aggregate, reduce the complexity of the loop checking the overlapped fields

[ghost]

Based on the fact that overlapping is only possible in unions and anonymous unions (yes ?) it's possible to avoid a nested control loop iff

1. the aggregate is not an union.
2. the aggregate doesn't contain any field declared in an anonymous union.

Little drawback : the second condition requires to add a field to the VarDeclaration class since AnonDeclaration (i.e anonymous union or anonymous struct) is implemented as an attribute.

[dlang-bot]

Thanks for your pull request and interest in making D better, @Basile-z! 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 annotated coverage diff directly on GitHub with CodeCov's browser extension
• 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

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + dmd#9288"

[kinke]

AFAICT this led to a regression, some overlapping fields in the following example apparently don't get their VarDeclaration.overlapped flag set:

struct SWithUnion
{
    char c;
    S nested;

    union
    {
        struct { ubyte ub = 6; ushort us = 33; align(8) ulong ul_dummy = void; ulong last = 123; }
        struct { uint ui1; uint ui2 = 84; ulong ul = 666; }
    }
}

[kinke]

2. the aggregate doesn't contain any field declared in an anonymous union

This goes more than one level deep, that's the problem, this code just looks at the direct variable children of anonymous unions (and thus skips the 2 nested anonymous structs in the example). It would have to descend into nested anonymous unions and structs too.

I take it this was an attempt to improve performance (with any noticeable gains?); having to descend into subtrees etc. just makes this more complex and brittle, so I recommend a revert for 2.085.

[WalterBright]

@kinke can you turn this into a bugzilla issue, and add a piece of code that fails under this PR?

[kinke]

I'm swamped with LDC work.

[WalterBright]

I understand, but you do have a test case that triggers the problem, it just needs to be purified.

[WalterBright]

@kinke I created a bugzilla for this, https://issues.dlang.org/show_bug.cgi?id=19685 can you see about the test case?

[kinke]

Done, thx for bugzillaing.

[ghost]

Well, revert seems to be appropriated here since the commit was supposed to do the same thing.

[ghost]

The problem is that align was not handled at all by this PR, so again, revert.

[wilzbach]

Link to the revert PR: #9373