Tracking Issue for Rust 2024: rustfmt assignment operators rhs indentation

[calebcartwright]

This is a tracking issue for changing the indentation level used in formatting assignment/op expressions where the left hand side expression has to be formatted across multiple lines, with the style/formatting changing to indent the right hand side expression relative to the last line of the left hand side expression

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 in nightly Rust 2024.
• Add documentation to the edition guide.
  • Tracking Issue for Rust 2024: rustfmt assignment operators rhs indentation #132380
• Add formatting for new syntax to the style guide.
  • See the nightly style procedure.
  • style-guide: When breaking binops handle multi-line first operand better #119838 (initially applied to all binary expressions regardless of the operator type)
  • style-guide: Only use the new binop heuristic for assignments #132369 (adjusted scope to only be applicable with assignment operators)
• Ensure ready for Rust 2024 stabilization.

Unresolved Questions

None.

Implementation history

• style-guide: When breaking binops handle multi-line first operand better #119838
• style-guide: Only use the new binop heuristic for assignments #132369
• rust-lang/style-team#189: rhs-should-use-indent-of-last-line-of-lhs  rustfmt#6305

cc @rust-lang/rustfmt @rust-lang/style

[calebcartwright]

the Style and Rustfmt teams are already aware of this, but given the late date I will note for transparency that we do already have this "implemented" in rustfmt and have been doing some testing on it (and it's that testing that surfaced some formatting diffs that led to us winding back the operators where the new rule should be applied).

I am intentionally going to keep the impl-incomplete label here though until we get the PR merged in