The flow tree (render object tree) in Servo (or any other browser engine) must use inheritance: we have a heterogeneous tree of objects that all share a common set of fields (position, intrinsic widths, collapsible margins, some various bits that store state during reflow), but they all use virtual methods because they must lay out their contents differently.
We can't use composition because we wouldn't get virtual methods. We can't use an interface because then we would be forced into virtual dispatch for all of those fields that are shared between flows.
Rust doesn't have OO yet either, so we're forced to hack around it in weird ways (usually via a small amount of unsafe code to simulate inheritance).
> I have yet to be presented with one of these "killer" is-a relationships that must exist. If the sole excuse is "performance", then sure I'll conceded. I guess I don't work in environments where member access is the performance bottleneck so I guess I can't relate.
A browser engine is exactly that sort of environment. Forcing all member access to go through virtual dispatch would murder the performance of any browser.
Note that this was exactly the sort of thing that OO was designed for in Simula: heterogeneous trees of objects that all share some common fields but have different virtual methods. This generalizes to GUI libraries, game worlds etc—in short, simulations :)
