Disallow build warnings for Rust

[dskloetd]

Motivation

Build warnings are distracting and confusing.

There is one type of build warning that can start appearing without any changes in the code: future incompatibility.
When there is a breaking change in a future version of a dependency, this will start causing build warnings without changes in the repository. We need to disable these warning in order to safely disallow any build warnings in Rust and prevent the introduction of new warnings.

Changes

1. Add RUSTFLAGS="-D warnings" in front of cargo test in CI to disallow build warning.
2. Disable the "future incompatibility" warning by adding a config.toml file.
3. Fix a warning by removing unused use candid::types::number; from rs/backend/src/accounts_store/schema/proxy/tests.rs.
4. Fix a warning by removing unused schema_label from rs/backend/src/state/partitions.rs

Tests

CI passes

Todos

• Add entry to changelog (if necessary).

[bitdivine]

Note that rust warns if a crate has breaking changes in a future version of Rust, even if that future version is not being used. That means that code that compiles with no warnings one day can issue warnings the next.

It is nice to get rid of warnings and the cleanup is legitimate and appreciated, but be aware that with this flag set you may get CI breaking suddenly without warning if you do this.... which is why it is not done yet. It's up to you.

Yes, I have had sealed docker builds, meant to produce the same artefacts for the next 20 years, break suddenly because of this :-D One needs to pull out the network cable, so to speak.

[dskloetd]

Thanks for the heads up! I'll have to think about this. Is it not possible to disable that behavior?

[dskloetd]

Looks like there is a way to disable it:
https://doc.rust-lang.org/cargo/reference/future-incompat-report.html#configuration

So the decision is whether we value not having warnings or knowing about future breakages.

[bitdivine]

Yes, as linked, and as you found. :-) IMO it would have been better if they had made it possible to make that an info rather than a warning. First world problems...

[mstrasinskis]

Rubber stamp, thanks!