Fix Issue 21885 - Bad diagnostic: struct is not copyable because it is annotated @disable

[RazvanN7]

I had to fix a few tests, but IMHO the errors are more accurate now.

In case a struct has a generated postblit that is disabled, it means that the struct has a field that has a disabled postblit. The patch searches for the first field that has a disabled postblit and outputs it. This code is similar to the one in buildPostblit, therefore an alternative solution could store the fields with disabled postblits when the postblit is constructed, however, I think it's better to redo the work since we are on a failing branch.

[dlang-bot]

Thanks for your pull request and interest in making D better, @RazvanN7! 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
✓	21885	enhancement	Bad diagnostic: struct is not copyable because it is annotated @disable

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#12513"

[Geod24]

Can we recurse, so that "because it has a disabled postblit" is shown as supplemental ?

[RazvanN7]

Sorry, I don't understand what you mean. Do you expect here to have:

Error: struct `fail21885.Outer` is not copyable because it has a disabled postblit.
          The postblit is disabled because field `i' is not copyable and no user defined postblit was provided.

?

[Geod24]

struct A { B b; }
struct B { C c; }
struct C { int a; @disable this(this); }
void foo (A a);

I mean that this should produce, if possible:

file.d:42: Error: struct `mod.A` is not copyable because field `b` is not copyable.
                           struct `mod.B` is not copyable because field `c` is not copyable
                           struct `mod.C` is not copyable because it has a disabled postblit

[RazvanN7]

I see. That would be easy to implement, but I think that this could be implemented behind a switch as for large structs the output could be quite verbose. Anyway, I would rather get this in and file your proposal as an enhancement request.

[nordlow]

I would prefer having an errorSupplemental printing

"disabled postblit is defined here"....

I guess that could be added postponed to @Geod24's enhancement request.