Avoid false-positives for yields with non-identical references (#1665)

Resolves #1663.

resources/test/fixtures/pyupgrade/UP028_1.py

@@ -111,3 +111,13 @@ def f():
     class C:
         def __init__(self):
             print(x)
+
+
+def f():
+    for x in y:
+        yield x, x + 1
+
+
+def f():
+    for x, y in z:
+        yield x, y, x + y

src/pyupgrade/plugins/rewrite_yield_from.rs

@@ -8,11 +8,25 @@ use crate::autofix::Fix;
 use crate::checkers::ast::Checker;
 use crate::registry::{Check, CheckKind};
 
+/// Return `true` if the two expressions are equivalent, and consistent solely
+/// of tuples and names.
+fn is_same_expr(a: &Expr, b: &Expr) -> bool {
+    match (&a.node, &b.node) {
+        (ExprKind::Name { id: a, .. }, ExprKind::Name { id: b, .. }) => a == b,
+        (ExprKind::Tuple { elts: a, .. }, ExprKind::Tuple { elts: b, .. }) => {
+            a.len() == b.len() && a.iter().zip(b).all(|(a, b)| is_same_expr(a, b))
+        }
+        _ => false,
+    }
+}
+
+/// Collect all named variables in an expression consisting solely of tuples and
+/// names.
 fn collect_names(expr: &Expr) -> Vec<&str> {
     match &expr.node {
         ExprKind::Name { id, .. } => vec![id],
         ExprKind::Tuple { elts, .. } => elts.iter().flat_map(collect_names).collect(),
-        _ => vec![],
+        _ => unreachable!("Expected: ExprKind::Name | ExprKind::Tuple"),
     }
 }
 
@@ -51,13 +65,12 @@ impl<'a> Visitor<'a> for YieldFromVisitor<'a> {
                 let body = &body[0];
                 if let StmtKind::Expr { value } = &body.node {
                     if let ExprKind::Yield { value: Some(value) } = &value.node {
-                        let names = collect_names(target);
-                        if names == collect_names(value) {
+                        if is_same_expr(target, value) {
                             self.yields.push(YieldFrom {
                                 stmt,
                                 body,
                                 iter,
-                                names,
+                                names: collect_names(target),
                             });
                         }
                     }