Tracking Issue for target modifiers

This is a tracking issue for the RFC "3716" (rust-lang/rfcs#3716).

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 target modifier infrastructure. #133138
• Make relevant flags into target modifiers.
  • -Zreg-struct-return #133138
  • -Zregparm #133138
  • Sanitizers. #138453
  • -Zfixed-x18 #140661
  • -Zpatchable-function-entry
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Details of the flag for opt-out.

Implementation history

• Implement target modifier infrastructure. #133138
• -Zfixed-x18 flag #140661
• -Zharden-sls flag #136597, MCP
• -Zx86-retpoline flag #135927, MCP
• https://github.com/rust-lang/rust/pull/150732

cc @ojeda @azhogin

Start with RFC 3716 and the unchecked checklist items in this tracking issue, then review the referenced implementation issues and pull requests. The rustc-dev-guide stabilization and documentation instructions define the remaining process; done means the outstanding target modifiers, documentation, stabilization work, and opt-out design are resolved.