To re-iterate what gankro said, we care a _lot_ about making sure that it's trivial to update your Rust. Your ABI compatibility comment is correct, but the rest of them aren't. We only break backwards compatibility in very few cases: if there's a soundness hole, or if we have to fix something in certain underspecified areas of the language. For those latter ones, if it's not a trivial fix, then it's a no-go, and even if it is, we do warnings until we don't see them in the ecosystem any longer.

But really, it's not about rules-lawyering over technicalities about what our policies are: the underlying attitude we share towards new Rust versions is that it should be trivial to upgrade. And we collect data to make sure that we're on target there: part of it is the train-based release strategy, part of it is stuff like testing everything on crates.io, part of it is things like the community survey, where most people said their code has never broken, and if it did, it was extremely easy to fix.