Lint for bool to integer casts in cast_lossless

[5225225]

The lint description says

Checks for casts between numerical types that may be replaced by safe conversion functions.

Which is strictly speaking being violated here, but it seems within the spirit of the lint. I think it is still a useful lint to have, and having a different lint for just this feels excessive. Thoughts?

Fixes #7947

changelog: Lint for bool to integer casts in [cast_lossless]

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @llogiq (or someone else) soon.

Please see the contribution instructions for more information.

[5225225]

Just realised this lint doesn't seem to check for the MSRV. Looks like all the From impls for bools were added in 1.28. (And the ones for integers are 1.5/1.6, old enough to not care about I suppose)

Worth adding a MSRV check for the bools?

[llogiq]

The linting part looks really thoughtfully done. Great job! However, I think the lint message doesn't exactly fit the cast. I doubt we ever get to the point where we get a u0 type that cannot be created from a bool.

So I think we should change the message for bool → int casts.

[5225225]

True, but then again, I can't really think of a better message. It can become silently lossy if you change the type (of the bool). But yeah, you're right, there's no possible change of the right hand side which would make it invalid.

error: casting `bool` to `i64` may become silently lossy if you later change the bool's type
error: casting `bool` to `i64` may become silently lossy if you later change the bool
error: casting `bool` to `i64` may become silently lossy if you later change the type of the bool

perhaps?

None of those seem obviously better than just leaving the message as is to me.

[llogiq]

I'm currently short on time, but I'll think of something.

[llogiq]

I don't think it's very likely that someone changes a value from bool to some integer type while forgetting some as conversions. How about

error: Casting `bool` to `u64` is more cleanly stated with `i64::from(_)`.

[5225225]

Okay, that's changed.

I used a literal _ in the message rather than actually showing the real expression since we show the real expression in the suggestion anyways.

[llogiq]

That's totally OK. No need to duplicate the suggestion. Thank you! @bors r+

[bors]

📌 Commit d4c8cb6 has been approved by llogiq

[bors]

⌛ Testing commit d4c8cb6 with merge 4f82dd8...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing 4f82dd8 to master...