Handle null pointer constness cast through methods

This covers two cases:

- `core::ptr::null::<T>().cast_mut()` -> `core::ptr::null_mut::<T>()`
- `core::ptr::null_mut::<T>().cast_const()` -> `core::ptr::null::<T>()`

clippy_lints/src/casts/mod.rs

@@ -813,6 +813,7 @@ impl<'tcx> LateLintPass<'tcx> for Casts {
         char_lit_as_u8::check(cx, expr);
         ptr_as_ptr::check(cx, expr, &self.msrv);
         cast_slice_different_sizes::check(cx, expr, &self.msrv);
+        ptr_cast_constness::check_null_ptr_cast_method(cx, expr);
     }
 
     extract_msrv_attr!(LateContext);

clippy_lints/src/casts/ptr_cast_constness.rs

@@ -82,3 +82,33 @@ pub(super) fn check<'tcx>(
         }
     }
 }
+
+pub(super) fn check_null_ptr_cast_method(cx: &LateContext<'_>, expr: &Expr<'_>) {
+    if let ExprKind::MethodCall(method, cast_expr, [], _) = expr.kind
+        && let ExprKind::Call(func, []) = cast_expr.kind
+        && let ExprKind::Path(QPath::Resolved(None, path)) = func.kind
+        && let Some(defid) = path.res.opt_def_id()
+    {
+        let mutable = method.ident.as_str() == "cast_mut";
+        if (cx.tcx.is_diagnostic_item(sym::ptr_null, defid) && mutable)
+            || (cx.tcx.is_diagnostic_item(sym::ptr_null_mut, defid) && method.ident.as_str() == "cast_const")
+        {
+            let mut app = Applicability::MachineApplicable;
+            let sugg = Sugg::hir_with_applicability(cx, cast_expr, "_", &mut app).to_string();
+            let Some((_, after_lt)) = sugg.split_once("::<") else {
+                return;
+            };
+            let method = if mutable { "null_mut" } else { "null" };
+            let prefix = std_or_core(cx).unwrap_or("::core");
+            span_lint_and_sugg(
+                cx,
+                PTR_CAST_CONSTNESS,
+                expr.span,
+                "changing constness of a null pointer",
+                format!("use `{method}()` directly instead"),
+                format!("{prefix}::ptr::{method}::<{after_lt}"),
+                app,
+            );
+        }
+    }
+}

tests/ui/ptr_cast_constness.fixed

@@ -74,10 +74,14 @@ fn null_pointers() {
     use std::ptr;
     let _ = std::ptr::null_mut::<String>();
     let _ = std::ptr::null::<u32>();
+    let _ = std::ptr::null_mut::<u32>();
+    let _ = std::ptr::null::<u32>();
 
     // Make sure the lint is triggered inside a macro
     let _ = inline!(std::ptr::null_mut::<u32>());
+    let _ = inline!(std::ptr::null_mut::<u32>());
 
     // Do not lint inside macros from external crates
     let _ = external!(ptr::null::<u32>() as *mut u32);
+    let _ = external!(ptr::null::<u32>().cast_mut());
 }

tests/ui/ptr_cast_constness.rs

@@ -74,10 +74,14 @@ fn null_pointers() {
     use std::ptr;
     let _ = ptr::null::<String>() as *mut String;
     let _ = ptr::null_mut::<u32>() as *const u32;
+    let _ = ptr::null::<u32>().cast_mut();
+    let _ = ptr::null_mut::<u32>().cast_const();
 
     // Make sure the lint is triggered inside a macro
     let _ = inline!(ptr::null::<u32>() as *mut u32);
+    let _ = inline!(ptr::null::<u32>().cast_mut());
 
     // Do not lint inside macros from external crates
     let _ = external!(ptr::null::<u32>() as *mut u32);
+    let _ = external!(ptr::null::<u32>().cast_mut());
 }

tests/ui/ptr_cast_constness.stderr

@@ -55,13 +55,33 @@ error: `as` casting to make a mutable null pointer into an immutable null pointe
 LL |     let _ = ptr::null_mut::<u32>() as *const u32;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `null()` directly instead: `std::ptr::null::<u32>()`
 
+error: changing constness of a null pointer
+  --> tests/ui/ptr_cast_constness.rs:77:13
+   |
+LL |     let _ = ptr::null::<u32>().cast_mut();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `null_mut()` directly instead: `std::ptr::null_mut::<u32>()`
+
+error: changing constness of a null pointer
+  --> tests/ui/ptr_cast_constness.rs:78:13
+   |
+LL |     let _ = ptr::null_mut::<u32>().cast_const();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `null()` directly instead: `std::ptr::null::<u32>()`
+
 error: `as` casting to make a const null pointer into a mutable null pointer
-  --> tests/ui/ptr_cast_constness.rs:79:21
+  --> tests/ui/ptr_cast_constness.rs:81:21
    |
 LL |     let _ = inline!(ptr::null::<u32>() as *mut u32);
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `null_mut()` directly instead: `std::ptr::null_mut::<u32>()`
    |
    = note: this error originates in the macro `__inline_mac_fn_null_pointers` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 10 previous errors
+error: changing constness of a null pointer
+  --> tests/ui/ptr_cast_constness.rs:82:21
+   |
+LL |     let _ = inline!(ptr::null::<u32>().cast_mut());
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `null_mut()` directly instead: `std::ptr::null_mut::<u32>()`
+   |
+   = note: this error originates in the macro `__inline_mac_fn_null_pointers` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 13 previous errors