Tracking issue for `style_edition`

Tracking issue for the rustfmt specifics of implementing/supporting RFC 3338, Style Evolution

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 link to this issue

Steps

• Create new configuration option style_edition
• Soft deprecate version option with auto-mapping to corresponding style_edition value
• Add --style-edition flag to cargo-fmt and rustfmt
• Update configuration mechanism to support varying option availability, stability, and default values based on style_edition
• Update CLIs (cargo-fmt & rustfmt) to support varying surface of option/arg availability and stability
• Update configuration documentation approaches to delineate by style edition

Associated Issues/Discussions

• #5650 - Design Discussion for style_edition

Unresolved Questions

• Should the rolling "vNext" variant of style_edition be made available on stable? I.e. the current latest formatting-stable version of style_edition would be 2021, where 2024 would be analogous to version=Two, so would 2024 only be available on nightly?

Implementation history

• https://github.com/rust-lang/rustfmt/pull/6247
• https://github.com/rust-lang/rustfmt/pull/6432
• https://github.com/rust-lang/rustfmt/pull/6431
• https://github.com/rust-lang/rust/pull/134929

Start by reviewing the unchecked tracking steps for configuration, CLI behavior, and documentation, then read the linked design discussion #5650 and the listed implementation history. A contribution is complete when one scoped style_edition task is implemented, validated, and reflected in the checklist.