Auto merge of #3154 - steveklabnik:gh3124, r=alexcrichton

Auto merge of #3154 - steveklabnik:gh3124, r=alexcrichton

upgrade semver

This is a spike at fixing #3124.

@alexcrichton I'm not sure how to actually emit the error message here. In the TOML example you linked me: https://github.com/rust-lang/cargo/blob/5593045ddef2744c1042dee0c0037c2ebcc1834e/src/cargo/util/toml.rs#L166

That takes a Config as an argument. This function does not. What's the right approach here?

Also, this code is messy. I am 99% sure I can write it nicer with combinators. This is just to get the discussion going.