Update the Rust Toolchain less often

[pyfisch]

As a casual contributor I find it annoying that I often need to recompile all of servo before I am able to work on anything. On my machine this takes around 45 min for a full debug build. This year the version of the Rust toolchain was changed 11 times already or every three weeks on average. Just now the toolchain was updated to 2018-08-28 from 2018-08-26. I assume this is because of some problem with the previous version but nothing is mentioned in the PRs.

I'd prefer if the toolchain was only updated rarely or if pre-built dependencies could be downloaded. I realize that this is probably not as much of an issue if you work daily on servo as the ratio between full recompiles and work on servo is much better.

Thanks for your consideration and feel free to close this if you think it is not an issue.

[tigercosmos]

I think pre-built dependencies is a good idea. Just like Firefox does.

[jdm]

We tend to update Servo when there is a breaking change caused by the most recent nightly. We have a nightly build that takes the latest Servo revision and the latest rustc nightly and checks if it builds; if we don't address breakage, we lose insight into any subsequent breakage that is introduced. I don't see a good way to avoid regular rustc updates as long as we remain on nightly rustc.

[SimonSapin]

We do try to limit the usage of unstable Rust language or standard library features, which are the ones that tend to cause breakage with new Nightlies: #5286.

These days most toolchain updates are due to changes to rustc internal APIs that are used in the script_plugins crate to implement a custom lint for GC rooting safety.

[pyfisch]

Thanks for the explanation.