Rebooting the __metada/__mutable discussion

[Zach Tollen]

On Friday, 8 April 2022 at 02:32:29 UTC, Paul Backus wrote:
> On Friday, 8 April 2022 at 01:46:35 UTC, Zach Tollen wrote:
>> On Friday, 8 April 2022 at 01:24:38 UTC, Paul Backus wrote:
>> In other words, if it's dangerous enough to start out as 
>> `@system`, how much additional danger are we adding by making 
>> it mutable?
>
> You're not adding any additional *danger*, I suppose, but it 
> would be a non-obvious special case in the type system to have 
> the `@system` attribute on variables interact with `immutable` 
> like this. As currently proposed by DIP 1035, they are 
> completely independent of each other.

It's only non-obvious if you define `@system` variables simply as 
"not usable from `@safe` code." If you defined them as "can also 
override the type system," it *would* be obvious.

The only question is whether this is a good language design. It 
is certainly easy to explain:

Q: "What are `@system` variables?"

A: "They are variables you can do anything you want with, as long 
as it's from `@system` code."

Q: "What are they for?"

A: "They're for doing things you can't do with regular variables, 
and also for marking regular variables as dangerous when the 
compiler can't figure that out on its own."