From c463a74f7bd4f8cd81a57c8a77faece2d2590a42 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Fri, 10 May 2024 20:04:00 -0700
Subject: [PATCH 1/2] Add tests of binop associativity

---
 tests/test_expr.rs | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/tests/test_expr.rs b/tests/test_expr.rs
index b908311d49..87fca8d936 100644
--- a/tests/test_expr.rs
+++ b/tests/test_expr.rs
@@ -577,6 +577,48 @@ 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,
+        },
+    }
+    "###);
+
+    // 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,
+    }
+    "###);
+}
+
 #[test]
 fn test_assign_range_precedence() {
     // Range has higher precedence as the right-hand of an assignment, but

From dc4ffde94ecac024785fd4fe75e7318fbc46be22 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Fri, 10 May 2024 20:05:41 -0700
Subject: [PATCH 2/2] Require parens for chained comparison binops

---
 src/expr.rs        | 14 ++++++++++++++
 tests/test_expr.rs | 14 ++------------
 2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/src/expr.rs b/src/expr.rs
index 4183ced263..8b44342eae 100644
--- a/src/expr.rs
+++ b/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 {
diff --git a/tests/test_expr.rs b/tests/test_expr.rs
index 87fca8d936..f39200d3f2 100644
--- a/tests/test_expr.rs
+++ b/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]