For expr return (_ = 42); unused_paren lint should not be triggered

fixes #131989

Author: surechen

compiler/rustc_lint/src/unused.rs

@@ -1063,6 +1063,8 @@ impl UnusedDelimLint for UnusedParens {
                                 _,
                                 _,
                             ) if node.is_lazy()))
+                    && !(ctx == UnusedDelimsCtx::ReturnValue
+                        && matches!(inner.kind, ast::ExprKind::Assign(_, _, _)))
                 {
                     self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos, is_kw)
                 }

tests/ui/lint/unused/unused-parens-assign-expr-in-ret-issue-131989.rs

@@ -0,0 +1,9 @@
+//@ check-pass
+#![warn(unused_parens)]
+
+fn foo() {
+    return (_ = 42);
+    // lint unused_parens should not be triggered here.
+}
+
+fn main() {}