One of the key requirements is that Buck2 had to be an (almost) drop-in replacement for Buck1 since there's no way we could reasonably rewrite all the millions of existing build rules to accommodate anything else.

Also Buck needs to support aggressive caching, and doing that reliably puts lots of other constraints on the build system (eg deterministic build actions via strong hermeticity) which lots of build systems don't really support. It's not clear to me whether waf does, for example (though if you squint it does look a bit like Buck's rule definitions in Starlark).

C is not a low level language - https://queue.acm.org/detail.cfm?id=3212479 - argues that this hasn't been true in a long time, since C was ported from the PDP-11.

Attempts to model C as being "close to the hardware" are at best misguided and usually hopelessly wrong. It's only possible by delving into the "implementation defined" parts of the spec but even then implementation s are often very bad at precisely defining what their semantics actually are.

Mono-tone -> The first cryptographic Merkle-tree distributed source control system, which was a large influence on both Git and Mercurial.

Monotone was written by Graydon Hoare, who later went on to design Rust (I think he was actually designing proto-Rust at the time), so the reference to Monotone is to both Mononoke's function and implementation.

The experience of using Mercurial within FB is quite different from the stock open source one. We have many local extensions, as well as integrations with the rest of our developer infrastructure. We're definitely optimizing for the linear-history monorepo model.

We don't see things as a competition between hg and git, but between good developer experiences and bad. Thus far, we've found it easier to improve the developer experience with hg than with git, so that have been our main focus.

But we feel we're reaching the limits of what we can do with hg, which is why we're investing in Mononoke. We feel that version control in general has been pretty stagnant, and we're hoping to do some neat stuff over the next few years (based on Mononoke and other projects).