fix issue 20682 - [DIP1000] wrong error: scope variable may not be co…

[aG0aep6G]

…pied into allocated memory

[dlang-bot]

Thanks for your pull request and interest in making D better, @aG0aep6G! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20682	normal	[DIP1000] wrong error: scope variable may not be copied into allocated memory

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#10935"

[aG0aep6G]

I think a better fix could be possible. In its current state, this PR only fixes a special case: If the escaping value has no pointers, then it can't possibly refer to the scope parameter. But there are also cases where the escaping value is a pointer, and it still can't refer to the scope parameter.

For example, this could also be allowed, as far as I understand:

ref int* f(return scope int** a) @safe
{
    return *a;
}

int* test(scope int** a) @safe
{
    // return *a; // already allowed
    return f(a); // could be made to work
}

But it seems that the escape checker currently doesn't compare types in that manner at all. Adding it would be a larger task with many pitfalls.

[aG0aep6G]

ref int* f(return scope int** a) @safe
{
    return *a;
}

int* test(scope int** a) @safe
{
    // return *a; // already allowed
    return f(a); // could be made to work
}

Making that work is beyond me at the moment. But I've found a better spot for the hasPointers check, and also fixed the same issue for cast.

This is good to go from my side.