Add warning when ptr-to-int cast would truncate

[jethrogb]

There should be a lint that warns if you cast a pointer to any integer smaller than usize. For example to prevent cases like this:

println!("{}",std::mem::size_of::<std::fs::File> as u32);

[Amanieu]

In this example, a warning should only be issued when compiling to a 64-bit target, since casting a pointer to u32 is perfectly valid on 32-bit targets. Maybe something like "Warning: cast to u32 may truncate pointer value". If truncation is really intended then the warning can be avoided by adding an intermediate usize cast.

[jethrogb]

There could also be a default-allow lint that complains about any pointer cast not to usize.

[nagisa]

Should be reported to clippy instead, I think. Cast possible truncation lint may be what you’re looking for.

[steveklabnik]

Since new lints have a big impact on users of rustc, the policy is that they should go through the RFC process like other user-facing changes. As such, I'm going to give this one a close, but if anyone comes across this ticket and wants this lint, consider adding it to clippy and/or writing up an RFC. Thanks!