Lesson 0002 — Cargo and Mutability

Why this matters for Tauri

Tauri projects are still Rust projects. Before the framework, you need the basic Rust development loop: edit, format, check, run. Cargo is Rust’s package manager and build system; the official Cargo Book describes it as the tool that downloads dependencies, compiles packages, and makes distributable packages [Cargo Book]. Tauri’s setup also starts by installing Rust and system dependencies [Tauri prerequisites].

Your machine already has Rust available in this workspace: rustc 1.94.0 and cargo 1.94.0.

The project loop

cargo new hello-cargo --bin
cd hello-cargo
cargo run
cargo check
cargo fmt

cargo run compiles and executes. cargo check is the fast “is this valid Rust?” command you will use constantly. cargo fmt standardizes style so you do not waste taste on formatting.

I created a small exercise project for you here:

rust-tauri-sprint/exercises/hello-cargo

Skill: read a compiler error without panic

Open rust-tauri-sprint/exercises/hello-cargo/src/main.rs. It currently contains:

fn main() {
    let app_name = "tiny-tauri-helper";
    let mut command_count = 1;

    println!("{app_name} has {command_count} command ready");

    command_count = command_count + 1;
    println!("after adding one: {command_count}");
}

Now deliberately break it:

Change let mut command_count = 1; to let command_count = 1;.
Run cargo check inside the exercise folder.
Find the compiler’s suggested fix.
Restore mut, then run cargo run.

The rule

In Rust, let x = ... creates an immutable binding. If you want to assign a new value to the same binding, you must write let mut x = .... The Rust Book introduces this early because it shapes how Rust code communicates intent [Rust Book: Variables and Mutability].

Rust	Read it as
`let count = 1;`	This name should not be reassigned.
`let mut count = 1;`	This name may be reassigned.
`count = count + 1;`	This requires the binding to be mutable.

Go comparison, but only once

Go default

count := 1
count = count + 1

Reassignment is ordinary unless the variable is not addressable or otherwise constrained.

Rust default

let mut count = 1;
count = count + 1;

Reassignment is opt-in at the binding site.

Retrieval check

Which command gives fast compiler feedback without running the program?

Which binding allows reassignment?

Your sprint task

Run the deliberate mut error.
Paste the key compiler line back to the agent.
In one sentence, explain why Rust rejected the code.

Primary reading: The Rust Book, Chapter 3.1: Variables and Mutability. Ask follow-up questions whenever the compiler wording feels dense.

Next lesson preview

Next: ownership. We will make a value move, watch Rust reject a use-after-move, and connect that to why Tauri command boundaries often prefer borrowed input and owned output.