Preserve surviving bindings for repeated unpack targets

src/decompiler.rs

@@ -42539,9 +42539,39 @@ fn cmp_from_index(idx: usize) -> CmpOp {
 // =====================  stack events, calls, functions, classes  =====================
 
 impl<'a> Ctx<'a> {
+    fn discarded_unpack_target(&self, remaining: usize) -> ExprRef {
+        // Repeated local targets can have their earlier stores replaced by
+        // POP_TOP. Their names may then be obfuscated while the surviving
+        // store keeps its slot. Reuse the next direct local store's spelling:
+        // the compiler elides the duplicate instead of creating an extra `_`
+        // local. Do not cross nested unpacking, side effects or control flow.
+        let name = if self.version.at_least(3, 13) {
+            self.idx_of.get(&self.cur_offset).and_then(|&index| {
+                let mut discarded = 1usize;
+                for next in &self.instrs[index + 1..] {
+                    match next.op {
+                        Op::NOP | Op::CACHE | Op::EXTENDED_ARG | Op::NOT_TAKEN => {},
+                        Op::POP_TOP => {
+                            discarded += 1;
+                            if discarded >= remaining { return None; }
+                        }
+                        Op::STORE_FAST => return self.code.varnames.get(next.arg as usize).cloned(),
+                        Op::STORE_FAST_STORE_FAST | Op::STORE_FAST_LOAD_FAST => {
+                            return self.code.varnames.get(((next.arg >> 4) & 0xF) as usize).cloned();
+                        }
+                        Op::STORE_DEREF => return self.code.deref_name(next.arg as usize).map(str::to_string),
+                        _ => return None,
+                    }
+                }
+                None
+            })
+        } else { None };
+        self.name_expr(name.as_deref().unwrap_or("_"))
+    }
+
     fn handle_pop_top(&mut self) {
-        // 3.14 `_` elision: the compiler replaces the store of a
-        // throwaway `_` name inside a tuple unpack with a POP_TOP
+        // 3.13+ repeated-target elision: the compiler replaces an
+        // overwritten local store inside a tuple unpack with a POP_TOP
         // discarding that slot (UNPACK_SEQUENCE; POP_TOP;
         // STORE_FAST_STORE_FAST). The open unpack frame must consume
         // the slot as a placeholder target or the tuple never
@@ -42552,8 +42582,9 @@ impl<'a> Ctx<'a> {
             if *remaining > 0 {
                 let is_slot = matches!(self.stack.last(), Some(Sv::E(e)) if Rc::ptr_eq(e, fv));
                 if is_slot {
+                    let target = self.discarded_unpack_target(*remaining);
                     if let Some(Sv::E(_)) = self.stack.pop() {
-                        self.collect_unpack_target(self.name_expr("_"));
+                        self.collect_unpack_target(target);
                     }
                     return;
                 }

tests/behavior/cases/b_repeated_unpack_bindings.py

@@ -0,0 +1,31 @@
+# MIN_VERSION: 3.13
+
+
+def underscore_loop(values):
+    result = []
+    for value, _, discarded, discarded in values:
+        result.append((value, discarded, _))
+    return result
+
+
+def underscore_list(values):
+    return [(value, discarded, _) for value, _, discarded, discarded in values]
+
+
+def nested_loop(values):
+    result = []
+    for value, (discarded, discarded) in values:
+        result.append((value, discarded))
+    return result
+
+
+def nested_list(values):
+    return [(value, discarded) for value, (discarded, discarded) in values]
+
+
+rows = [(1, 2, 3, 4), (5, 6, 7, 8)]
+print(underscore_loop(rows))
+print(underscore_list(rows))
+nested = [(1, (2, 3)), (4, (5, 6))]
+print(nested_loop(nested))
+print(nested_list(nested))

tests/fixtures/repeated_unpack_bindings.py

@@ -0,0 +1,35 @@
+# Versioned .pyc files are consumed by tests/integration.rs.
+
+# CPython 3.13 starts replacing overwritten stores with POP_TOP.
+
+def repeated_loop(values):
+    result = []
+    for value, discarded, discarded, discarded in values:
+        result.append(value is not None)
+    return result
+
+
+def repeated_list(values):
+    return [value is not None for value, discarded, discarded, discarded in values]
+
+
+def underscore_loop(values):
+    result = []
+    for value, _, discarded, discarded in values:
+        result.append((value, discarded, _))
+    return result
+
+
+def underscore_list(values):
+    return [(value, discarded, _) for value, _, discarded, discarded in values]
+
+
+def nested_loop(values):
+    result = []
+    for value, (discarded, discarded) in values:
+        result.append((value, discarded))
+    return result
+
+
+def nested_list(values):
+    return [(value, discarded) for value, (discarded, discarded) in values]

tests/integration.rs

@@ -169,3 +169,34 @@ fn py314_hello() {
         "py3.14 hello",
     );
 }
+
+#[test]
+fn overwritten_unpack_slots_reuse_the_surviving_binding() {
+    for version in ["3.12", "3.13", "3.14"] {
+        let (src, clean) = decompile_fixture(&format!("repeated_unpack_bindings.{version}.pyc"));
+        assert!(clean, "Python {version}: output marked incomplete:\n{src}");
+        assert_contains(&src, &[
+            r#"def repeated_loop(values):
+    result = []
+    for value, discarded, discarded, discarded in values:
+        result.append(value is not None)
+    return result"#,
+            r#"def repeated_list(values):
+    return [value is not None for value, discarded, discarded, discarded in values]"#,
+            r#"def underscore_loop(values):
+    result = []
+    for value, _, discarded, discarded in values:
+        result.append((value, discarded, _))
+    return result"#,
+            r#"def underscore_list(values):
+    return [(value, discarded, _) for value, _, discarded, discarded in values]"#,
+            r#"def nested_loop(values):
+    result = []
+    for value, (discarded, discarded) in values:
+        result.append((value, discarded))
+    return result"#,
+            r#"def nested_list(values):
+    return [(value, discarded) for value, (discarded, discarded) in values]"#,
+        ], version);
+    }
+}