Avoid the need to "make clean" manually when building Rust

[catamorphism]

Sometimes (perhaps when pulling patches, perhaps not), metadata gets out of date and isn't rebuilt automatically. #2318 is just one example. If the build system works properly, ideally it would never be necessary to manually "make clean".

This is confusing for a user since the error message (for example, "nonexhaustive match failure" in #2318) says nothing about how to fix the problem.

[msullivan]

Do we have a procedure for reproducing this?

[gwillen]

Well, if you check out a version that has 0.2 in the metadata, then make, then check out a version that has 0.3, then make check, the checks fail.

Also, make clean does not actually make very clean. I'm filing another bug about that.

[bblum]

I believe this is still an issue. Often even with just daily pulls from master, I get "multiple matching crates for std" and have to make clean.

This isn't a backwards-compatibility issue. Re-nominating for production-ready.

[brson]

cc #2419

[pnkfelix]

accepted for production ready

[emberian]

I think @pnkfelix fixed this with #7637, closing