Rollup merge of #119242 - BenWiederhake:dev-from-nanos, r=joshtriplett

Suggest less bug-prone construction of Duration in docs

std::time::Duration has a well-known quirk: Duration::as_nanos() returns u128 [1], but Duration::from_nanos() takes u64 [2]. So these methods cannot easily roundtrip [3]. It is not possible to simply accept u128 in from_nanos [4], because it requires breaking other API [5].

It seems to me that callers have basically only two options:
1. `Duration::from_nanos(d.as_nanos() as u64)`, which is the "obvious" and buggy approach.
2. `Duration::new(d.as_secs(), d.subsecs_nanos())`, which only becomes apparent after reading and digesting the entire Duration struct documentation.

I suggest that the documentation of `from_nanos` is changed to make option 2 more easily discoverable.

There are two major usecases for this:
- "Weird math" operations that should not be supported directly by `Duration`, like squaring.
- "Disconnected roundtrips", where the u128 value is passed through various other stack frames, and perhaps reconstructed into a Duration on a different machine.

In both cases, it seems like a good idea to not tempt people into thinking "Eh, u64 is good enough, what could possibly go wrong!". That's why I want to add a note that points out the similarly-easy and *safe* way to reconstruct a Duration.

[1] https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.as_nanos
[2] https://doc.rust-lang.org/stable/std/time/struct.Duration.html#method.from_nanos
[3] https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=fa6bab2b6b72f20c14b5243610ea1dde
[4] https://github.com/rust-lang/rust/issues/103332
[5] https://github.com/rust-lang/rust/issues/51107#issuecomment-392353166

library/core/src/time.rs

@@ -273,6 +273,11 @@ pub const fn from_micros(micros: u64) -> Duration {
 
     /// Creates a new `Duration` from the specified number of nanoseconds.
     ///
+    /// Note: Using this on the return value of `as_nanos()` might cause unexpected behavior:
+    /// `as_nanos()` returns a u128, and can return values that do not fit in u64, e.g. 585 years.
+    /// Instead, consider using the pattern `Duration::new(d.as_secs(), d.subsec_nanos())`
+    /// if you cannot copy/clone the Duration directly.
+    ///
     /// # Examples
     ///
     /// ```