Tracking Issue for `gen` blocks and functions

[oli-obk]

This is a tracking issue for the RFC "#3513" (rust-lang/rfcs#3513).
The feature gate for the issue is #![feature(gen_blocks)].

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 (happening in Implement gen blocks in the 2024 edition #116447)
• Lots of diagnostics work
• Make the Iterators fused (so they keep returning None after the first None instead of panicking (which is what they do now))
• implement FusedIterator - Implement FusedIterator for gen block #122829
• Adjust documentation (see instructions on rustc-dev-guide)
• Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Whether to implement Iterator directly or, e.g., IntoIterator or IntoGenerator.
• Whether to do what is needed for self-referential gen blocks prior to stabilization.
• Whether we should use some keyword other than gen.
  • Bikeshedding the 'gen' keyword & syntax #123731
• Whether we should reserve gen as a full keyword or as a contextual one.
• Whether we should opportunistically implement size_hint.
• Whether we should implement other traits such as DoubleEndedIterator or ExactSizeIterator.
• What to do about Rust 2015 and Rust 2018 where k#gen may or may not be workable.

Related issues

• feature(gen_blocks): cannot use the ? operator in a gen block that returns () #117486

Implementation history

• Implement gen blocks in the 2024 edition #116447
• Add support for gen fn #118457
• CHANGE: Rename Rng::gen to avoid conflicting with a keyword in Rust 2024 rust-random/rand#1435 (thanks @PatchMixolydic for talking with rand folks!)

[kanashimia]

The feature gate for the issue is #![feature(gen)]

It was renamed to #![feature(gen_blocks)] in the implementation PR

[ChayimFriedman2]

If gen blocks are fused, shouldn't they implement FusedIterator?