What breaks in the language every month? That’s not the experience our users report. There are some areas where we do reserve the right to tweak things, but we take great care to ensure that these breakages are theoretical, rather than an actual pain. We have large (hundreds of thousands of LOC, and maybe even a few 1MM ones) code bases in production in the wild now, we cannot afford to break things willy-nilly.
You are right that we are far, but that’s because it’s a monumental task, and it’s fundamentally unfair to expect equivalence from a young language. It is fair to say that it is a drawback.
The language doesn't break as much today, but what constitutes "idiomatic" Rust is constantly changing. I don't use Rust but I spend a lot of time with people who do and am echoing what I've heard from them, and seen for myself as a casual user of Rust software and occasional drive-by contributor.
It doesn't have to be a monumental task. Rust is simply too big, and getting bigger.
But C never set out to be what it is today. It was a long time before anyone thought a spec was worth writing. For a language with the stated design goals of Rust, a specification and alternative implementations should be priority #1. You can't eliminate undefined behavior if you don't define any!
You are right that we are far, but that’s because it’s a monumental task, and it’s fundamentally unfair to expect equivalence from a young language. It is fair to say that it is a drawback.