Tracking Issue for export_function_ordinals

This is a tracking issue for the RFC "3641" (rust-lang/rfcs#3641).
The feature gate for the issue is #![feature(export_function_ordinals)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Accept an RFC
  • https://github.com/rust-lang/rfcs/pull/3641
• Implement the RFC
• Adjust documentation (see instructions on rustc-dev-guide)
• Style updates for any new syntax (nightly-style-procedure)
  • Style team decision on new formatting
  • Formatting for new syntax has been added to the Style Guide
  • (non-blocking) Formatting has been implemented in rustfmt
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Can ordinals be skipped? If you specify ordinals 1, 3, 4, should this throw an error as 2 is skipped?
• If ordinals 1, 3 are specified, and you have another exported function, should it use 2 (the next unused ordinal) or 4 (the next in the sequence)?
• Instead of implementing this proposal, Could the usage of the .def file be changed to allow other functions to stay exported, even if they aren't included in the .def file?

Implementation history

Start with RFC 3641 and the export_function_ordinals feature gate. Review the tracking checklist, including implementation, documentation, style, and stabilization steps, then examine the unresolved ordinal-sequencing questions. Done means the RFC is implemented, documentation and style work are addressed, and a stabilization PR is completed.