Add primary associated type for MarkupVisitor

[Kyle-Ye]

Bug/issue #, if applicable:

Summary

Add primary associated type for MarkupVisitor

Update: Bump the minimal supported Swift version to Swift 5.7

Alternatives

The current implementation of PAT support for MarkupVisitor has a lot of duplicated code since PAT was introduced by Swift 5.7.

We could drop such duplication if we can update this Package's min supported Swift version to Swift 5.7.

See #78

Testing

See MarkupVisitorTests.swift

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[QuietMisdreavus]

I think there's a broader discussion to be had about upgrading the minimum Swift version for Swift-DocC and its dependencies. This could probably be a Documentation Workgroup topic, IMO.

[Kyle-Ye]

I think there's a broader discussion to be had about upgrading the minimum Swift version for Swift-DocC and its dependencies. This could probably be a Documentation Workgroup topic, IMO.

If we can upgrade min-supported version to Swift 5.7. I think this PR is fairly easy to be merged.

If we decide to maintain a Swift version lower than Swift 5.7, it looks like we need to maintain such huge duplication of code to get PAT support.

Do you think this PR is generally fine even if we still need to target a swift version lower than 5.7?

[Kyle-Ye]

Follow the swift repo's style, I just added 2 labels for this repo so that we can better tag this for the future PR.

I'll remove the "pending discussion" label to "approved" when the meeting notes is posted on the forum.

[Kyle-Ye]

@swift-ci please test

[Kyle-Ye]

I think there's a broader discussion to be had about upgrading the minimum Swift version for Swift-DocC and its dependencies. This could probably be a Documentation Workgroup topic, IMO.

The Swift 5.7 bump was discussed and got approved at last week's meeting.

I think this PR is now ready for review. @QuietMisdreavus

[Kyle-Ye]

For PAT support, it unlocks more usage for the downstream user. Basically the same as Combine add PAT for Publisher protocol Publisher<Output, Failure>
Before the PR, we can only use some MarkupVisitor and can't do anything for the result of visit method since it is unbound.

var vistor: some MarkupVisitor = EmptyWalker()
let result = vistor.visit(Text(""))
// Can't do anything on result

After the PR, we can unlock something very interesting for the downstream user. See the latest test case. 84a2048
(The original test case use MarkupWalker whose Result type is Void and may not fully express the idea. I've update the test case to show the PAT's usage)

You may also have a check here https://www.swiftbysundell.com/articles/opaque-return-types-primary-associated-types/ which has a great explanation for PAT.

[Kyle-Ye]

@swift-ci please test

[Kyle-Ye]

@swift-ci please test

[Kyle-Ye]

@swift-ci please test