Suggest ptr::null, ptr::null_mut over casting.

[Stebalien]

Add a lint to suggest converting all instances of:

0 as *const T;
0 as *mut T;

To:

std::ptr::null();
std::ptr::null_mut();

See rust-lang/rust#34456

[oli-obk]

These are also in core. We should probably figure out whether no_std is set and suggest core::ptr::* in that case.

[mcarton]

We should probably figure out whether no_std is set and suggest core::ptr::* in that case.

This could be done for a lot a paths in suggestions.

[llogiq]

Sounds like we could use a utility function that walks the parent chain to look for #[no_std].

[mcarton]

You don't even need to walk the parent chain, the current crate is available as LateContext::krate which has attributes.

[llogiq]

True. I had forgotten that this attribute is crate-level only.

[alilleybrinker]

I'm happy to give this a shot.

[mcarton]

Great! Feel free to ask if you have any question.