Tracking Issue for RFC 3239: Add target configuration

[pnkfelix]

This is a tracking issue for the RFC "3239" (rust-lang/rfcs#3239).
The feature gate for the issue is #![feature(cfg_target)].

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.

Steps

• Implement the RFC (RFC3239: Implement cfg(target) - Part 1 #96909 and RFC3239: Implement cfg(target) - Part 2 #96913)
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• How do we ensure a project does not miss configurations similar to the ones
  being #[cfg]-ed against with this feature? Perhaps this should be added as a
  Clippy lint that's off by default.

Implementation history

[Urgau]

@rustbot claim