Ever wondered why Cargo selects the *newest* versions of crates? Or what that might have to do with putting compiler versions in Cargo.toml? Well I have just the blog post for you: 
http://aturon.github.io/2018/07/25/cargo-version-selection/ …
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I've been generally worried in the past that we put too much faith in a green crater run, treating it as a tool that says "yes this definitely does not break things" when all it can do is tell you "yes this definitely *does* break things", but not the opposite.
In my personal opinion, wording like that strengthens that false trust. Note: I am not trying to diminish crater's usefulness as a tool. It's incredibly powerful and I'm glad that we have it and use it as frequently as we do. Made some similar points in https://github.com/rust-lang-nursery/crater/pull/276 …