Rollup merge of #83553 - jfrimmel:addr-of, r=m-ou-se

Update `ptr` docs with regards to `ptr::addr_of!`

This updates the documentation since `ptr::addr_of!` and `ptr::addr_of_mut!` are now stable. One might remove the distinction between the sections `# On packed structs` and `# Examples`, as the old section on packed structs was primarily to prevent users of doing undefined behavior, which is not necessary anymore.

Technically there is now wrong/outdated documentation on stable, but I don't think this is worth a point release 😉

Fixes #83509.

``````````@rustbot`````````` modify labels: T-doc

library/std/src/primitive_docs.rs

@@ -445,7 +445,27 @@ mod prim_unit {}
 /// Note that here the call to [`drop`] is for clarity - it indicates
 /// that we are done with the given value and it should be destroyed.
 ///
-/// ## 3. Get it from C.
+/// ## 3. Create it using `ptr::addr_of!`
+///
+/// Instead of coercing a reference to a raw pointer, you can use the macros
+/// [`ptr::addr_of!`] (for `*const T`) and [`ptr::addr_of_mut!`] (for `*mut T`).
+/// These macros allow you to create raw pointers to fields to which you cannot
+/// create a reference (without causing undefined behaviour), such as an
+/// unaligned field. This might be necessary if packed structs or uninitialized
+/// memory is involved.
+///
+/// ```
+/// #[derive(Debug, Default, Copy, Clone)]
+/// #[repr(C, packed)]
+/// struct S {
+///     aligned: u8,
+///     unaligned: u32,
+/// }
+/// let s = S::default();
+/// let p = std::ptr::addr_of!(s.unaligned); // not allowed with coercion
+/// ```
+///
+/// ## 4. Get it from C.
 ///
 /// ```
 /// # #![feature(rustc_private)]