"cargo check" should re-use "cargo build" artifacts

[RalfJung]

Right now, even if you have all your dependencies compiled already via "cargo build", doing "cargo check" re-builds all of them. That seems somewhat redundant, and it is certainly frustrating given that the expectation for check is to be fast -- but because of this, it can actually be much slower than "build". (This is even more true if you stick to nightly, where all your dependencies are re-built daily anyway because your toolchain keeps changing.)

I asked about this on #rust, and from what I understood, fixing this requires two pieces. (But this may be entirely wrong! Please correct me.)

• Change rustc so that when it does --emit=metadata, it can handle both an rlib and an rmeta for dependencies. (Or maybe that's already the case?)
• Change cargo to tell rustc re-use an up-to-date rlib if there is no rmeta, rather than building a new rmeta.

[RalfJung]

Actually, the first part seems to already be the case. I took the command given by cargo check -v, replaced some of the rmeta by rlib (had to change the hash for some of them), and things still seemed to work.

[alexcrichton]

I believe this is a dupe of #3501 so closing in favor of that, but thanks for the report!

Unfortunately it's pretty tricky to share the implementation details here today, but it's definitely possible and I hope to implement it one day in Cargo! I believe we've got all the compiler support necessary and we just need changes on the Cargo side of things.