Merge pull request #1643 from dtolnay/chainedcompare

Require parens for chained comparison binops

src/expr.rs

@@ -1400,6 +1400,13 @@ pub(crate) mod parsing {
                 if precedence < base {
                     break;
                 }
+                if precedence == Precedence::Compare {
+                    if let Expr::Binary(lhs) = &lhs {
+                        if Precedence::of(&lhs.op) == Precedence::Compare {
+                            break;
+                        }
+                    }
+                }
                 input.advance_to(&ahead);
                 let right = parse_binop_rhs(input, allow_struct, precedence)?;
                 lhs = Expr::Binary(ExprBinary {
@@ -1455,6 +1462,13 @@ pub(crate) mod parsing {
                 if precedence < base {
                     break;
                 }
+                if precedence == Precedence::Compare {
+                    if let Expr::Binary(lhs) = &lhs {
+                        if Precedence::of(&lhs.op) == Precedence::Compare {
+                            break;
+                        }
+                    }
+                }
                 input.advance_to(&ahead);
                 let right = parse_binop_rhs(input, precedence)?;
                 lhs = Expr::Binary(ExprBinary {

tests/test_expr.rs

@@ -577,6 +577,38 @@ fn test_tuple_comma() {
     "###);
 }
 
+#[test]
+fn test_binop_associativity() {
+    // Left to right.
+    snapshot!("() + () + ()" as Expr, @r###"
+    Expr::Binary {
+        left: Expr::Binary {
+            left: Expr::Tuple,
+            op: BinOp::Add,
+            right: Expr::Tuple,
+        },
+        op: BinOp::Add,
+        right: Expr::Tuple,
+    }
+    "###);
+
+    // Right to left.
+    snapshot!("() += () += ()" as Expr, @r###"
+    Expr::Binary {
+        left: Expr::Tuple,
+        op: BinOp::AddAssign,
+        right: Expr::Binary {
+            left: Expr::Tuple,
+            op: BinOp::AddAssign,
+            right: Expr::Tuple,
+        },
+    }
+    "###);
+
+    // Parenthesization is required.
+    syn::parse_str::<Expr>("() == () == ()").unwrap_err();
+}
+
 #[test]
 fn test_assign_range_precedence() {
     // Range has higher precedence as the right-hand of an assignment, but