A variable is a component of a stack frame, either a named function parameter, an anonymous temporary, or a named local variable.

A local variable (or stack-local allocation) holds a value directly, allocated within the stack's memory. The value is a part of the stack frame.

Local variables are immutable unless declared otherwise. For example: let mut x = ....

Function parameters are immutable unless declared with mut. The mut keyword applies only to the following parameter. For example: |mut x, y| and fn f(mut x: Box<i32>, y: Box<i32>) declare one mutable variable x and one immutable variable y.

Methods that take either self or Box<Self> can optionally place them in a mutable variable by prefixing them with mut (similar to regular arguments). For example:

# #![allow(unused_variables)]
#fn main() {
trait Changer: Sized {
    fn change(mut self) {}
    fn modify(mut self: Box<Self>) {}
}
#}

Local variables are not initialized when allocated. Instead, the entire frame worth of local variables are allocated, on frame-entry, in an uninitialized state. Subsequent statements within a function may or may not initialize the local variables. Local variables can be used only after they have been initialized; this is enforced by the compiler.