> How easy is it to add Rust to a C++ code base? If the answer is 'not at all',

Its not clear that you even read the linked post on GitHub before commenting.

The first post in that GitHub thread says: "This should be thought of as a "port" instead of a "rewrite" because we would not start from scratch; instead we would translate C++ to Rust, incrementally, module by module"

> then the transition is unlikely to happen anytime soon (it hasn't happened in the decade that Rust has been around)

As of a year ago, 10% of FireFox had been ported over (https://news.ycombinator.com/item?id=30744057). This is the overall effort started in 2014 https://wiki.mozilla.org/Oxidation