Add #[rustc_never_returns_null_ptr]

And look for it in the useless_ptr_null_checks lint

compiler/rustc_feature/src/builtin_attrs.rs

@@ -698,6 +698,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_pass_by_value, Normal, template!(Word), ErrorFollowing,
         "#[rustc_pass_by_value] is used to mark types that must be passed by value instead of reference."
     ),
+    rustc_attr!(
+        rustc_never_returns_null_ptr, Normal, template!(Word), ErrorFollowing,
+        "#[rustc_never_returns_null_ptr] is used to mark functions returning non-null pointers."
+    ),
     rustc_attr!(
         rustc_coherence_is_core, AttributeType::CrateLevel, template!(Word), ErrorFollowing, @only_local: true,
         "#![rustc_coherence_is_core] allows inherent methods on builtin types, only intended to be used in `core`."

compiler/rustc_lint/src/ptr_nulls.rs

@@ -49,7 +49,7 @@ fn ptr_cast_chain<'a>(cx: &'a LateContext<'_>, mut e: &'a Expr<'a>) -> Option<&'
         } else if let ExprKind::Call(path, [arg]) = e.kind
             && let ExprKind::Path(ref qpath) = path.kind
             && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
-            && matches!(cx.tcx.get_diagnostic_name(def_id), Some(sym::ptr_from_ref | sym::ptr_from_mut)) {
+            && cx.tcx.has_attr(def_id, sym::rustc_never_returns_null_ptr) {
             had_at_least_one_cast = true;
             arg
         } else if had_at_least_one_cast {

compiler/rustc_passes/src/check_attr.rs

@@ -139,6 +139,9 @@ impl CheckAttrVisitor<'_> {
                     self.check_rustc_std_internal_symbol(&attr, span, target)
                 }
                 sym::naked => self.check_naked(hir_id, attr, span, target),
+                sym::rustc_never_returns_null_ptr => {
+                    self.check_applied_to_fn_or_method(hir_id, attr, span, target)
+                }
                 sym::rustc_legacy_const_generics => {
                     self.check_rustc_legacy_const_generics(hir_id, &attr, span, target, item)
                 }

compiler/rustc_span/src/symbol.rs

@@ -1311,6 +1311,7 @@ symbols! {
         rustc_main,
         rustc_mir,
         rustc_must_implement_one_of,
+        rustc_never_returns_null_ptr,
         rustc_nonnull_optimization_guaranteed,
         rustc_nounwind,
         rustc_object_lifetime_default,