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soft_body_attachments

A particle can also be attached directly to a rigid-body (the SoftBodyAttachments component), which is the simplest way of combining the two kinds of bodies: a rope tied to a swinging ball, a flag attached to its pole, etc. Unlike pinning, an attachment is a two-way point-to-point constraint: the rigid-body holds the particle, and the particle pulls the rigid-body back. The anchor is the position the particle has when the attachment is created, expressed in the local frame of the rigid-body. Note that a particle attached twice keeps both of its attachments, and that an attachment is undone with its removal from the SoftBodyAttachments component (removing the component detaches every particle):

Each entry of the SoftBodyAttachments component attaches one particle to the entity of a rigid-body (or of a cluster). Each time the component changes, the attachments of the soft-body are updated to match it: the attachments that didn't change keep their anchor, and the ones targeting an entity which rigid-body isn't created yet are retried on the next frames:

// Attach the last particle of a rope to a rigid box, at the particle's position.
let weight = commands
    .spawn((
        Transform::from_xyz(12.0, 8.6, 0.0),
        RigidBody::Dynamic,
        Collider::cuboid(0.3, 0.3),
        ColliderMassProperties::Density(2.0),
    ))
    .id();
commands.spawn((
    Rope,
    Transform::from_xyz(8.0, 9.0, 0.0),
    SoftBody::rope(Vec2::ZERO, Vec2::new(4.0, 0.0), 25).map(|builder| {
        builder
            .pinned_particles([0])
            .softness(SpringCoefficients::new(40.0, 1.0))
    }),
    SoftBodyAttachments(vec![SoftBodyAttachment {
        particle: 24,
        body: weight,
    }]),
));