Oh hey I recently saw a comment which discussed this exact issue: https://news.ycombinator.com/item?id=47752915

99% of the time this situation is okay, because Cargo allows you to have both 0.1 and 0.2 in the same project as dependencies. It's just packages that call out to external dependencies, like libc, where it enforces the single version rule.

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mort96 2 days ago | parent [-]

You can have both 0.1 and 0.2 in the same project, but you really don't want to.

	▲	steveklabnik 2 days ago \| parent [-]
		Most of the time, it works so well people don't even notice. The only time you run into a problem is if you try and use values with a type from 0.1 with a function that takes a 0.2 as an argument, or whatever. Then you get a type error.