Set unnecessary_cast suggestion to MaybeIncorrect for pointer casts

Removing casts may cause type inference to stop working which requires
manual intervention

Author: Alexendoo

clippy_lints/src/casts/unnecessary_cast.rs

@@ -56,7 +56,7 @@ pub(super) fn check<'tcx>(
                 &format!("casting raw pointers to the same type and constness is unnecessary (`{cast_from}` -> `{cast_to}`)"),
                 "try",
                 cast_str.clone(),
-                Applicability::MachineApplicable,
+                Applicability::MaybeIncorrect,
             );
         }
     }

tests/ui/unnecessary_cast_unfixable.rs

@@ -0,0 +1,22 @@
+#![warn(clippy::unnecessary_cast)]
+
+fn main() {
+    let _ = std::ptr::null() as *const u8;
+}
+
+mod issue11113 {
+    #[repr(C)]
+    struct Vtbl {
+        query: unsafe extern "system" fn(),
+    }
+
+    struct TearOff {
+        object: *mut std::ffi::c_void,
+    }
+
+    impl TearOff {
+        unsafe fn query(&self) {
+            ((*(*(self.object as *mut *mut _) as *mut Vtbl)).query)()
+        }
+    }
+}

tests/ui/unnecessary_cast_unfixable.stderr

@@ -0,0 +1,16 @@
+error: casting raw pointers to the same type and constness is unnecessary (`*const u8` -> `*const u8`)
+  --> $DIR/unnecessary_cast_unfixable.rs:4:13
+   |
+LL |     let _ = std::ptr::null() as *const u8;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::ptr::null()`
+   |
+   = note: `-D clippy::unnecessary-cast` implied by `-D warnings`
+
+error: casting raw pointers to the same type and constness is unnecessary (`*mut issue11113::Vtbl` -> `*mut issue11113::Vtbl`)
+  --> $DIR/unnecessary_cast_unfixable.rs:19:16
+   |
+LL |             ((*(*(self.object as *mut *mut _) as *mut Vtbl)).query)()
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `*(self.object as *mut *mut _)`
+
+error: aborting due to 2 previous errors
+