No I think two sequences is exactly what's asked for. What I'm wondering is whether this is a warning when cargo fails to restore the crates and must use incompatible versions. I'd only be surprised if the behavior was a clean compile + a runtime crash. Because that's usually not the design chosen in Rust.

> If the frog numbering code is using either counter, it gets unique IDs

I guess this is the issue: what I'm imagining is that the counter crate has a static mutable state. That's not a problem with atomics, but it's an antipattern (And especially so in Rust). The solution of course is to NOT use static mutable state at all. Whoever wants to create either a Frog or a Wizard must pass in a universe, from which it can grab the next ID, so it's a counter instance rather than get_next_static_id().

My question isn't "do we really get two memory locations" (of course we do) my question is: how afraid should one be about this, i.e. what is the behavior of the compiler/cargo or other linters when it comes to warnings etc? Are there any non-contrived scenarios where bumping a version of a dependency causes a problem at runtime (only)?