Rollup merge of #136502 - yotamofek:pr/fmt-from-fn-must-use, r=dtolnay

Mark `std::fmt::from_fn` as `#[must_use]`

While working on #135494 I managed to shoot my own foot a few times by forgetting to actually use the result of `fmt::from_fn`, so I think a `#[must_use]` could be appropriate!

Didn't have a good message to put in the attr so left it blank, still unstable so we can come back to it I guess?

cc #117729 (and a huge +1 for getting it stabilized, it's very useful IMHO)

Author: fmease