But, how often is it that dynamic dispatch can be optimized into static dispatch? I imagine it probably cannot be done with most of DOM handling... Isn't Rust's static polymorphic dispatch more-or-less equivalent to what you can do in C++ with templates?
For a long time there were Servo devs pushing for Rust to feature struct inheritance in order to safely and efficiently implement the DOM. Various proposals were written and prototypes implemented, but they all received criticism from the community (often of the form "this seems like a feature that solves a problem that only Servo will ever have") and no single proposal was a clear winner over the others. Today the Servo devs hack around the issue via `unsafe` blocks and transmuting things willy-nilly, but they're still dissatisfied with it.
Right... I'm partially familiar with the development, but AFAIK, the basic goals are basically thin pointers and cheap dynamic dispatch using v-tables, not elimination of dynamic dispatch in favor of static dispatch.
My goal was to emphasize that you are correct that not all useful instances of dynamic dispatch can be reduced to static dispatch. With regard to the DOM specifically, however, there is some concern that the structure itself was designed and standardized such that it can only be implemented efficiently by assuming some sort of inheritance scheme, effectively baking the typical OO approach into the web standard. Opponents of inheritance being added to Rust have claimed that this is a unique case, and that such a feature would not pull its weight in any non-DOM scenarios (or worse, that it would be redundant with Rust's trait system and that you would fall into C++'s trap of nobody agreeing on which subsets of the language to use). Given the nebulousness of the proposals in this space, I have no dog in this fight at the moment.
My other goal was to show that Rust gives you enough tools to manually emulate an efficient inheritance-style dynamic dispatch scheme, even if it can't fully prove that your implementation is safe.
