This is a tracking issue for the RFC "Unnamed fields of struct and union type" (rust-lang/rfcs#2102).
The feature gate for the issue is #![feature(unnamed_fields)].

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 (see Tracking issue for RFC 2102, "Unnamed fields of struct and union type" #49804 (comment) for mentoring instructions)
• Adjust documentation (see instructions on forge)
• Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
• Stabilization PR (see instructions on forge)

Unresolved questions

• The RFC was accepted ages ago, and implementation turned out a lot harder than what was likely anticipated. As written, the RFC adds a fundamentally new "kind of type" to the language that's a hybrid of a struct and a union, with non-trivial consequences across many parts of the compiler (and the language, e.g. for macros accessing all fields of a type). Also see the discussion here, these meeting notes, and this comment.

Implementation history

• Parse unnamed fields and anonymous structs or unions (no-recovery) #115131
• Lowering unnamed fields and anonymous adt #115367
• Add more checks for unnamed_fields during HIR analysis #121198
• Retire the unnamed_fields feature for now #131045