Don't gate the feature twice

compiler/rustc_ast/src/ast.rs

@@ -676,6 +676,19 @@ impl Pat {
         });
         could_be_never_pattern
     }
+
+    /// Whether this contains a `!` pattern. This in particular means that a feature gate error will
+    /// be raised if the feature is off. Used to avoid gating the feature twice.
+    pub fn contains_never_pattern(&self) -> bool {
+        let mut contains_never_pattern = false;
+        self.walk(&mut |pat| {
+            if matches!(pat.kind, PatKind::Never) {
+                contains_never_pattern = true;
+            }
+            true
+        });
+        contains_never_pattern
+    }
 }
 
 /// A single field in a struct pattern.