[lint request] unnecessary_slice_transmute

Some people transmute a (T* [const, mut], usize) into a &[T]/&mut [T]/* const[T]/*mut [T] unnecessarily and vice-versa, instead of using the from_raw_parts methods and or (slice.as_ptr(), slice.len()) (yeah, really).

We should probably cover also the case where the pointer type and the element type of the slice differ. A pointer is a pointer, so the transmute won't care, but they should just be casting the stuff around and using the proper methods instead of relying on the unspecified slice layout.

We should detect this and tell people to use the proper methods instead. We probably want to detect the incorrect (usize, T* [const, mut]) cases as well, and tell people to also use the correct methods instead.

If people are doing this inside a macro... we should warn anyways.

No source files, tests, or entry points are named in the issue. Start by reading Clippy's existing transmute lint implementations and the Rust APIs for slice construction and pointer/length conversion; done means the requested slice transmute patterns, including reversed tuple order, differing element types, and macro usage, are detected with appropriate guidance.