Tracking Issue for `iter` macro

[tmandry]

This is a tracking issue for the iter macro lang team experiment.
The feature gate for the issue is #![feature(iter_macro)].

Links to the discussion are in this #t-lang/gen topic, with more context starting here: #137725 (comment).

See also:

• Tracking Issue for gen blocks and functions (gen_blocks) #117078

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. Discussion comments will get marked as off-topic or deleted. Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement the macro
• Experiment with it in nightly
• Write an RFC: RFC: Add iter! macro rfcs#3861

Unresolved Questions

• Which syntaxes we support:
  • iter!(|| { ... })
  • iter! { ... }
• Which traits the return value of each syntax implements:
  • Iterator
  • IntoIterator
  • Fn() -> impl Iterator
• pin!() (and any other unsafe code relying on local variables having a fixed location) in a coroutine or gen block is unsound #153688

Implementation history

• Add iter macro #137725