Use clippy to avoid unchecked casts and arithmetic

[RalfJung]

@oli-obk writes

There's a clippy lint for this if we want to make sure we don't add more of these in the future: https://rust-lang.github.io/rust-clippy/master/index.html#integer_arithmetic

AFAIK there is also one for casts, right?

Is there a good way to also subject the miri engine in librustc_mir/interpret to these checks?

[oli-obk]

There's a whole group of cast lints.

Unfortunately we aren't quite at the stage where we can run clippy lints on rustc. Though @matthiaskrgr has been doing this, and it may actually work well nowadays.

[matthiaskrgr]

Yes, one can get it to work but it is very fragile and I am currently waiting on clippy being updated :)
https://github.com/matthiaskrgr/rust/commits/fix_xpy_clippy

edit: the branch is probably somewhat I'll see if I can get it to work if clippy update lands tonight.

[oli-obk]

The minimal bar would be to add it to our CI like with rustfmt. #2091 updates miri to be clean with clippy

[RalfJung]

We now have clippy on CI. So we can resolve this issue by adding

#![warn(
    clippy::cast_sign_loss,
    clippy::cast_possible_truncation,
    clippy::cast_possible_wrap,
    clippy::cast_lossless
)]

and fixing the warnings.

This doesn't help for the code in the rustc repo though.