Warn against passing a pointer derived from a 0 or 1 sized array to from_raw_parts

[retep998]

If there is a fixed size array with a length of 0 or 1, and as_ptr() or as_ptr_mut() is called on that array and the result passed to slice::from_raw_parts or slice::from_raw_parts_mut, then emit a lint.

This is almost definitely a sign of UB: rust-lang/unsafe-code-guidelines#134 (comment)

[Shnatsel]

This is a somewhat common pattern, as it turns out. Some examples of its use on crates.io: rust-lang/unsafe-code-guidelines#134 (comment)

[RalfJung]

If there is a fixed size array with a length of 0 or 1, and as_ptr() or as_ptr_mut() is called on that array and the result passed to slice::from_raw_parts or slice::from_raw_parts_mut, then emit a lint.

As a slight refinement, the lint could also check that the length passed to from_raw_parts is not a constant 0 or 1. (But that seems unlikely.)