const vs immutable - when declaring a variable

[Steven Schveighoffer]

On Friday, 17 October 2025 at 19:49:07 UTC, Brother Bill wrote:
> Is there any difference when declaring a variable of using 
> const vs immutable.
>
> Example:
> ```
> const     int a = 3;
> immutable int b = 4;
>
> const     string c = "Greetings!";
> immutable string d = "D Programmers!";
> ```

Yes, a `const` declaration types the value as `const`, whereas an 
`immutable` declaration types the value as `immutable`.

While this is somewhat of an obvious explanation, the differences 
are subtle.

* A `const` reference type can point at either a mutable or 
`immutable` value. So regardless of where the value is stored, 
this has implications on where you can pass the value. You cannot 
pass a `const` reference to an `immutable` parameter.
* A non-reference type can be stored as either `const` or 
`immutable`, and it's effectively the same thing (value types 
such as `int` can be freely implicitly converted between 
mutability).
* Taking the *address of* a `const` or `immutable` value keeps 
the mutability when passing around, and this has implications as 
to what functions can accept such parameters.

My recommendation is to use `immutable` for declaring constants. 
Use `const` when you will assign to data that might be either 
`immutable` or mutable.

-Steve