rust: add basic Rust platform

[scorphus]

Initial support to Rust. @jonnyguio, here we go!

Refs #20

[scorphus]

On Dec 3, 2018, 5:40 PM GMT+1, @jonnyguio wrote in the referenced issue:

• [...] How to offer the Rust Platform. By choosing the toolchain? Choosing the version (Rust 2015, Rust 2018)?

I think we should stick to current stable/beta/nightly toolchain specification.

• Which components of Rust it should be installed?

Good question. What you suggest?

• How the Rust code should be compiled and installed in each deploy? [...] [should it] be consistent with the other languages? [Should] the executable be in /home/application/current? [...]

Rust is pretty picky here. This sounds like too much work just to comply and be consistent.

Regarding rust-lang/rustup#466, looks like it's fixed, we need to confirm.

[jonnyguio]

I think we should stick to current stable/beta/nightly toolchain specification.

Sounds good. If we use the rust-toolchain file, we should be able to offer any toolchain chosen by the user. But, the toolchain have to be downloaded and installed, and that would increase the deploy time. I think it's a good way to go, we only need to be clear about it on the documentation.

Good question. What you suggest?

I'd suggest only rustc, cargo, and rust-std components. There's a recent effort to allow different installation profiles in rustup, but it's not ready. Also, to enforce removing the docs, we would need to manually delete the folder, since it's not possible to remove the component after installing it. For starters, that shouldn't be a problem. We should just be aware that we must update it in the future as soon as the profile feature (or anything that suits our need) is available.

About the previous topic too, maybe the platform should already have installed the stable, beta and nightly toolchains, but

Rust is pretty picky here. This sounds like too much work just to comply and be consistent.

Not knowing where your application data is located inside the unit can be a big hassle. We could easily set where to install using the --root flag in the cargo install command.