ci: Deny warnings

[mkroening]

build.warnings is stable since Rust 1.97.0 and allows controlling rustc and clippy warnings (previously RUSTFLAGS=-Dwarnings) as well as rustdoc warnings (previously RUSTDOCFLAGS=-Dwarnings) and the new Cargo warnings.

Apparently CI has only failed on errors before, not on warnings.

[martin-hughes]

One for @IsaacWoods - I'm happy to deny all warnings, but ought to check with the boss...

[mkroening]

It seems like all warnings are gone for now.

Note that this will make CI fail whenever new lints are added to Rust or Clippy. I can completely understand if you prefer not dealing with that regularly. In that case, feel free to close this. :)

[IsaacWoods]

Thanks for the PR @mkroening!

Note that this will make CI fail whenever new lints are added to Rust or Clippy.

I'm in two minds on this - I do think there's value in us trying to keep the crate warning free, including ensuring new PRs are warning-free before merging, now that we're in this position (with thanks to you!). However, I think failing CI on warnings (given the above) is likely to just have us sit with broken main and no artifacts for potentially prolonged periods.

If warnings creep back in without this, we could have another think, but for now I'll close this. Thanks :)