Rollup merge of #129760 - cuviper:old-timey, r=compiler-errors

Make the "detect-old-time" UI test more representative

The test code did have an inference failure, but that would have failed
on Rust 1.79 and earlier too. Now it is rewritten to be specifically
affected by 1.80's `impl FromIterator<_> for Box<str>`.

Author: matthiaskrgr