Auto merge of #61864 - lzutao:ptr-null, r=sfackler

Make use of `ptr::null(_mut)` instead of casting zero

There are few places that I don't replace the zero casting pointer with `ptr::null`
or `ptr::null_mut`:
```bash
% git grep -E '[ ([{]0 as \*'
src/libcore/ptr/mod.rs:216:pub const fn null<T>() -> *const T { 0 as *const T }
src/libcore/ptr/mod.rs:231:pub const fn null_mut<T>() -> *mut T { 0 as *mut T }
src/test/run-pass/consts/const-cast-ptr-int.rs:12:static a: TestStruct = TestStruct{x: 0 as *const u8};
src/test/ui/issues/issue-45730.rs:5:    let x: *const _ = 0 as *const _; //~ ERROR cannot cast
src/test/ui/issues/issue-45730.rs:8:    let x = 0 as *const i32 as *const _ as *mut _; //~ ERROR cannot cast
src/test/ui/issues/issue-45730.stderr:14:LL |     let x: *const _ = 0 as *const _;
src/test/ui/issues/issue-45730.stderr:24:LL |     let x = 0 as *const i32 as *const _ as *mut _;
src/test/ui/lint/lint-forbid-internal-unsafe.rs:15:    println!("{}", evil!(*(0 as *const u8)));
src/test/ui/order-dependent-cast-inference.rs:5:    let mut y = 0 as *const _;
src/test/ui/order-dependent-cast-inference.stderr:4:LL |     let mut y = 0 as *const _;
```

r? @sfackler

0 files changed, 0 insertions, 0 deletions