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

2 files changed, 2 insertions, 2 deletions

diff --git a/src/libstd/sys/wasi/args.rs b/src/libstd/sys/wasi/args.rs
index 9c8e59e4fb5..8b4b354d9fc 100644
--- a/src/libstd/sys/wasi/args.rs
+++ b/src/libstd/sys/wasi/args.rs
@@ -32,7 +32,7 @@ fn maybe_args() -> io::Result<Args> {
         let (mut argc, mut argv_buf_size) = (0, 0);
         cvt_wasi(libc::__wasi_args_sizes_get(&mut argc, &mut argv_buf_size))?;
 
-        let mut argc = vec![0 as *mut libc::c_char; argc];
+        let mut argc = vec![core::ptr::null_mut::<libc::c_char>(); argc];
         let mut argv_buf = vec![0; argv_buf_size];
         cvt_wasi(libc::__wasi_args_get(argc.as_mut_ptr(), argv_buf.as_mut_ptr()))?;
 
diff --git a/src/libstd/sys/wasm/thread_local_atomics.rs b/src/libstd/sys/wasm/thread_local_atomics.rs
index b408ad0d5c1..3dc0bb24553 100644
--- a/src/libstd/sys/wasm/thread_local_atomics.rs
+++ b/src/libstd/sys/wasm/thread_local_atomics.rs
@@ -11,7 +11,7 @@ struct ThreadControlBlock {
 impl ThreadControlBlock {
     fn new() -> ThreadControlBlock {
         ThreadControlBlock {
-            keys: [0 as *mut u8; MAX_KEYS],
+            keys: [core::ptr::null_mut(); MAX_KEYS],
         }
     }