How do I declare multiple mutable variables at the same time?

Question

I can declare multiple constants like this:

let (a, b, c) = (1, 0.0, 3);

But why can't I do this with mutable variables?

let mut (a, b, c) = (1, 0.0, 3); throws a compile error:

error: expected identifier, found `(`
 --> <anon>:2:13
2 |>     let mut (a, b, c) = (1, 0.0, 3);
  |>             ^

Answer 1

The proper syntax is

let (mut a, mut b, mut c) = (1, 0.0, 3);

Mutability is a property of the binding, and a, b, and c are all different bindings, each bound to a specific element of the tuple after the pattern has been matched. Thus they can be individually made mutable.

If you wanted to specify the type, you could do that too:

let (mut a, mut b, mut c): (u8, f32, i32) = (1, 0.0, 3); 

For numeric literals, you could also use the suffix form:

let (mut a, mut b, mut c) = (1u8, 0.0f32, 3i32);

Of course, there's no reason to do this for the example code; it's much simpler to just have 3 separate statements.

declare multiple constants

These aren't constants, they are just immutable variables. A const is a different concept.