add more cases of UB

Author: RalfJung

src/what-unsafe-does.md

@@ -22,16 +22,21 @@ language cares about is preventing the following things:
 * Breaking the [pointer aliasing rules][]
 * Producing invalid primitive values (either alone or as a field of a compound
   type such as `enum`/`struct`/array/tuple):
-    * dangling/null/unaligned references
+    * dangling/null/unaligned references, references that do themselves point to
+      invalid values, or fat references (to a dynamically sized type) with
+      invalid metadata
     * null `fn` pointers
     * a `bool` that isn't 0 or 1
     * an undefined `enum` discriminant
     * a `char` outside the ranges [0x0, 0xD7FF] and [0xE000, 0x10FFFF]
     * a non-utf8 `str`
+    * an uninitialized integer (`i*`/`u*`) or floating point value (`f*`)
     * an invalid library type with custom invalid values, such as a `NonNull` or
       `NonZero*` that is 0
 * Unwinding into another language
 * Causing a [data race][race]
+* Executing code compiled with platform features that the current platform does
+  not support (see [`target_feature`])
 
 "Producing" a value happens any time a value is assigned, passed to a
 function/primitive operation or returned from a function/primitive operation.
@@ -69,3 +74,4 @@ these problems are considered impractical to categorically prevent.
 [pointer aliasing rules]: references.html
 [uninitialized memory]: uninitialized.html
 [race]: races.html
+[`target_feature`]: ../reference/attributes/codegen.html#the-target_feature-attribute