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)

library/core/src/fmt/builders.rs

@@ -1228,6 +1228,7 @@ impl<'a, 'b: 'a> DebugMap<'a, 'b> {
 /// assert_eq!(format!("{:?}", wrapped), "'a'");
 /// ```
 #[unstable(feature = "debug_closure_helpers", issue = "117729")]
+#[must_use = "returns a type implementing Debug and Display, which do not have any effects unless they are used"]
 pub fn from_fn<F: Fn(&mut fmt::Formatter<'_>) -> fmt::Result>(f: F) -> FromFn<F> {
     FromFn(f)
 }