Fix comments, use constant instead of raw constant_context

Author: Niki4tap

clippy_lints/src/fn_null_check.rs

@@ -56,6 +56,8 @@ fn is_fn_ptr_cast(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
 impl<'tcx> LateLintPass<'tcx> for FnNullCheck {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
         match expr.kind {
+            // Catching:
+            // (fn_ptr as *<const/mut> <ty>).is_null()
             ExprKind::MethodCall(method_name, receiver, _, _)
                 if method_name.ident.as_str() == "is_null" && is_fn_ptr_cast(cx, receiver) =>
             {

clippy_lints/src/transmute/transmute_null_to_fn.rs

@@ -1,4 +1,4 @@
-use clippy_utils::consts::{constant_context, Constant};
+use clippy_utils::consts::{constant, Constant};
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::{is_integer_literal, is_path_diagnostic_item};
 use rustc_hir::{Expr, ExprKind};
@@ -25,12 +25,12 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, arg: &'t
         return false;
     }
 
-    // Catching:
-    // transmute over constants that resolve to `null`.
-    let mut const_eval_context = constant_context(cx, cx.typeck_results());
-
     match arg.kind {
-        ExprKind::Path(ref _qpath) if matches!(const_eval_context.expr(arg), Some(Constant::RawPtr(0))) => {
+        // Catching:
+        // transmute over constants that resolve to `null`.
+        ExprKind::Path(ref _qpath)
+            if matches!(constant(cx, cx.typeck_results(), arg), Some((Constant::RawPtr(0), _))) =>
+        {
             lint_expr(cx, expr);
             true
         },