add lint deref_nullptr detecting when a null ptr is dereferenced
fixes#83856
changelog: add lint that detect code like
```rust
unsafe {
&*core::ptr::null::<i32>()
};
unsafe {
addr_of!(std::ptr::null::<i32>())
};
let x: i32 = unsafe {*core::ptr::null()};
let x: i32 = unsafe {*core::ptr::null_mut()};
unsafe {*(0 as *const i32)};
unsafe {*(core::ptr::null() as *const i32)};
```
```
warning: Dereferencing a null pointer causes undefined behavior
--> src\main.rs:5:26
|
5 | let x: i32 = unsafe {*core::ptr::null()};
| ^^^^^^^^^^^^^^^^^^
| |
| a null pointer is dereferenced
| this code causes undefined behavior when executed
|
= note: `#[warn(deref_nullptr)]` on by default
```
Limitation:
It does not detect code like
```rust
const ZERO: usize = 0;
unsafe {*(ZERO as *const i32)};
```
or code where `0` is not directly a literal
7537b20626