Other Interesting Rust Syntax

Miscellaneous findings.

Return values

One thing I found strange about rust is the way values are returned from functions. You can just use a return keyword like most langs, but one unique thing about rust is that a semicolon can be the difference between an implicit return and a simple “value invocation” which does not return anything.

Take these two examples:

pub fn doStuffAndReturnIt() {
    // do stuff
    let x = 5;
    x
}

pub fn doStuffAndThenDoNothing() {
    // do stuff
    let x = 5;
    x;
}

In Rust, if the last expression in a function has no semicolon, it becomes the return value. So in the first example, we return x, in the second example, we return () (the unit type) and the value of x is discarded.

mut

In rust, variables are immutable by default. Make a mutable variable with the mut keyword.

let x = 5; // immutable
let mut y = 10; // mutable

! on stuff

In rust, macros can be used in a c-esque way, or they can be invoked while hidden as normal functions by using a ! and calling it like a function. That’s a bad sentence. Here’s a better example:

println!("Hello, World!");  // This is a macro

Other examples of common macros:

• vec![]: Creates a vector.
• format!(): Creates a formatted string.
• panic!(): Triggers a panic (terminating the program).