Invert suggestion if pointer is tested for non-nullness (#15015)

Fixes rust-lang/rust-clippy#15010

----

changelog: [`cmp_null`]: emit proper suggestion when pointer is checked
for non-nullness

Author: llogiq

clippy_lints/src/ptr.rs

@@ -268,14 +268,15 @@ impl<'tcx> LateLintPass<'tcx> for Ptr {
                 (false, false, true) if let Some(sugg) = Sugg::hir_opt(cx, l) => sugg.maybe_paren(),
                 _ => return check_ptr_eq(cx, expr, op.node, l, r),
             };
+            let invert = if op.node == BinOpKind::Eq { "" } else { "!" };
 
             span_lint_and_sugg(
                 cx,
                 CMP_NULL,
                 expr.span,
                 "comparing with null is better expressed by the `.is_null()` method",
                 "try",
-                format!("{non_null_path_snippet}.is_null()"),
+                format!("{invert}{non_null_path_snippet}.is_null()",),
                 Applicability::MachineApplicable,
             );
         }

tests/ui/cmp_null.fixed

@@ -33,3 +33,9 @@ fn main() {
     let _ = (x as *const ()).is_null();
     //~^ cmp_null
 }
+
+fn issue15010() {
+    let f: *mut i32 = std::ptr::null_mut();
+    debug_assert!(!f.is_null());
+    //~^ cmp_null
+}

tests/ui/cmp_null.rs

@@ -33,3 +33,9 @@ fn main() {
     let _ = x as *const () == ptr::null();
     //~^ cmp_null
 }
+
+fn issue15010() {
+    let f: *mut i32 = std::ptr::null_mut();
+    debug_assert!(f != std::ptr::null_mut());
+    //~^ cmp_null
+}

tests/ui/cmp_null.stderr

@@ -31,5 +31,11 @@ error: comparing with null is better expressed by the `.is_null()` method
 LL |     let _ = x as *const () == ptr::null();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `(x as *const ()).is_null()`
 
-error: aborting due to 5 previous errors
+error: comparing with null is better expressed by the `.is_null()` method
+  --> tests/ui/cmp_null.rs:39:19
+   |
+LL |     debug_assert!(f != std::ptr::null_mut());
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `!f.is_null()`
+
+error: aborting due to 6 previous errors