Warn on transmutes that could use pre-defined methods instead

[clarfonthey]

I've been rather exhaustively adding methods to the standard library that allow "safe" transmutes that we know work, like between [u8] and str (see: rust-lang/rust#41119 and rust-lang/rust#40380). It'd be nice if we had a framework to detect transmutes specifically between these types and suggest other methods instead.

For example, &str -> &u8 is str::as_bytes, and the reverse is str::from_utf8_unchecked. If someone does a raw transmute here, they should be redirected to the standard library functions that do these conversions.

I've been slowly adding methods to do less common conversions, like &mut [u8] -> &mut str. In general it makes sense to have some sort of framework to recognise transmutes and suggest to replace them with more explicit functions, especially as these other methods get stabilised.

If we want to go all-out, we could detect functions that do these conversions and suggest them instead of transmutes.

Current list:

• transmute::<&str, &[u8]> => str::as_bytes
• transmute::<&mut str, &mut [u8]> => str::as_mut_bytes
• transmute::<&[u8], &str> => std::str::from_utf8_unchecked
• transmute::<&mut [u8], &mut str> => std::str::from_utf8_unchecked_mut
• transmute::<&CStr, &[u8]> => CStr::to_bytes_with_nul
• transmute::<&[u8], &CStr> => CStr::from_bytes_with_nul_unchecked
• transmute::<f32, u32> => f32::to_bits
• transmute::<f64, u64> => f64::to_bits
• transmute::<u32, f32> => f32::from_bits
• transmute::<u64, f64> => f64::from_bits

[clarfonthey]

This should also include pointer conversions, such as *(x as *const [u8] as *const str) for example.

[oli-obk]

@clarcharr since you are the most qualified to talk about these, can you create a list of transformations that you'd like to see?

[clarfonthey]

Wrote a list up for anyone who's interested in implementing this. Added float versions too after noticing rust-lang/rust#50464.

[alex]

An extension of this with the recent Rust release is using transmute in place of to_le_bytes() and from_le_bytes(). Like the others, clippy warning about these would be good since it'd reduce unsafe usage in the ecosystem.