Concept Flow - Immutable variables

Declare variable with let

↓

Variable is immutable by default

↓

Try to change variable value

↓

Compiler error: cannot assign to immutable variable

↓

Use variable as is or declare mutable with let mut

In Rust, variables declared with let are immutable by default. Trying to change their value causes a compile error. To change a variable, declare it mutable with let mut.

Execution Sample

Rust

fn main() {
    let x = 5;
    // x = 6; // error: cannot assign to immutable variable
    println!("x = {}", x);
}

This code declares an immutable variable x with value 5 and prints it. Trying to assign a new value to x causes a compile error.

Execution Table

Step	Action	Variable	Value	Result/Output
1	Declare x with let x = 5;	x	5	x is set to 5
2	Attempt to assign x = 6;	x	5	Compile error: cannot assign to immutable variable
3	Print x	x	5	Output: x = 5

💡 Execution stops at compile error on step 2 because x is immutable

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
x	undefined	5	5 (unchanged, assignment failed)	5

Key Moments - 2 Insights

Why can't we assign a new value to x after declaring it with let?

Because variables declared with let are immutable by default in Rust, so the compiler prevents changing their value as shown in step 2 of the execution_table.

How can we make a variable mutable so we can change its value?

By declaring it with let mut instead of let, which tells Rust the variable can be changed.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of x after step 1?

A undefined

B 6

C 5

D 0

💡 Hint

Check the 'Value' column for step 1 in the execution_table

At which step does the compiler error occur?

A Step 1

B Step 2

C Step 3

D No error

💡 Hint

Look at the 'Result/Output' column in the execution_table for the error message

If we change 'let x = 5;' to 'let mut x = 5;', what happens at step 2?

A x changes to 6 successfully

B Program crashes

C Compile error remains

D x remains 5

💡 Hint

Recall that 'let mut' allows variable reassignment, so step 2 assignment succeeds

Concept Snapshot

Rust variables declared with let are immutable by default.
Trying to assign a new value causes a compile error.
Use let mut to declare mutable variables.
Immutable variables help prevent bugs by avoiding unintended changes.

Full Transcript

In Rust, when you declare a variable with let, it is immutable by default. This means you cannot change its value later. For example, if you write let x = 5;, x holds the value 5 and cannot be changed. If you try to assign x = 6;, the compiler will give an error and stop the program from compiling. To allow changing a variable's value, you must declare it mutable using let mut x = 5;. Then you can assign new values to x. This behavior helps keep your code safe and predictable by preventing accidental changes to variables.