How can Rust know if a variable is mutable?

Question

The following C# code compiles fine:

static readonly List<int> list = new List<int>();
static void Main(string[] args)
{
    list.Add(1);
    list.Add(2);
    list.Add(3);
}

If I write similar code in Rust, it won't compile because it cannot borrow immutable v as mutable:

let v = Vec::new();
v.push(1);
v.push(2);
v.push(3);

How does the push function know v is immutable?

Answer 1

All variables are immutable by default. You must explicitly tell the compiler which variables are mutable though the mut keyword:

let mut v = Vec::new();
v.push(1);
v.push(2);
v.push(3);

Vec::push is defined to require a mutable reference to the vector (&mut self):

fn push(&mut self, value: T)

This uses method syntax, but is conceptually the same as:

fn push(&mut Vec<T>, value: T)

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I highly recommend that you read The Rust Programming Language, second edition. It covers this beginner question as well as many other beginner questions you will have.