Lint for multiple versions of dependencies

[ysimonson]

Addresses #2665. Implementation notes:

• I put this in the nursery as I'm not sure if in practice this'll end up being noisy. This is now in a new lint group for cargo-related lints: clippy_cargo.
• Tests are failing for me for both this branch and master (rustc 1.27.0-nightly (bd40cbbe1 2018-04-14 / macos.)
• Manually tested against both crates that do not have this issue (e.g. clippy itself) and crates that do have this issue (e.g. https://github.com/sharkdp/fd)

[ysimonson]

@phansch thoughts on keeping this in nursery vs a new clippy_cargo group?

[phansch]

It's more of a categorization thing, because the cargo lints affect a tool and not rust code.
A clippy_cargo group would also allow non-cargo projects to enable or disable the cargo lints all at once. (Although we should probably encourage the use of cargo)

[phansch]

No need for the "Warn on" here. It should just say what's wrong directly (in all lowercase, as tools are using that text)

[ysimonson]

OK, I put it in a clippy_cargo lint group. It looks like it's disabled by default anyways.

[phansch]

lgtm!

@oli-obk Would you like to have a look as well?

[oli-obk]

We should create a new lint group cargo in the declare_clippy_lint macro and use that instead of nursery. Otherwise the lint update Python script will move the lint back to the nursery

[oli-obk]

And now we need to update the python script :/

https://github.com/rust-lang-nursery/rust-clippy/blob/a47734c41d5ebc73bf0b457cf8f9075841132c28/util/update_lints.py#L126

Could you also add a mention of the new lint group to the README?

[oli-obk]

We'll need to figure out a way to test these lints someday...

[oli-obk]

Lgtm now. Please squash the commits into one. Then it's good to go

[ysimonson]

Sorry for the dumb question, but is there a straight-forward way to collapse this to one commit, given I merged from master mid-way through to resolve conflicts? The only ways I can think of is to either squash merge (which I wouldn't be able to do) or open a new PR.

[oli-obk]

You can rebase over the master branch. That'll kill all the merge commits. If you use git rebase -i origin/master you can squash commits at the same time

[oli-obk]

Ok, now let's advertise this awesome lint. iirc there were a lot of ppl asking for it in different places, like cargo issues and irc.