fix: avoid adding parameters for variables defined in the selection during helper extraction (#2373)

Summary:
This PR fixes an issue where the “Extract into helper" functionality incorrectly added variables as parameters to the extracted function when those variables were actually defined inside the selection.

Specifically, variables on the left-hand side of augmented assignments (e.g., `total` in `total += price`) were unconditionally added to the extracted method's parameter list, even if the variable was initialised earlier in the same selection (e.g., `total = 0`).

The fix simply checks `store_refs` for a prior assignment of the variable within the selection before adding it as a parameter.

Fixes #2335

Pull Request resolved: #2373

Test Plan:
1. Added a regression test to cover the changed behaviour.
2. Manual testing:
2.1. Locally built pyrefly to `/some-dir/pyrefly`
2.2. In editor settings, add the following setting: `”pyrefly.lspPath”: ”/some-dir/pyrefly”`
2.3. Manually test the “Extract into helper” functionality to verify fix. See below for reference.

https://github.com/user-attachments/assets/58b18f2a-262b-468b-8f28-1a6e7ee35ebe

Reviewed By: grievejia

Differential Revision: D93156987

Pulled By: kinto0

fbshipit-source-id: b027e9a978a99585349efca736993268eecd8184

Author: self-sasi

pyrefly/lib/state/lsp/quick_fixes/extract_function.rs

@@ -72,8 +72,13 @@ pub(crate) fn extract_function_code_actions(
             continue;
         }
         if ident.synthetic_load {
-            seen_params.insert(ident.name.clone());
-            params.push(ident.name.clone());
+            let defined_earlier_in_selection = store_refs
+                .iter()
+                .any(|store| store.name == ident.name && store.position < ident.position);
+            if !defined_earlier_in_selection {
+                seen_params.insert(ident.name.clone());
+                params.push(ident.name.clone());
+            }
             continue;
         }
         let defs = transaction.find_definition(handle, ident.position, FindPreference::default());

pyrefly/lib/test/lsp/code_actions.rs

@@ -1486,6 +1486,65 @@ class Outer:
     assert_eq!(expected.trim(), updated.trim());
 }
 
+#[test]
+fn extract_function_excludes_vars_defined_in_selection() {
+    // variables defined within the selection should not become parameters, even
+    // when they are later used in augmented assignments (e.g., total += price).
+    let code = r#"
+def calculate_total_price(prices: list[int]) -> float:
+    # EXTRACT-START
+    total = 0
+    for price in prices:
+        total += price
+    with_tax = total * 1.085
+    # EXTRACT-END
+    return with_tax
+"#;
+    let updated = apply_first_extract_action(code).expect("expected extract refactor action");
+    let expected = r#"
+def extracted_function(prices):
+    total = 0
+    for price in prices:
+        total += price
+    with_tax = total * 1.085
+    return with_tax
+
+def calculate_total_price(prices: list[int]) -> float:
+    # EXTRACT-START
+    with_tax = extracted_function(prices)
+    # EXTRACT-END
+    return with_tax
+"#;
+    assert_eq!(expected.trim(), updated.trim());
+}
+
+#[test]
+fn extract_function_includes_var_from_augmented_assign_without_prior_def() {
+    // When the selection contains only an augmented assignment (e.g., x += 1)
+    // without a prior definition of that variable, the variable must still be
+    // added as a parameter to the extracted function.
+    let code = r#"
+def update(x: int) -> int:
+    # EXTRACT-START
+    x += 1
+    # EXTRACT-END
+    return x
+"#;
+    let updated = apply_first_extract_action(code).expect("expected extract refactor action");
+    let expected = r#"
+def extracted_function(x):
+    x += 1
+    return x
+
+def update(x: int) -> int:
+    # EXTRACT-START
+    x = extracted_function(x)
+    # EXTRACT-END
+    return x
+"#;
+    assert_eq!(expected.trim(), updated.trim());
+}
+
 #[test]
 fn extract_variable_basic_refactor() {
     let code = r#"