From a08dedf76bdde8340b7fca0dcc01420338cb7afd Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Mon, 30 Dec 2024 19:43:23 -0800
Subject: [PATCH] Fix ranges bailout edge cases with assignment

---
 src/expr.rs        | 26 ++++++++++++++++++++++++--
 tests/test_expr.rs | 25 ++++++++++++++++++++++++-
 2 files changed, 48 insertions(+), 3 deletions(-)

diff --git a/src/expr.rs b/src/expr.rs
index cde51cdebb..49f050e2a6 100644
--- a/src/expr.rs
+++ b/src/expr.rs
@@ -1324,6 +1324,11 @@ pub(crate) mod parsing {
                 if precedence < base {
                     break;
                 }
+                if precedence == Precedence::Assign {
+                    if let Expr::Range(_) = lhs {
+                        break;
+                    }
+                }
                 if precedence == Precedence::Compare {
                     if let Expr::Binary(lhs) = &lhs {
                         if Precedence::of_binop(&lhs.op) == Precedence::Compare {
@@ -1339,7 +1344,13 @@ pub(crate) mod parsing {
                     op,
                     right,
                 });
-            } else if Precedence::Assign >= base && input.peek(Token![=]) && !input.peek(Token![=>])
+            } else if Precedence::Assign >= base
+                && input.peek(Token![=])
+                && !input.peek(Token![=>])
+                && match lhs {
+                    Expr::Range(_) => false,
+                    _ => true,
+                }
             {
                 let eq_token: Token![=] = input.parse()?;
                 let right = parse_binop_rhs(input, allow_struct, Precedence::Assign)?;
@@ -2874,7 +2885,18 @@ pub(crate) mod parsing {
                 || input.peek(Token![.]) && !input.peek(Token![..])
                 || input.peek(Token![?])
                 || input.peek(Token![=>])
-                || !allow_struct.0 && input.peek(token::Brace))
+                || !allow_struct.0 && input.peek(token::Brace)
+                || input.peek(Token![=]) && !input.peek(Token![==])
+                || input.peek(Token![+=])
+                || input.peek(Token![-=])
+                || input.peek(Token![*=])
+                || input.peek(Token![/=])
+                || input.peek(Token![%=])
+                || input.peek(Token![^=])
+                || input.peek(Token![&=])
+                || input.peek(Token![|=])
+                || input.peek(Token![<<=])
+                || input.peek(Token![>>=]))
         {
             Ok(None)
         } else {
diff --git a/tests/test_expr.rs b/tests/test_expr.rs
index 5783c778b2..1bcc0941fa 100644
--- a/tests/test_expr.rs
+++ b/tests/test_expr.rs
@@ -408,7 +408,7 @@ fn test_range_precedence() {
 }
 
 #[test]
-fn test_ranges_dots_bailout() {
+fn test_ranges_bailout() {
     syn::parse_str::<Expr>(".. ?").unwrap_err();
     syn::parse_str::<Expr>(".. .field").unwrap_err();
 
@@ -476,6 +476,29 @@ fn test_ranges_dots_bailout() {
         member: Member::Named("field"),
     }
     "#);
+
+    snapshot!("return .. = ()" as Expr, @r"
+    Expr::Assign {
+        left: Expr::Return {
+            expr: Some(Expr::Range {
+                limits: RangeLimits::HalfOpen,
+            }),
+        },
+        right: Expr::Tuple,
+    }
+    ");
+
+    snapshot!("return .. += ()" as Expr, @r"
+    Expr::Binary {
+        left: Expr::Return {
+            expr: Some(Expr::Range {
+                limits: RangeLimits::HalfOpen,
+            }),
+        },
+        op: BinOp::AddAssign,
+        right: Expr::Tuple,
+    }
+    ");
 }
 
 #[test]