bug(syntax): `Ord` on `ast::Position` is inconsistent with `Eq`

[langston-barrett]

What version of regex are you using?

0.8.11

Describe the bug at a high level.

Ord for Position is keyed off of offset only, but its Eq and PartialEq are derived, so they can disagree.

regex/regex-syntax/src/ast/mod.rs

Line 372 in 72d650c

self.offset.cmp(&other.offset)

This affects Span as well, since it stores Positions.

The methods of this trait must be consistent with each other and with those of PartialEq.

https://doc.rust-lang.org/std/cmp/trait.PartialOrd.html

What are the steps to reproduce the behavior?

use std::collections::BTreeSet;
use regex_syntax::ast::{Position, Span};

fn main() {
    let a = Position { offset: 0, line: 1, column: 1 };
    let b = Position { offset: 0, line: 9, column: 9 };

    assert_ne!(a, b);
    assert_eq!(a.cmp(&b), std::cmp::Ordering::Equal);

    let mut set = BTreeSet::new();
    set.insert(a);
    set.insert(b);
    assert_eq!(set.len(), 1);
}

What is the actual behavior?

BTreeSet is confused, all assertions pass.

What is the expected behavior?

Both a and b are inserted.

Found with a fuzzer I'm developing.