Shadowing

Instead of changing variable properties, it's often more readable to overwrite a variable with a different value (for example, a changed copy of the original). This technique is called shadowing.

Typically, this is used to reuse a variable name, even though the actual value has changed, to work in the current situation. This snippet sanitizes String and, by using the same name throughout the function, it's always clear that it's the input parameter that is changed:

fn sanitize(s: String) -> String {
    let s = s.trim();
    let s = s.replace(" ", "_");
    s
} 

While this is akin to changing the value of a variable, shadowing does not replace mutability, especially when it's less costly to actually change properties of that variable; Rust has a specific design pattern for that!