Enum Attachment 

pub enum Attachment {
    Body(usize),
    World,
}

What one side of a pairwise quantity is attached to: a body in the model, or the static world.

Why this type exists

Contacts, constraints and relative Jacobians all need to express “body i against body j, where j may be the ground”. The engine long spelled that as a plain usize with usize::MAX meaning the world — an in-band sentinel, and a hazardous one: usize::MAX is a perfectly valid-looking index, so a stale or miscomputed index that happens to land on it is silently reinterpreted as “the world” instead of panicking. The failure is not a crash but a contact that quietly stops pushing back on one side.

Attachment makes the two cases distinct at the type level, so the compiler forces every consumer to say which it means. It carries no sentinel and cannot be constructed from a bare index by accident — there is deliberately no From<usize> impl, because that would reintroduce exactly the ambiguity this type removes.

Variants

Body(usize)

A body in the model, by index.

World

The static world: infinitely massive, never moves, and contributes no columns to a Jacobian.

Implementations

impl Attachment

pub fn body(self) -> Option<usize>

The body index, or None for Attachment::World.

pub fn is_world(self) -> bool

Whether this is the static world.

Trait Implementations

impl Clone for Attachment

fn clone(&self) -> Attachment

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for Attachment

impl Debug for Attachment

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for Attachment

impl Hash for Attachment

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for Attachment

fn eq(&self, other: &Attachment) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for Attachment