> Changing the size of int shouldn't be a breaking change in Go since pointer arithmetic isn't allowed and everyone should use fixed-size variables when you're relying on this behavior.

All of the "breaking changes" in Rust have been of this form.

We're very up front about any possible breakage. When we talk about "breaking changes", we mean "a change that shouldn't be breaking, but could be breaking if code was relying on this bug/implementation-specific behavior". Those types of changes—changes that might cause breakage in practice but do not change the language definition—are often not considered "breaking changes" in Go. But in Rust we often call them "breaking changes", because we are concerned first and foremost about the practical considerations of changes we make, not just whether we are technically allowed to make them per the letter of the law.