Built-in Rust types

What you'll learn: Rust's fundamental types (i32, u64, f64, bool, char), type inference, explicit type annotations, and how they compare to C/C++ primitive types. No implicit conversions — Rust requires explicit casts.

• Rust has type inference, but also allows explicit specification of the type

• Rust permits arbitrary use of _ between numbers for ease of reading

Rust type specification and assignment

• Rust uses the let keyword to assign values to variables. The type of the variable can be optionally specified after a :

fn main() {
    let x : i32 = 42;
    // These two assignments are logically equivalent
    let y : u32 = 42;
    let z = 42u32;
}

• Function parameters and return values (if any) require an explicit type. The following takes a u8 parameter and returns u32

fn foo(x : u8) -> u32
{
    return x as u32 * x as u32;
}

• Unused variables are prefixed with _ to avoid compiler warnings

Rust type specification and inference

• Rust can automatically infer the type of the variable based on the context.
• ▶ Try it in the Rust Playground

fn secret_of_life_u32(x : u32) {
    println!("The u32 secret_of_life is {}", x);
}

fn secret_of_life_u8(x : u8) {
    println!("The u8 secret_of_life is {}", x);
}

fn main() {
    let a = 42; // The let keyword assigns a value; type of a is u32
    let b = 42; // The let keyword assigns a value; inferred type of b is u8
    secret_of_life_u32(a);
    secret_of_life_u8(b);
}

Rust variables and mutability

• Rust variables are immutable by default unless the mut keyword is used to denote that a variable is mutable. For example, the following code will not compile unless the let a = 42 is changed to let mut a = 42

fn main() {
    let a = 42; // Must be changed to let mut a = 42 to permit the assignment below 
    a = 43;  // Will not compile unless the above is changed
}

• Rust permits the reuse of the variable names (shadowing)

fn main() {
    let a = 42;
    {
        let a = 43; //OK: Different variable with the same name
    }
    // a = 43; // Not permitted
    let a = 43; // Ok: New variable and assignment
}