How can I create a function with a variable number of arguments?

In general, you can't - Rust does not support variadic functions, except when interoperating with C code that uses varargs.

In this case, since all of your arguments are the same type, you can accept a slice:

fn foo(args: &[&str]) {
for arg in args {
println!("{}", arg);
}
}


fn main() {
foo(&["hello", "world", "I", "am", "arguments"]);
}

(Playground)

Beyond that, you can explicitly accept optional arguments:

fn foo(name: &str, age: Option<u8>) {
match age {
Some(age) => println!("{} is {}.", name, age),
None      => println!("Who knows how old {} is?", name),
}
}


fn main() {
foo("Sally", Some(27));
foo("Bill", None);
}

(Playground)

If you need to accept many arguments, optional or not, you can implement a builder:

struct Arguments<'a> {
name: &'a str,
age: Option<u8>,
}


impl<'a> Arguments<'a> {
fn new(name: &'a str) -> Arguments<'a> {
Arguments {
name: name,
age: None
}
}


fn age(self, age: u8) -> Self {
Arguments {
age: Some(age),
..self
}
}
}


fn foo(arg: Arguments) {
match arg.age {
Some(age) => println!("{} is {}.", arg.name, age),
None      => println!("Who knows how old {} is?", arg.name),
}
}


fn main() {
foo(Arguments::new("Sally").age(27));
foo(Arguments::new("Bill"));
}

(Playground)

fn variable_func<T>(_vargs: &[T]) {}


fn main() {
variable_func(&[1]);
variable_func(&[1, 2]);
variable_func(&["A", "B", "C"]);
}

In the general case where your types may differ, you can use a macro:

macro_rules! print_all {
($($args:expr),*) => \{\{
$(
println!("{}", $args);
)*
}}
}


fn main() {
print_all!(1, 2, "Hello");
}

Playground

As another example, if you want to fold over the arguments, you can do something like this:

macro_rules! sum {
($($args:expr),*) => \{\{
let result = 0;
$(
let result = result + $args;
)*
result
}}
}


fn main() {
assert_eq!(sum!(1, 2, 3), 6);
}

Playground

See the Rust book's simplified implementation of vec![...] for another example.