Sure, but according to semver it's also totally fine to change a function that returns a Result to start returning Err in cases that used to be Ok. Semver might be ae to project from your Rust code not compiling after you update, but it doesn't guarantee it will do the same thing the next time you run it. While changes like that could still happen in a patch release, I'd argue that you're losing nothing by forgoing new API features if all you're doing is recompiling the existing code you have without making any changes, so only getting patches and manually updating for anything else is a better default. (That said, one of the sibling comments pointed out I was actually wrong about the implicit behavior of Cargo dependencies, so what I recommended doesn't protect from anything, but not for the reasons it sounds like you were thinking).

Some people might argue that changing a function to return an error where it didn't previously would be a breaking change; I'd argue that those people are wrong about what semver means. From what I can tell, people having their own mental model of semver that conflicts with the actual specification is pretty common. Most of the time when I've had coworkers claim that semver says something that actively conflicts with what it says, after I point out the part of the spec that says something else, they end up still advocating for what they originally had said. This is fine, because there's nothing inherently wrong with a version schema other than semver, but I try to push back when the term itself gets used incorrectly because it makes discussions much more difficult than they need to be.