Tracking Issue for Rust 2024: rustfmt single line where clauses

[calebcartwright]

This is a tracking issue for changing the formatting of where clauses to be formatted on a single line in some additional contexts, provided they meet certain constraints (short, single bound, and no = on aliases)

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.
  • 2024: Add chapter on single-line where clauses edition-guide#340
  • Revert "2024: Add chapter on single-line where clauses" edition-guide#344
• Add formatting for new syntax to the style guide.
  • style-guide: Format single associated type where clauses on the same line #119515
  • Revert #119515 single line where clause style guide #134974
• Ensure ready for Rust 2024 stabilization.

Unresolved Questions

None.

Implementation history

• style-guide: Format single associated type where clauses on the same line #119515

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

[calebcartwright]

Similar to my comment in #132380 (comment), I will note that we have already done the development for this and have been testing and reviewing the testing results with the style team.

Going to keep the impl-incomplete label until implementation is merged though

[joshtriplett]

rust-lang/edition-guide#340

[ytmimi]

@calebcartwright was the plan to default where_single_line=true for style_edition=2024 in rustfmt? Should be a one line change:

https://github.com/rust-lang/rustfmt/blob/9f8fcc2f3e7c7ff48412c30cfda67306d8e3320f/src/config/options.rs#L622

-    WhereSingleLine, bool, _ => false;
+    WhereSingleLine, bool, Edition2024 =>  true, _ => false;

That said, I took a look at #119515, and rustfmt doesn't yet format single line where clauses for associated types:

If an associated type is short, has no =, and has a where clause with only
one entry, format the entire type declaration including the where clause on
the same line if it fits:

type Item<'a> where Self: 'a;
type Item<'a>: PartialEq + Send where Self: 'a;

If the associated type has a =, or if the where clause contains multiple
entries, format it across multiple lines as with a type alias.

[traviscross]

Reminder: There are only 12 days left until the branch date for Rust 1.85. Anything not in master at that point will not make it into Rust 2024 unless it is accepted by compiler according to the conventions for backports to beta.

I.e., this item is in jeopardy.