Re-enable cast_lossless lint by default

[Shnatsel]

The cast_lossless lint has been made pedantic following #4528, with the reasoning being "It will only become a problem if the code changes in the future".

I am a happy user of this lint, and I find that converting always-lossless casts to from() calls as the lint suggests clarifies the code a great deal: it makes it explicit where truncation may happen, which makes the code much easier to understand.

As such, I believe this lint is a good candidate for clippy::style category. Perhaps its description could be revised to make the readability aspect more explicit.

[Shnatsel]

Example from real code to back this up:
Before: https://github.com/fschutt/fastblur/blob/c80b518e9633605546352895fb963dde39ce40f6/src/blur.rs#L203-L213
After: https://github.com/fschutt/fastblur/blob/df2032bf50a30186aa569959e960292b3215fa8d/src/blur.rs#L203-L213

[jhpratt]

it makes it explicit where truncation may happen

That's what the "lossless" part is. Truncation can't happen with the current types. It would lint if the types changes in the future such that truncation is possible. As such, I am personally opposed to this.

[llogiq]

I don't think that enabling this lint will be very popular. At best, it improves style (but not much, and it makes the code longer, too), and at worst it is seen as a false positive.

[flip1995]

I agree with @llogiq. IMO pedantic is the most fitting group for this lint.