Add null coalescing operator.

src/parser.rs

@@ -1917,8 +1917,8 @@ impl Engine {
                     }
                 }
             }
-            // ??? && ??? = rhs, ??? || ??? = rhs
-            Expr::And(..) | Expr::Or(..) => Err(LexError::ImproperSymbol(
+            // ??? && ??? = rhs, ??? || ??? = rhs, xxx ?? xxx = rhs
+            Expr::And(..) | Expr::Or(..) | Expr::Coalesce(..) => Err(LexError::ImproperSymbol(
                 "=".to_string(),
                 "Possibly a typo of '=='?".to_string(),
             )
@@ -2223,6 +2223,18 @@ impl Engine {
                         pos,
                     )
                 }
+                Token::DoubleQuestion => {
+                    let rhs = args.pop().unwrap();
+                    let current_lhs = args.pop().unwrap();
+                    Expr::Coalesce(
+                        BinaryExpr {
+                            lhs: current_lhs,
+                            rhs,
+                        }
+                        .into(),
+                        pos,
+                    )
+                }
                 Token::In => {
                     // Swap the arguments
                     let current_lhs = args.remove(0);