new lint: `endless_iter`

What it does

1. We can create an endless Iterator easily by calling the Iterator::cycle() method or by Range literal construction without an upper bound.
   There is a corner case; the cycle call on an empty iterator will not create an endless iterator.

2. The endless property will be passed by many methods without an early return branch, e.g.:

• map
• filter
• skip
• skip_while
• map_while
• zip_longest
• ...

4. We also have some finalizers that will not early return:

• min
• max
• last
• count
• for_each
• reduce
• collect *
• sum *
• product *
• ...

There are also some false negative cases because FromIter, Sum and Product are customizable, so there may be some early-return implementation.

We can detect such cases.

Another acceptable name for the lint is infinite_iter, both are good.

Lint Name

endless_iter

Category

correctness

Advantage

Avoid some endless loops

Drawbacks

Some false negative cases

Example

(1..).map(|x| x + 2).for_each(|x| dbg!(x))

The issue names no files, tests, or entry points. Start with the Rust example and the listed iterator methods and finalizers; done means the new correctness lint detects applicable endless iterator chains while accounting for the documented false negatives.