adds error attribute note

Author: damirka

book/src/move-basics/assert-and-abort.md

@@ -45,7 +45,8 @@ The code above will, of course, abort with abort code `1`.
 The `assert!` macro is a built-in macro that can be used to assert a condition. If the condition is
 false, the transaction will abort with the given abort code. The `assert!` macro is a convenient way
 to abort a transaction if a condition is not met. The macro shortens the code otherwise written with
-an `if` expression + `abort`. The `code` argument is required and has to be a `u64` value.
+an `if` expression + `abort`. The `code` argument is optional, but has to be a `u64` value or an
+`#[error]` (see below for more information).
 
 ```move
 {{#include ../../../packages/samples/sources/move-basics/assert-and-abort.move:assert}}
@@ -63,6 +64,16 @@ code and make it easier to understand the abort scenarios.
 {{#include ../../../packages/samples/sources/move-basics/assert-and-abort.move:error_const}}
 ```
 
+## Error messages
+
+Move 2024 introduces a special type of error constant, marked with the `#[error]` attribute. This
+attribute allows the error constant to be of type `vector<u8>` and can be used to store an error
+message.
+
+```move
+{{#include ../../../packages/samples/sources/move-basics/assert-and-abort.move:error_attribute}}
+```
+
 ## Further reading
 
 - [Abort and Assert](/reference/abort-and-assert.html) in the Move Reference.

packages/samples/sources/move-basics/assert-and-abort.move

@@ -50,3 +50,22 @@ public fun update_record(/* ... , */ user_has_access: bool, field_exists: bool)
     /* ... */
 }
 // ANCHOR_END: error_const
+
+public struct User { is_authorized: bool, value: u64 }
+
+// ANCHOR: error_attribute
+#[error]
+const ENotAuthorized: vector<u8> = b"The user is not authorized to perform this action";
+
+#[error]
+const EValueTooLow: vector<u8> = b"The value is too low, it should be at least 10";
+
+/// Performs an action on behalf of the user.
+public fun update_value(user: &mut User, value: u64) {
+    assert!(user.is_authorized, ENotAuthorized);
+    assert!(value >= 10, EValueTooLow);
+
+    user.value = value;
+}
+// ANCHOR_END: error_attribute
+}

packages/samples/sources/move-basics/importing-modules-members.move

@@ -0,0 +1,15 @@
+// Copyright (c) Mysten Labs, Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+#[allow(unused_use)]
+// ANCHOR: members
+module book::more_imports;
+
+use book::module_one::new;       // imports the `new` function from the `module_one` module
+use book::module_one::Character; // importing the `Character` struct from the `module_one` module
+
+/// Calls the `new` function from the `module_one` module.
+public fun create_character(): Character {
+    new()
+}
+// ANCHOR_END: members