The title of this post is not (entirely) rhetorical. I'm puzzled by Mozilla's decision to burn several of its full-time engineers' cycles on the Rust programming language. Maybe someone can enlighten me. I offer the following observations with all due humility.
As a problem of engineering management, programming language development can be analyzed in terms of cost, risk, and return. I do not see how Rust makes sense for Mozilla along any of these dimensions.
Cost: Getting a new programming language off the ground only requires a handful of people, if you have the right people. For giant technology corporations, which have thousands of engineers on staff, that's a tiny relative overhead, well within the margin of a sensible research budget. By contrast, for Mozilla, a lean, low-headcount organization, it's a major opportunity cost. A handful of Mozilla's top full-time developers represents a big bite out of Mozilla's engineering throughput.
Return: The returns to adopting a better language are roughly proportional to the developer-hours spent writing code with it. As a organization with both low headcount and few projects, Mozilla will have difficulty recouping its investment:
- Consider Google and Go. Suppose 5 core Go developers work on the language for 2 years; this costs about 5 * 2000 * 2 = 20k developer-hours. This investment can be recouped by saving 1000 Google engineers an average of 2 hours per week for just 10 weeks, or equivalently by providing a quality increase equivalent to 2 more hours per week of design, testing, etc. Viewed this way, Go has a plausibly huge upside for Google. But Mozilla will never have 1000 full-time engineers using Rust; it has a few hundred employees total, not all of whom are engineers, and few of whom might be writing Rust full-time in the foreseeable future. Perform a similar calculation using any remotely plausible numbers, and the case for the Rust's return to Mozilla comes out pretty weak. Either Rust will take far too long to pay off, or it must be orders of magnitude better than the next-best alternative (and the latter seems unlikely).
- It is easier to realize the benefits of a better language when starting a new, greenfield project than when rewriting legacy code. In fact, when rewriting legacy code, developers incur more costs than benefits until a tipping point is reached. Google, Microsoft, or Amazon, to take a few examples, each start dozens of substantial projects and hundreds of little projects per year. At such companies, it is conceivable that, by winning over new projects at their inception, a new language could achieve an annual adoption rate of hundreds of developers per year. By contrast, Mozilla can't start more than few projects per year without losing focus. So the rate at which Mozilla can realize the benefit of a new language is constrained even further than their headcount would suggest. And indeed the core developers of Firefox, Mozilla's biggest project by far, will not benefit from Rust anytime soon.
Now, in spite of all the above, one might disregard risk, cost, and return if one had a truly unique opportunity. If you're the only organization in the world that might plausibly deliver on some project of unique importance, then it might be worth rolling the dice: "We have to save the world from X; nobody else will." Perhaps reasonable people can disagree, but I can't see how Rust qualifies. As a "fast, safe, concurrent, static, systems language", Rust is quite comparable to Go or D; even if one grants, for argument's sake, that it's better than those languages in some ways, it occupies a similar space. So Mozilla's opportunity is simply that it has the money and freedom to pay a few software engineers to work on Rust full-time. The "unique opportunity" rationale for overriding cost/risk/reward does not seem to apply.
Alternatively, one might rebut all of the above calculations by saying that Rust is a research project, not strictly an engineering project. But it is a curious research project. Research justifies its existence by hoping to advance computer science in a fundamental way. Rust conversely aims to minimize novelty, as its FAQ states: "Old, established techniques are better [than novelty]". I cannot think of a fundamental computer science question that is investigated by Rust's development. It is research only in the sense that it is a speculative engineering project without an obvious short-term payoff.
Furthermore, all of the above must be considered in light of Mozilla's current position. Mozilla is fighting to maintain relevance in a world where all the major desktop and mobile operating system vendors are shipping high-quality web browsers. And Mozilla is fighting with far less resources than its competitors. Mozilla punches far above its weight, and for that it should be proud, but I doubt that it has the luxury of diffusing its focus at this time. Mozilla is not like the IBM or Microsoft of old; it cannot plow huge surplus profit margins into basic research without subtracting needed resources from product engineering.
Therefore, in my opinion, Mozilla should spin out the Rust project to community maintenance, and try hard to convince its paid engineers to work on something with a cost/risk/return/opportunity profile better suited to Mozilla's place in the world. Rust developers are experts in programming language implementation, so ES6, asmjs, or Firefox and Firefox OS developer tools all seem like promising alternatives.
Or, if Mozilla really believes that reducing dependence on C++ is a major strategic priority, these developers might investigate rewriting Firefox components in another language — one for which they won't have to do most of the heavy lifting in toolchain and library development. Delegating language development to someone else makes far more sense than doing it in-house. By way of analogy, Mozilla does not try to innovate in datacenter design. To do so would be ludicrous, as Google, Amazon, Facebook, etc. are all far better-positioned to do so. Instead, Mozilla is happy to reap the fruits of robust multi-party competition in datacenters. They should take a similar approach to programming language design and implementation (outside of the web client platform, where they are unavoidably a key contributor).
A third alternative might be to find a deep-pocketed external sponsor to take on the heavy lifting for Rust. Samsung made some noise a while ago about sponsoring Rust; but how many active Rust contributors come out of Samsung's headcount rather than Mozilla's? I believe that the key cost of Rust development for Mozilla is the opportunity cost of expert Mozilla developers' labor. Until Samsung or some comparable organization credibly commits not only financial sponsorship but long-term ownership and headcount to Rust development, its costs will not be truly offloaded from Mozilla.
p.s. I say all of the above as someone who likes what I've seen of Rust, purely as a programming language. I would be happy if some organization took up the Rust banner and carried it forward. But I do not think Mozilla is a logical primary sponsor for it.