fix Nomicon transmute UB

Author: RalfJung

src/transmutes.md

@@ -12,28 +12,42 @@ to have the same size. The ways to cause Undefined Behavior with this are mind
 boggling.
 
 * First and foremost, creating an instance of *any* type with an invalid state
-  is going to cause arbitrary chaos that can't really be predicted.
+  is going to cause arbitrary chaos that can't really be predicted. Do not
+  transmute `3` to `bool`. Even if you never *do* anything with the `bool`. Just
+  don't.
 * Transmute has an overloaded return type. If you do not specify the return type
   it may produce a surprising type to satisfy inference.
-* Making a primitive with an invalid value is UB
-* Transmuting between non-repr(C) types is UB
-* Transmuting an & to &mut is UB
-    * Transmuting an & to &mut is *always* UB
-    * No you can't do it
-    * No you're not special
+* Transmuting an & to &mut is UB.
+    * Transmuting an & to &mut is *always* UB.
+    * No you can't do it.
+    * No you're not special.
 * Transmuting to a reference without an explicitly provided lifetime
   produces an [unbounded lifetime]
+* When transmuting between different compound types, you have to make sure they
+  are laid out the same way! If layouts differ, the wrong fields are going to
+  get filled with the wrong data, which will make you unhappy and can also be UB
+  (see above).
+
+    So how do you know if the layouts are the same? For `repr(C)` types and
+  `repr(transparent)` types, layout is precisely defined. But for your
+  run-of-the-mill `repr(Rust)`, it is not. Even different instances of the same
+  generic type can have wildly different layout. `Vec<i32>` and `Vec<u32>`
+  *might* have their fields in the same order, or they might not. The details of
+  what exactly is and is not guaranteed for data layout are still being worked
+  out over [at the UCG WG][ucg-layout].
 
 [`mem::transmute_copy<T, U>`][transmute_copy] somehow manages to be *even more*
 wildly unsafe than this. It copies `size_of<U>` bytes out of an `&T` and
 interprets them as a `U`.  The size check that `mem::transmute` has is gone (as
 it may be valid to copy out a prefix), though it is Undefined Behavior for `U`
 to be larger than `T`.
 
-Also of course you can get most of the functionality of these functions using
-pointer casts.
+Also of course you can get all of the functionality of these functions using raw
+pointer casts or `union`s, but without any of the lints or other basic sanity
+checks. Raw pointer casts and `union`s do not magically avoid the above rules.
 
 
 [unbounded lifetime]: unbounded-lifetimes.html
 [transmute]: ../std/mem/fn.transmute.html
 [transmute_copy]: ../std/mem/fn.transmute_copy.html
+[ucg-layout]: https://rust-lang.github.io/unsafe-code-guidelines/layout.html