Not a Rust user but this all sounds remarkably painful. Is it common? The only other compiled, type-safe language I use regularly is C# (via Unity) and I don't recall this level of upheaval.
If only three or four of my dependencies use the Foo crate, then I can just wait for all of them to upgrade to Foo 2.0, or I can do it myself and submit pull requests. In this case, I don't really care whether Foo's maintainer uses the semver trick.
However, if I'm developing a really big project like Servo or rustc, and Foo is a foundational crate that is used by dozens of my (transitive) dependencies, then waiting for all of them to upgrade (or doing the work to upgrade all of them myself) starts to become prohibitive. Now a bit of work by Foo's maintainer can save a lot of time for large downstream consumers.
An example of a “foundational” crate that has used the semver trick is `num-traits`, which has over a thousand reverse dependencies: https://crates.io/crates/num-traits/0.1.43
The 'libc' crate was a perfect storm of all three. It was still at 0.x, and the whole point of the library is to define types/bindings for others to use.
It also requires the FOSS model of independent owners, I guess, since that's why some consumers haven't upgraded. Inside a company and especially in a monorepo, someone - probably the person making the breaking change - would have just upgraded everyone at once. And a redistributor of FOSS like a Linux distro would have probably just patched everyone at once or held back upgrades until they could - in fact, this is the same shape of problem as upgrading from OpenSSL 1.0 to 1.1, or GTK 2 to 3, or libstdc++'s new ABI, or whatever.
Ideally yes, in practice not always. The easy way to break it is of course political problems, but it's perfectly possible to be stuck in a situation where unfortunate design decisions early on leave you stuck unable to make breaking changes even if everyone is on the same side (at any rate, not without taking a massive service disruption that would violate our contracts with customers).
The whole thing is clearly not great (not just in C#). In particular - many of these problems are entirely artificial. If the type system were sufficiently dynamic, or magically psychic - the problems often go away. E.g. in the rust example - void_c didn't actually change, it was just incompatible because rustc isn't psychic. Even seemingly serious problems like an interface (~ in rust a trait) gaining a new method aren't necessarily fundamentally breaking - as long as you're not calling it. Even truly breaking changes like "best-solution-finder returns a different result and has an incompatible API" might not be breaking between in the case of a diamond dependency pattern as long as the method's side effects allow running both versions - one for each transitive dependency requesting that specific version. Even cases where the break is "real" like a rename, it'd be trivial to consider a shim that allows old consumers to use the new api.
In the vast, vast majority of cases in my experience this kind of breakage is a problem due to technicalities. That doesn't mean I have a clue how to solve that, but it does beg the question: isn't it possible for a dependency resolution system to do fundamentally better, here?
Good heavens was that a mess. It had been a nice 15+ years of easy-peasy with dependency management prior to that in MS-land, though.
It seemed that a fair amount of the problems were MS learning the pitfalls of how they packaged System assemblies for NuGet. Then the compatibility shim mostly brought things back to what we had been used to in .Net (works) - even easier, in fact, now that automatic assembly binding in builds isn't a minefield.
If I’m following correctly, it would only occur if a library is using a struct defined in a dependency in a public API you are using and you also use the same dependency elsewhere in your application. Upgrading the version elsewhere in your application could break passing that struct.
Most of the time I see public APIs which either base types or struct a defined by the library itself.