make rust warnings break the build

[dmose]

In Slack, I proposed that we make warnings break the CI builds so that they don't start creeping into our code and training us to ignore them. @rfk said :

+1 to warnings as errors; I think we have this in some places already in appservices, e.g. clippy lints as errors
it's occasionally a PITA, such as when a new version of rust gets released with new warnings that break CI, but it helps us keep up to speed with best practices

What are other folks thoughts?

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[dmose]

So, amusingly, the reason this took a bit of debugging is that RUSTFLAGS doesn't matter when the warning is coming from cargo, like the license warning was. As per rust-lang/cargo#8424, cargo doesn't yet have equivalent functionality.

That said, I just tested this on Rust code by adding a dead function to a file, and sure enough, it does turn circleci red for rustc warnings, which still seems worth doing.