No null — `Option<T>`

Rust has no null. Absence is a value of type Option<T>:


enum Option<T> { Some(T), None }

Kotlin


val name: String? = null
val name: String? = "Ada"

Rust


let name: Option<String> = None;
let name: Option<String> = Some("Ada".to_string());

Handling it — several ergonomic tools that map onto Kotlin habits:


// Kotlin: name?.length                     // map over Some
let len: Option<usize> = name.map(|n| n.len());
 
// Kotlin: name ?: "default"                 // provide a fallback
let n = name.unwrap_or("default".to_string());
let n = name.unwrap_or_else(|| expensive());
 
// Kotlin: name!!                            // assert non-null (panics if None)
let n = name.unwrap();          // or .expect("name must be set")
 
// Kotlin: if (name != null) { use(name) }   // smart-cast style
if let Some(n) = &name {
    println!("{n}");
}

? also works on Option inside a function returning Option — early-returns None.

No null — Option<T>

No null — `Option<T>`