Rust binaries update

[lilianmoraru]

The Rust version in rust/repositories.bzl seems a bit old.
Would you guys please consider updating to Rust 1.20+(it seems that the archive is stored on Google's storage).
Besides the features that were released meanwhile(before the 1.20 release), 1.20 has some important things like:

• @pcwalton(Servo team) finished up the stack probes support in LLVM(for x86/x86_64) and the Rust support for them landed in 1.20 - this prevents stack overflows from going outside the application's memory.
• -Z relro-level=[full,partial,off] - again, for security-aware software.

[mfarrugi]

#56 is open to update, but it would also be good to make this pluggable. Has this already been thought about?

[katre]

These can definitely be merged to update the default, but the point of the new toolchains system in Bazel is that you can define your own toolchain, with the new Rust release, and use that instead of the ones provided in the rules by default. You just need to copy the relevant rust_toolchain and toolchain rules, and instead of calling rust_repositories() in your WORKSPACE, call register_toolchains with the toolchains you've defined.

I'll assign #56 to myself and get it merged, however.

[mfarrugi]

Just to be clear, pasting repositories.bzl and updating parameters is the best way to do this?

[katre]

Yes, that should be fine.

[katre]

PR #56 is merged, so I am also closing this issue.