Correct use of 'nul' 'null' and capitalization in the book

src/doc/book/ffi.md

@@ -521,14 +521,14 @@ against `libc` and `libm` by default.
 
 # The "nullable pointer optimization"
 
-Certain types are defined to not be `null`. This includes references (`&T`,
+Certain types are defined to not be NULL. This includes references (`&T`,
 `&mut T`), boxes (`Box<T>`), and function pointers (`extern "abi" fn()`).
-When interfacing with C, pointers that might be null are often used.
+When interfacing with C, pointers that might be NULL are often used.
 As a special case, a generic `enum` that contains exactly two variants, one of
 which contains no data and the other containing a single field, is eligible
 for the "nullable pointer optimization". When such an enum is instantiated
 with one of the non-nullable types, it is represented as a single pointer,
-and the non-data variant is represented as the null pointer. So
+and the non-data variant is represented as the NULL pointer. So
 `Option<extern "C" fn(c_int) -> c_int>` is how one represents a nullable
 function pointer using the C ABI.
 

src/doc/book/raw-pointers.md

@@ -17,7 +17,7 @@ Here are some things to remember about raw pointers that are different than
 other pointer types. They:
 
 - are not guaranteed to point to valid memory and are not even
-  guaranteed to be non-null (unlike both `Box` and `&`);
+  guaranteed to be non-NULL (unlike both `Box` and `&`);
 - do not have any automatic clean-up, unlike `Box`, and so require
   manual resource management;
 - are plain-old-data, that is, they don't move ownership, again unlike

src/doc/book/strings.md

@@ -9,7 +9,7 @@ strings also work differently than in some other systems languages, such as C.
 Let’s dig into the details. A ‘string’ is a sequence of Unicode scalar values
 encoded as a stream of UTF-8 bytes. All strings are guaranteed to be a valid
 encoding of UTF-8 sequences. Additionally, unlike some systems languages,
-strings are not null-terminated and can contain null bytes.
+strings are not NUL-terminated and can contain NUL bytes.
 
 Rust has two main types of strings: `&str` and `String`. Let’s talk about
 `&str` first. These are called ‘string slices’. A string slice has a fixed

src/doc/book/unsafe.md

@@ -63,7 +63,7 @@ In addition, the following are all undefined behaviors in Rust, and must be
 avoided, even when writing `unsafe` code:
 
 * Data races
-* Dereferencing a null/dangling raw pointer
+* Dereferencing a NULL/dangling raw pointer
 * Reads of [undef][undef] (uninitialized) memory
 * Breaking the [pointer aliasing rules][aliasing] with raw pointers.
 * `&mut T` and `&T` follow LLVM’s scoped [noalias][noalias] model, except if
@@ -77,7 +77,7 @@ avoided, even when writing `unsafe` code:
   * Using `std::ptr::copy_nonoverlapping_memory` (`memcpy32`/`memcpy64`
     intrinsics) on overlapping buffers
 * Invalid values in primitive types, even in private fields/locals:
-  * Null/dangling references or boxes
+  * NULL/dangling references or boxes
   * A value other than `false` (0) or `true` (1) in a `bool`
   * A discriminant in an `enum` not included in its type definition
   * A value in a `char` which is a surrogate or above `char::MAX`