Put all metadata calls into GC

[schveiguy]

The GC is now fully responsible for storing and managing all metadata in GC blocks. The runtime will now just use GC calls to manage the metadata.

This allows GCs to innovate with how to store the metadata and not make any assumptions about the storage and retrieval of metadata.

[dlang-bot]

Thanks for your pull request, @schveiguy!

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 run digger -- build "master + dmd#20833"

[schveiguy]

Unsure what to do about the macos failure. I've rerun it, and it still fails "Error: The operation was canceled."

What does that mean?

[kinke]

Unsure what to do about the macos failure. I've rerun it, and it still fails "Error: The operation was canceled."

What does that mean?

It means the job was killed after hitting the 40 mins time-out - the Phobos tests shouldn't take more than a few minutes, but apparently hang from time to time on macOS. AFAICT, unrelated to your changes, as master is red too: https://github.com/dlang/dmd/actions/runs/13211623619/job/36885753688 (with latest DMD as host compiler, not bootstrap DMD as here).