cargo: enable ‘-D warnings’ by default

[mina86]

Making ‘-D warnings’ the default may be a minor annoyence to developers
who now will need to fix warnings even while they work on the feature
(as opposed to only having to do that once they are ready to send the
code for review) but it does simplify the code a small amount.

Issue: #6226

[bowenwang1996]

Is there a reason for removing the check from CI? I suppose we should still check for warnings in CI to make sure they are fixed

[mina86]

With the flag being set in .cargo/config.toml setting RUSTFLAGS is now redundant.

[matklad]

I don't think we should do this. To me, it seems that while the annoyance can be minor, it can also be very significant.

For example, I often do quick&dirty exploratory work (building the code with two versions of some library, etc) where I have a ton of warnings.

Even if it were true that the annoyance is minor, i'd argue that fixing a minor, repeated annoyance for everyone is well worth to spend a couple of extra CI lines on.

There's also a point that we are setting RUST_BACKTRACE=1 anyway, so it's not like we make anything meaningfully simpler, we still have the repeated "set up common env" step. It seems to be that the actual problem here is that we can't set env-variable once, like we can do with github actions:

https://github.com/rust-analyzer/rust-analyzer/blob/9b1978a3ed405c2a5ec34703914ec1878b599e14/.github/workflows/ci.yaml#L10-L16

[mina86]

It seems to be that the actual problem here is that we can't set env-variable once, like we can do with github actions:

Actually it can: #6269