EncodeWide::size_hint doesn't account for `self.extra`

[programmerjake]

rust/library/std/src/sys_common/wtf8.rs

Lines 854 to 860 in 4d3ce2e

	fn size_hint(&self) -> (usize, Option<usize>) {
	let (low, high) = self.code_points.size_hint();
	// every code point gets either one u16 or two u16,
	// so this iterator is between 1 or 2 times as
	// long as the underlying iterator.
	(low, high.and_then(|n| n.checked_mul(2)))
	}

I haven't tested it, but, on Windows the following test should pass with the current code, demonstrating how size_hint is currently broken:

#[test]
fn test() {
    let os_str: &OsStr = "\u{12345}".as_ref();
    let mut iter = os_str.encode_wide();
    iter.next().unwrap();
    // at this point iter.code_points is empty and iter.extra is non-zero
    assert_eq!((0, Some(0)), iter.size_hint());
    // size_hint returned 0, yet we didn't reach the end:
    iter.next().unwrap();
}

[ChrisDenton]

As expected the test code does not panic on Windows. The size hint is initially only 1.

[fee1-dead]

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