bug: Incorrect size_hint on http::HeaderMap::iter #869

Version

http 1.5.0

Platform

Linux x86_64

Summary

size_hint may return 1 when the iterator actually has 0 elements left.

the implementation should provide a correct estimation, because otherwise it would be a violation of the trait’s protocol.

https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.size_hint

This is likely not security-relevant, because this hint

must not be trusted to e.g., omit bounds checks in unsafe code

Code Sample

// Cargo.toml: http = "=1.5.0"
fn main() {
    let mut m: http::HeaderMap<http::HeaderValue> = Default::default();
    m.insert(http::header::HOST, http::HeaderValue::from_static("x"));
    let mut it = m.iter();
    let (lo, _) = std::iter::Iterator::size_hint(&it); let a = it.next(); // (1,_), Some
    let (lo, _) = std::iter::Iterator::size_hint(&it); let b = it.next(); // (1,_), None ← bug
    assert!(!(lo >= 1 && b.is_none()), "size_hint lower bound {lo} > 0 but next() == None");
    let _ = a;
}

Expected Behavior

size_hint is accurate

Actual Behavior

it is not accurate

Additional Context

Bug found by a fuzzer I'm working on, minimal repro from Claude. Please let me know if this bug isn't helpful, and how I can do better. Thank you for maintaining such a foundational crate!