If it’s known that these methods can cause UB if used incorrectly, and are actually pub, would it be appropriate to call the module __private_unsafe?

Otherwise, people might just consider it a potential semver-breaking module, not an actual safety issue.

would it be appropriate to call the module __private_unsafe

Disagreed. First of all, users should not use private APIs. I think the distinction between "safe" and "unsafe" private modules is misleading. (Even if it is a "safe" private module, it is not "fine" for users to use it.)

a potential semver-breaking module, not an actual safety issue.

Disagreed. Such APIs are usually called unstable instead of private.

That said, I don't think those who would abuse the private API care about documentation or convention...

That said, I don't think those who would abuse the private API care about documentation or convention...

Given this point, I think it would be difficult to completely deal with users who want to abuse the private API unless the yielder::pair is marked as unsafe -- added a commit to make yielder::pair unsafe.

added a commit to make yielder::pair unsafe.

Since this is adding an unsafe call inside of quote!, would this cause an issue if the user applies #![forbid(unsafe_code)]?

Since this is adding an unsafe call inside of quote!, would this cause an issue if the user applies #![forbid(unsafe_code)]?

No, unsafe_code will not trigger on unsafe keyword generated by external macros (except when the unsafe token has a span other than the call-site). (rust-lang/rust#52467)