remove redundant counters for remaining elements in set and frozenset iteration

[davidhewitt]

Related to #5661

This PR removes counters for remaining values from set and frozenset iterators; the underlying Python iterators already have this information and we can query it when needed.

While touching the code, I replaced some .unwrap() calls to .expect() to improve self-documentation slightly.

[LilyFirefly]

Do we need to call self.__length_hint__() here? (Or whatever the Rust/C name of that is.)

[davidhewitt]

Changed to call ExactSizeIterator::len to show where this value is coming from (that len does call size_hint of the underlying iterator, yes).

[Icxolu]

@davidhewitt Looks like there is some issue with this. On current main I get two FrozenSet test failures when using the abi3 feature:

❯ cargo test --package pyo3 --lib -F abi3 -- types::frozenset::tests --nocapture
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.19s
     Running unittests src\lib.rs (target\x86_64-pc-windows-gnullvm\debug\deps\pyo3-9eb94db30d3f0e35.exe)

running 8 tests
test types::frozenset::tests::test_frozenset_builder ... 
thread 'types::frozenset::tests::test_iter_count' (8760) panicked at src\types\frozenset.rs:348:13:
assertion `left == right` failed
  left: 0
 right: 3
oknote: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

test types::frozenset::tests::test_iter_count ... FAILED
thread 'types::frozenset::tests::test_frozenset_iter_size_hint' (9804) panicked at src\types\frozenset.rs:315:13:
assertion `left == right` failed
  left: 0
 right: 1

test types::frozenset::tests::test_frozenset_iter ... ok
test types::frozenset::tests::test_frozenset_iter_bound ... ok
test types::frozenset::tests::test_frozenset_empty ... ok
test types::frozenset::tests::test_frozenset_iter_size_hint ... FAILED
test types::frozenset::tests::test_frozenset_contains ... ok
test types::frozenset::tests::test_frozenset_new_and_len ... ok

failures:

failures:
    types::frozenset::tests::test_frozenset_iter_size_hint
    types::frozenset::tests::test_iter_count

test result: FAILED. 6 passed; 2 failed; 0 ignored; 0 measured; 763 filtered out; finished in 0.03s

[Icxolu]

I think the problem is that Bound<'py, PyIterator> only overrides Iterator::size_hint on the unlimited API. On the limited api the default from Iterator is used, which returns (0, None). Not sure why CI did not catch that tho...

[davidhewitt]

#5727 should have enabled Iterator::size_hint on abi3? 🤔

[Icxolu]

Oh, weird it looks like local git history just skipped #5727 even though it had this on in it... Not sure how that happed 🤔 but I fixed it now and the issue went away. Sorry for the noise.

[davidhewitt]

No prob, strange git bug!