[metadata prespecialization] Not always on for stdlib.

[nate-chandler]

Prespecializing metadata in the stdlib looks to be exposing a bug in its implementation. Disabling for now.

[nate-chandler]

@swift-ci please smoke test

[compnerd]

@nate-chandler do you happen to have a reference to a SR or a test case or any explanation of what has a bug (its unclear if the comment means the standard library or the metadata prespecialization - I suspect you meant the latter) and what the consequence of the bug is?

[mikeash]

@compnerd Still figuring that out, unfortunately. We have a crasher that goes away when this is disabled but we have not yet figured out exactly why.

[compnerd]

@mikeash ah, would be nice to mention the crasher.

I didn't mean to imply that the issue is explain, more the result of the failure - e.g. this type of pattern will cause a crash or your cat will will spontaneously combust if you use this compiler - so that others can possibly correlate the change.

[mikeash]

@compnerd I understand! I'm not even sure how to describe it generally, other than "very weird crash that looks like a memory smasher or thread safety error."

[tbkka]

The common element I've seen for this is resumeMetadataCompletion calling swift_runtime_unreachable(). Specifically, the following three frames in a stack trace:

swift_runtime_unreachable()
swift::resumeMetadataCompletion(swift::MetadataCompletionQueueEntry*)::ResumeCallbacks::forOtherMetadata(swift::TargetMetadata<swift::InProcess> const*) &&
swift::resumeMetadataCompletion(swift::MetadataCompletionQueueEntry*)