Cargo support

[nrc]

Need --save-analysis for Cargo. Then support for using cargo instead of rustc in DXR.

[nrc]

So, you can easily do this it turns out: cargo rustc -Zsave-analysis. I think you could the just set RUSTC = cargo rustc, and everything should work.

Need to test and document this, if it works.

[snf]

The problem is cargo rustc didn't make it to 1.0 (afaik) so we must have to wait until 1.1 or use nightly or do something like servo:

run cargo build --verbose and then add the additional flags to the line before rerunning the command.

[nrc]

For the near future, DXR will require nightly Rust in any case - we have to frequently fix bugs in the compiler's analysis in order to be able to index large programs. Hopefully once DXR gets more use, save-analysis will be more stable and the release branch will be more usable.