`.insert_children/.insert_related` inserting first element last

[databasedav]

Bevy version

0.16.0

What you did

repro:

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(MinimalPlugins)
        .add_systems(
            Startup,
            (
                |world: &mut World| {
                    let child = |i: usize| {
                        world.spawn(Name::new(i.to_string())).id()
                    };
                    let children = (0..3).map(child).collect::<Vec<_>>();
                    let mut parent = world.spawn_empty();
                    parent.insert_children(0, &children);
                    // for child in children {
                    //     parent.add_child(child);
                    // }
                    let children = parent.get::<Children>().unwrap().into_iter().copied().collect::<Vec<_>>();
                    for child in children {
                        println!("{:?}", world.entity(child).get::<Name>().unwrap());
                    }
                },
            ),
        )
        .run();
}

outputs

"1"
"2"
"0"

but should output this

"0"
"1"
"2"

which it does when using the commented out .add_child for loop

works fine in 0.15

[SkiFire13]

This line seems to be the issue:

bevy/crates/bevy_ecs/src/relationship/relationship_source_collection.rs

Line 223 in 12aba64

let index = index.min(self.len().saturating_sub(1));

It caps the index to insert on at len - 1, but inserting at len is perfectly valid (it will insert after the last element!).

Note that the same issue is present in the place function right below.

I'm currently not at a PC, so this is up for grab for anyone who wants an easy PR :)