some edits

Author: RalfJung

src/what-unsafe-does.md

@@ -23,25 +23,25 @@ language cares about is preventing the following things:
 * Causing a [data race][race]
 * Executing code compiled with [target features][] that the current thread of execution does
   not support
-* Producing invalid primitive values (either alone or as a field of a compound
-  type such as `enum`/`struct`/array/tuple):
+* Producing invalid values (either alone or as a field of a compound type such
+  as `enum`/`struct`/array/tuple):
     * a `bool` that isn't 0 or 1
-    * an undefined `enum` discriminant
-    * null `fn` pointers
+    * an `enum` with an invalid discriminant
+    * a null `fn` pointer
     * a `char` outside the ranges [0x0, 0xD7FF] and [0xE000, 0x10FFFF]
     * a `!` (all values are invalid for this type)
-    * dangling/unaligned references, references that do themselves point to
-      invalid values, or wide references (to a dynamically sized type) with
-      invalid metadata
+    * a reference that is dangling, unaligned, points to an invalid value, or
+      that has invalid metadata (if wide)
         * slice metadata is invalid if the slice has a total size larger than
           `isize::MAX` bytes in memory
         * `dyn Trait` metadata is invalid if it is not a pointer to a vtable for
           `Trait` that matches the actual dynamic trait the reference points to
-    * a non-utf8 `str`
+    * a `str` that isn't valid UTF-8
     * an integer (`i*`/`u*`), floating point value (`f*`), or raw pointer read from
       [uninitialized memory][]
-    * an invalid library type with custom invalid values, such as a `NonNull` or
-      the `NonZero` family of types, that is 0
+    * a type with custom invalid values that is one of those values, such as a
+      `NonNull` that is null. (Requesting custom invalid values is an unstable
+      feature, but some stable libstd types, like `NonNull`, make use of it.)
 
 "Producing" a value happens any time a value is assigned, passed to a
 function/primitive operation or returned from a function/primitive operation.
@@ -50,7 +50,9 @@ A reference/pointer is "dangling" if it is null or not all of the bytes it
 points to are part of the same allocation (so in particular they all have to be
 part of *some* allocation). The span of bytes it points to is determined by the
 pointer value and the size of the pointee type. As a consequence, if the span is
-empty, "dangling" is the same as "non-null".
+empty, "dangling" is the same as "non-null". Note that slices point to their
+entire range, so it's very important that the length metadata is never too
+large. If for some reason this is too cumbersome, consider using raw pointers.
 
 That's it. That's all the causes of Undefined Behavior baked into Rust. Of
 course, unsafe functions and traits are free to declare arbitrary other