As noted in the semver spec, pre-release versions do not have the same stability requirements as proper releases. Due to this, I believe that cargo should allow selecting multiple pre-release versions of a dependency without causing an error.

Not allowing this causes real problems, as can be seen with the "mime_guess" crate, which accidentally released a 1.0 version before its public dependency (mime) reached 1.0.

The only way for "mime_guess" to follow semver going forward would be to either release a new major version on every patch release of "mime", or (the solution chosen) to switch to only publishing pre-release versions (2.0-alpha1, 2.0-alpha2, etc.) with the hope that "mime" will soon have a 1.0 release and then mime_guess 2.0 proper can be published.

However, this causes headaches for the average user who depends on common crates like "hyper" and "reqwest", because these must now have dependencies on pre-release versions of "mime_guess", which universally conflict with each other.

I think this will be a problem going forward too, as crate versions are forever, so as soon as you publish a 1.0 of a crate, there's no return to allowing multiple versions to be linked.