I might not have been clear enough. I was just trying to show how the current syntax fits in with `:`, and why your suggestions for `: mut` would not fit with that. All of these are valid Rust code:

    let (mut x, ref y, &z): (int, int, &int) = (1, 2, &3);
    let (mut x, ref y, &z): (_, _, &_) = (1, 2, &3);
    let (mut x, ref y, &z) = (1, 2, &3);

The types are still the same in each case. No conversions or dynamic typing is happening in any of them - they are all statically typed.

Rust, like Haskell and ML, uses an advanced form of type reconstruction called Hindley-Milner[0] type inference. This is a constraint solving algorithm that can deduce types from very little information. Sometimes you do need to add some annotations though to help the inference along a little, but that is very rare (although more common in Rust than in Haskell or ML).

The grammar could be expressed as:

    'let' <pattern> (':' <type>)? '=' <expr>

Where `<type>` is the type of the expression to the right of the `=`. As I said before: in practice it is very rare to explicitly annotate a `let` binding.

[0]: http://en.wikipedia.org/wiki/Hindley-Milner