Disallow "partial" borrows of `@mut`

[nikomatsakis]

Right now if you have an @mut data structure and you borrow a sub-field, you wind up freezing the whole box and its confusing. We should only permit you to borrow the whole @mut, which makes this particular error a bit harder to do.

[nikomatsakis]

Example is something like the following:

fn handleCollision(game:@mut Game) {
    unsafe {
        for game.paddles.each |paddle| {
            let diff = (game.puck.position - paddle.position);
            if (diff.length() < game.puck.radius+paddle.radius) {
                let normal = diff.normalizeOrZero();
                let impact = normal * normal.dot(game.puck.velocity - paddle.velocity);
                game.puck.velocity += impact * 2.;
            }
        };
    }
}

[pcwalton]

+1

[graydon]

assigning bug; change assignment if you disagree

[nikomatsakis]

We are not doing this anymore.