`TreeCopier` doesn't preserve attachments

[Duhemm]

While working on embedding the doc in Tasty, I noticed that the TreeCopier subclasses don't copy over the attachments of the trees.

Is this on purpose?

Doc comments are stored as attachments on MemberDef trees, and cannot be recovered in the REPL for instance, because they're discarded when the trees are modified.

[Duhemm]

It looks like they're not copied over on purpose: it appears that copying them breaks a lot of things.

Should the docs be stored somewhere else that will not be lost down the line? Should we only copy the doc comment in TreeCopier?

[smarter]

It looks like they're not copied over on purpose: it appears that copying them breaks a lot of things.

What kind of failures are you getting?

Should the docs be stored somewhere else that will not be lost down the line? Should we only copy the doc comment in TreeCopier?

Maybe we need two kinds of attachments: sticky and non-sticky.

[Duhemm]

@smarter Here's sbt's log for dotty-compiler/testOnly dotty.tools.dotc.CompilationTests -- --tests=compilePos:
https://gist.github.com/Duhemm/5eff50bcf9981aa941773d252e22e8eb

Maybe we need two kinds of attachments: sticky and non-sticky.

Sounds good! I'll look into that.