Experimental Tracking Issue for Next-Gen Transmute

This is a tracking issue for experimental work related to https://github.com/rust-lang/rfcs/pull/3844 and https://github.com/rust-lang/libs-team/issues/772 about reworking transmute to give it more flexibility.

Note that this is not about "safe transmute"; for that see #99571.

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

• Implement the RFC (cc @rust-lang/XXX -- can anyone write up mentoring
  instructions?)
• 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

• list all the "unresolved questions" found in the RFC to ensure they are
  not forgotten.

Implementation history

• https://github.com/rust-lang/rust/pull/155084

Start with RFC 3844, the linked rust-lang/libs-team issue #772, and implementation PR #155084 to understand the proposed transmute changes and current progress. Completion follows the checklist: implement the RFC, update documentation and style guidance, resolve unresolved questions, and complete stabilization.