Add documentation on coding

Includes the bit on conditional compilation from [1].

[1] #32 (comment)

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

Documentation/rust/coding.rst

@@ -0,0 +1,41 @@
+.. _rust_coding:
+
+Coding
+======
+
+This document describes how to write Rust code in the kernel.
+
+
+Coding style
+------------
+
+For the moment, we are following the idiomatic Rust style. For instance,
+we use 4 spaces for indentation rather than tabs.
+
+
+Abstractions vs. bindings
+-------------------------
+
+We don't have abstractions for all the kernel internal APIs and concepts,
+but we would like to expand coverage as time goes on. Unless there is
+a good reason not to, always use the abstractions instead of calling
+the C bindings directly.
+
+If you are writing some code that requires a call to an internal kernel API
+or concept that isn't abstracted yet, consider providing an (ideally safe)
+abstraction for everyone to use.
+
+
+Conditional compilation
+-----------------------
+
+Rust code has access to conditional compilation based on the kernel
+configuration:
+
+.. code-block:: rust
+
+	#[cfg(CONFIG_X)]      // `CONFIG_X` is enabled (`y` or `m`)
+	#[cfg(CONFIG_X="y")]  // `CONFIG_X` is enabled as a built-in (`y`)
+	#[cfg(CONFIG_X="m")]  // `CONFIG_X` is enabled as a module   (`m`)
+	#[cfg(not(CONFIG_X))] // `CONFIG_X` is disabled
+

Documentation/rust/index.rst

@@ -7,6 +7,7 @@ Documentation related to Rust within the kernel. If you are starting out, read t
     :maxdepth: 1
 
     quick-start
+    coding
 
 .. only::  subproject and html