Require parens for chained comparison binops

Author: dtolnay

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

@@ -605,18 +605,8 @@ fn test_binop_associativity() {
     }
     "###);
 
-    // FIXME: this should fail to parse. Parenthesization is required.
-    snapshot!("() == () == ()" as Expr, @r###"
-    Expr::Binary {
-        left: Expr::Binary {
-            left: Expr::Tuple,
-            op: BinOp::Eq,
-            right: Expr::Tuple,
-        },
-        op: BinOp::Eq,
-        right: Expr::Tuple,
-    }
-    "###);
+    // Parenthesization is required.
+    syn::parse_str::<Expr>("() == () == ()").unwrap_err();
 }
 
 #[test]