Preserve separate evaluations in assignment grouping

src/decompiler.rs

@@ -21174,7 +21174,10 @@ let reopen = self
             let group = std::mem::take(&mut self.pending_stores);
             self.last_store_line = None;
             self.pending_unpack_wrap.clear();
-            let all_same = group.windows(2).all(|w| expr_eq(&w[0].1, &w[1].1));
+            // COPY/DUP preserves the identity of one evaluated stack value.
+            // Structurally equal expressions can be independent evaluations:
+            // `left, right = read(), read()` must still call read twice.
+            let all_same = group.windows(2).all(|w| Rc::ptr_eq(&w[0].1, &w[1].1));
             if all_same {
                 let value = group[0].1.clone();
                 let targets: Vec<ExprRef> = group.iter().map(|(t, _)| t.clone()).collect();

tests/behavior/cases/b_assignment_identity.py

@@ -0,0 +1,20 @@
+# MIN_VERSION: 3.9
+
+
+def separate(read):
+    left, right = read(), read()
+    return left, right
+
+
+def shared(read):
+    left = right = read()
+    return left, right
+
+
+events = []
+def read():
+    events.append("read")
+    return len(events)
+
+print(separate(read), events)
+print(shared(read), events)

tests/fixtures/assignment_identity.py

@@ -0,0 +1,20 @@
+# Separate evaluations must not become one COPY-backed chained assignment.
+
+def separate(read):
+    left, right = read(), read()
+    return left, right
+
+
+def shared(read):
+    left = right = read()
+    return left, right
+
+
+def separate_none():
+    left, right = None, None
+    return left, right
+
+
+def shared_none():
+    right = left = None
+    return left, right

tests/integration.rs

@@ -106,6 +106,26 @@ fn py310_basics() {
     assert_contains(&src, &["while i < 10:", "v = 1 < x < 100"], "py3.10 basics");
 }
 
+#[test]
+fn assignment_identity() {
+    for version in ["3.11", "3.12", "3.13", "3.14"] {
+        let fixture = format!("assignment_identity.{version}.pyc");
+        let (src, clean) = decompile_fixture(&fixture);
+        assert!(clean, "{fixture}: output marked incomplete:\n{src}");
+        assert_contains(
+            &src,
+            &[
+                "def separate(read):\n    left, right = read(), read()\n    return left, right",
+                "def shared(read):\n    left = right = read()\n    return left, right",
+                "def separate_none():\n    left, right = None, None\n    return left, right",
+                "def shared_none():\n    right = left = None\n    return left, right",
+            ],
+            &fixture,
+        );
+        assert_eq!(src.matches("read()").count(), 3, "{fixture}:\n{src}");
+    }
+}
+
 #[test]
 fn py311_comprehensions() {
     let (src, clean) = decompile_fixture("comprehensions.3.11.pyc");