On Borrow Checking

[Dukc]

On Tuesday, 29 April 2025 at 17:12:41 UTC, Walter Bright wrote:
> I was quite intrigued with the borrow checker, and set about 
> learning about it. While D cannot be retrofitted with a borrow 
> checker, it can be enhanced with it. A borrow checker has 
> nothing tying it to the Rust syntax, so it should work.

That's right.

>
> So I implemented a borrow checker for D, and it is enabled by 
> adding the `@live` annotation for a function, which turns on 
> the borrow checker for that function. There are no syntax or 
> semantic changes to the language, other than laying on a borrow 
> checker.

There's a difference.

In Rust, as I understand it, if you have a function like

```Rust
fn free(ptr: MyCustomPointer)
{   // ...
}
```

it is 100% safe to use. The compiler will not let you double-free 
or use after free, unless you use the `unsafe` block to do so.

But you can't have

```D
@trusted void free(MyCustomPointer ptr)
{   // ...
}
```

in D because it would be unsafe to use from a non-`@live` 
function.

If we had a way to say that "this function can be called from 
`@safe`, if and only if it's usage is guarded with `@live`" then 
it would be equal to the Rust borrow checker.