Add section on safety checks (#6)

src/chapter_3/the_accounts_struct.md

@@ -1,4 +1,5 @@
 # The Accounts Struct
+
 The Accounts struct is where you define which accounts your instruction expects and which constraints these accounts should adhere to. You do this via two constructs: Types and constraints.
 
 ## Types
@@ -46,7 +47,6 @@ Most importantly, the `#[account]` attribute sets the owner of that data to the
 
 #### Using `Account<'a, T>` with non-anchor program accounts
 
-
 There may be cases where you want your program to interact with a non-Anchor program. You can still get all the benefits of `Account` but you have to write a custom wrapper type instead of using `#[account]`. For instance, Anchor provides wrapper types for the token program accounts so they can be used with `Account`.
 
 ```rust,ignore
@@ -97,12 +97,14 @@ Check out the [reference for the Account type](https://docs.rs/anchor-lang/lates
 Account types can do a lot of work for you but they're not dynamic enough to handle all the security checks a secure program requires.
 
 Add constraints to an account with the following format:
+
 ```rust,ignore
 #[account(<constraints>)]
 pub account: AccountType
 ```
 
 Some constraints support custom Errors (we will explore errors [later](./errors.md)):
+
 ```rust,ignore
 #[account(...,<constraint> @ MyError::MyErrorVariant, ...)]
 pub account: AccountType
@@ -125,3 +127,38 @@ pub struct SetData<'info> {
 ```
 
 You can find information about all constraints in the reference. We will cover some of the most important ones in the milestone project at the end of the Essentials section.
+
+## Safety checks
+
+Two of the Anchor account types, [AccountInfo](https://docs.rs/anchor-lang/latest/anchor_lang/accounts/account_info/index.html) and [UncheckedAccount](https://docs.rs/anchor-lang/latest/anchor_lang/accounts/unchecked_account/index.html) do not implement any checks on the account being passed. Anchor implements safety checks that encourage additional documentation describing why additional checks are not necesssary.
+
+Attempting to build a program containing the following excerpt with `anchor build`:
+
+```rust,ignore
+#[derive(Accounts)]
+pub struct Initialize<'info> {
+    pub potentially_dangerous: UncheckedAccount<'info>
+}
+```
+
+will result in an error similar to the following:
+
+```
+Error:
+        /anchor/tests/unchecked/programs/unchecked/src/lib.rs:15:8
+        Struct field "potentially_dangerous" is unsafe, but is not documented.
+        Please add a `/// SAFETY:` doc comment explaining why no checks through types are necessary.
+        See https://book.anchor-lang.com/chapter_3/the_accounts_struct.html#safety-checks for more information.
+```
+
+To fix this, write a doc comment describing the potential security implications, e.g.:
+
+```rust,ignore
+#[derive(Accounts)]
+pub struct Initialize<'info> {
+    /// SAFETY: This is not dangerous because we don't read or write from this account
+    pub potentially_dangerous: UncheckedAccount<'info>
+}
+```
+
+Note the doc comment needs to be a [line or block doc comment](https://doc.rust-lang.org/reference/comments.html#doc-comments) (/// or /\*\*) to be interepreted as doc attribute by Rust. Double slash comments (//) are not interpreted as such.