Suggest `.cast()` instead of `as` for pointer casts

What it does

Suggests using the more-scoped cast methods
https://doc.rust-lang.org/std/primitive.pointer.html#method.cast
https://doc.rust-lang.org/std/primitive.pointer.html#method.cast-1
for pointer casting instead of the more-general as.

Categories (optional)

• Kind: style

(One could argue that this is more pedantic, as that's what https://rust-lang.github.io/rust-clippy/master/index.html#cast_lossless is. But I think that pointer casts are generally around unsafe code, so the pedantry is more generally appropriate.)

This helps avoid mistakes where the as cast is accidentally converting between *const _ and *mut _, for example. For pointers it's often tempting to slap on as _ to make something work, but that's a very broad conversion.

(This is not applicable for *const T <-> *mut T casts, which this lint should not warn about.)

Drawbacks

None.

Example

From https://users.rust-lang.org/t/converting-between-unsized-transparent-types-safely-in-place/67680/19?u=scottmcm

impl<T> From<OrderedVecSet<T>> for Rc<OrderedVecSetUnsized<T>>
where
    T: Ord,
{
    fn from(set: OrderedVecSet<T>) -> Self {
        let rc: Rc<[T]> = set.0.into();
        unsafe { Rc::from_raw(Rc::into_raw(rc) as _) }
    }
}

Could be written as:

impl<T> From<OrderedVecSet<T>> for Rc<OrderedVecSetUnsized<T>>
where
    T: Ord,
{
    fn from(set: OrderedVecSet<T>) -> Self {
        let rc: Rc<[T]> = set.0.into();
        unsafe { Rc::from_raw(Rc::into_raw(rc).cast()) }
    }
}

Start by reading the Rust pointer::cast documentation linked in the issue and the example conversions. Identify the Clippy lint entry point and its tests for style lints, then verify the warning applies to pointer casts using as while excluding *const T to *mut T and the reverse. Done means the lint suggests .cast() with coverage for the shown cases.