move all compat notes to their own section

Author: pietroalbini

posts/2022-09-22-Rust-1.64.0.md

@@ -48,35 +48,6 @@ working with C strings. Rust 1.64 also provides
 for working with owned C strings using only the `alloc` crate, rather than the
 full `std` library.
 
-### New internal layout for network structures
-
-The standard library network data structures
-[`Ipv4Addr`](https://doc.rust-lang.org/std/net/struct.Ipv4Addr.html),
-[`Ipv6Addr`](https://doc.rust-lang.org/std/net/struct.Ipv6Addr.html),
-[`SocketAddrV4`](https://doc.rust-lang.org/std/net/struct.SocketAddrV4.html),
-and
-[`SocketAddrV6`](https://doc.rust-lang.org/std/net/struct.SocketAddrV6.html)
-now use a more compact, memory-efficient layout, rather than using the memory
-layout of the underlying OS data structure.
-
-Normally, such a change would be an entirely internal detail of the standard
-library, with the only user-visible effect being less memory usage. However,
-some crates relied on the internal implementation details of the standard
-library, by using
-[`std::mem::transmute`](https://doc.rust-lang.org/std/mem/fn.transmute.html) or
-similar to access the internal representation and pass it directly to OS
-functions.
-
-Such internal implementation details of the standard library are *never*
-considered a stable interface. Nonetheless, because multiple crates relied on
-this, we first worked with the authors of all of the still-maintained crates
-doing so to arrange for the release of new versions, and to yank the old ones,
-before making this change. New releases for known affected crates have been out
-for typically over a year, and the vast majority of impacted users should be
-able to mitigate with a `cargo update` if needed."
-
-See <https://github.com/rust-lang/rust/pull/78802> for more details.
-
 ### Enhancing `.await` with `IntoFuture`
 
 Rust 1.64 stabilizes the
@@ -186,14 +157,6 @@ In the future, we hope to provide simpler APIs that require less use of
 low-level details like `Poll` and `Pin`, but in the meantime, these helpers
 make it easier to write such code.
 
-### Minimum required kernel and glibc versions increased
-
-As [previously
-announced](https://blog.rust-lang.org/2022/08/01/Increasing-glibc-kernel-requirements.html),
-`linux` targets now require at least Linux kernel 3.2 (except for targets which
-already required a newer kernel), and `linux-gnu` targets now require glibc
-2.17 (except for targets which already required a newer glibc).
-
 ### `rust-analyzer` now available via `rustup`, no longer in preview
 
 Users of `rust-analyzer` can now obtain it via `rustup` on the stable and beta
@@ -209,6 +172,27 @@ These APIs are now usable in const contexts:
 
 - ...
 
+### Compatibility notes
+
+Rust 1.64 includes some changes that might prevent you from upgrading:
+
+* As [previously
+  announced](https://blog.rust-lang.org/2022/08/01/Increasing-glibc-kernel-requirements.html),
+  `linux` targets now require at least Linux kernel 3.2 (except for targets which
+  already required a newer kernel), and `linux-gnu` targets now require glibc
+  2.17 (except for targets which already required a newer glibc).
+
+* Rust 1.64.0 changes the memory layout of `Ipv4Addr`, `Ipv6Addr`,
+  `SocketAddrV4` and `SocketAddrV6` to be more compact and memory efficient.
+   This internal representation was never exposed, but some crates relied on
+   it anyway by using `std::mem::transmute`, resulting in invalid memory
+   accesses. Such internal implementation details of the standard library are
+   *never* considered a stable interface. To limit the damage, we worked with
+   the authors of all of the still-maintained crates doing so to release fixed
+   versions, which have been out for more than a year. The vast majority of
+   impacted users should be able to mitigate with a `cargo update`.
+
+
 ### Other changes
 
 There are other changes in the Rust 1.64 release, including: