Variables
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 like: let mut x = ....
Function parameters are immutable unless declared with mut. The mut keyword
applies only to the following parameter (so |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):
trait Changer: Sized {
fn change(mut self) {}
fn modify(mut self: Box<Self>) {}
}
Local variables are not initialized when allocated; the entire frame worth of local variables are allocated at once, 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.