rust: support `srctree`-relative links

Some of our links use relative paths in order to point to files in the
source tree, e.g.:

    //! C header: [`include/linux/printk.h`](../../../../include/linux/printk.h)
    /// [`struct mutex`]: ../../../../include/linux/mutex.h

These are problematic because they are hard to maintain and do not support
`O=` builds.

Instead, provide support for `srctree`-relative links, e.g.:

    //! C header: [`include/linux/printk.h`](srctree/include/linux/printk.h)
    /// [`struct mutex`]: srctree/include/linux/mutex.h

The links are fixed after `rustdoc` generation to be based on the absolute
path to the source tree.

Essentially, this is the automatic version of Tomonori's fix [1],
suggested by Gary [2].

Suggested-by: Gary Guo <gary@garyguo.net>
Reported-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Closes: https://lore.kernel.org/r/20231026.204058.2167744626131849993.fujita.tomonori@gmail.com [1]
Fixes: 48fadf4400 ("docs: Move rustdoc output, cross-reference it")
Link: https://lore.kernel.org/rust-for-linux/20231026154525.6d14b495@eugeo/ [2]
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20231215235428.243211-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Documentation/rust/coding-guidelines.rst

@@ -177,6 +177,19 @@ please take a look at the ``rustdoc`` book at:
 
 	https://doc.rust-lang.org/rustdoc/how-to-write-documentation.html
 
+In addition, the kernel supports creating links relative to the source tree by
+prefixing the link destination with ``srctree/``. For instance:
+
+.. code-block:: rust
+
+	//! C header: [`include/linux/printk.h`](srctree/include/linux/printk.h)
+
+or:
+
+.. code-block:: rust
+
+	/// [`struct mutex`]: srctree/include/linux/mutex.h
+
 
 Naming
 ------

rust/Makefile

@@ -99,7 +99,8 @@ rustdoc: rustdoc-core rustdoc-macros rustdoc-compiler_builtins \
 	$(Q)find $(rustdoc_output) -name '*.html' -type f -print0 | xargs -0 sed -Ei \
 		-e 's:rust-logo-[0-9a-f]+\.svg:logo.svg:g' \
 		-e 's:favicon-[0-9a-f]+\.svg:logo.svg:g' \
-		-e 's:<link rel="alternate icon" type="image/png" href="[/.]+/static\.files/favicon-(16x16|32x32)-[0-9a-f]+\.png">::g'
+		-e 's:<link rel="alternate icon" type="image/png" href="[/.]+/static\.files/favicon-(16x16|32x32)-[0-9a-f]+\.png">::g' \
+		-e 's:<a href="srctree/([^"]+)">:<a href="$(abs_srctree)/\1">:g'
 	$(Q)for f in $(rustdoc_output)/static.files/rustdoc-*.css; do \
 		echo ".logo-container > img { object-fit: contain; }" >> $$f; done
 

rust/kernel/error.rs

@@ -2,7 +2,7 @@
 
 //! Kernel errors.
 //!
-//! C header: [`include/uapi/asm-generic/errno-base.h`](../../../include/uapi/asm-generic/errno-base.h)
+//! C header: [`include/uapi/asm-generic/errno-base.h`](srctree/include/uapi/asm-generic/errno-base.h)
 
 use crate::str::CStr;
 

rust/kernel/ioctl.rs

@@ -2,7 +2,7 @@
 
 //! ioctl() number definitions
 //!
-//! C header: [`include/asm-generic/ioctl.h`](../../../../include/asm-generic/ioctl.h)
+//! C header: [`include/asm-generic/ioctl.h`](srctree/include/asm-generic/ioctl.h)
 
 #![allow(non_snake_case)]
 

rust/kernel/kunit.rs

@@ -2,7 +2,7 @@
 
 //! KUnit-based macros for Rust unit tests.
 //!
-//! C header: [`include/kunit/test.h`](../../../../../include/kunit/test.h)
+//! C header: [`include/kunit/test.h`](srctree/include/kunit/test.h)
 //!
 //! Reference: <https://docs.kernel.org/dev-tools/kunit/index.html>
 

rust/kernel/print.rs

@@ -2,7 +2,7 @@
 
 //! Printing facilities.
 //!
-//! C header: [`include/linux/printk.h`](../../../../include/linux/printk.h)
+//! C header: [`include/linux/printk.h`](srctree/include/linux/printk.h)
 //!
 //! Reference: <https://www.kernel.org/doc/html/latest/core-api/printk-basics.html>
 
@@ -48,7 +48,7 @@ pub mod format_strings {
     /// The format string is always the same for a given level, i.e. for a
     /// given `prefix`, which are the kernel's `KERN_*` constants.
     ///
-    /// [`_printk`]: ../../../../include/linux/printk.h
+    /// [`_printk`]: srctree/include/linux/printk.h
     const fn generate(is_cont: bool, prefix: &[u8; 3]) -> [u8; LENGTH] {
         // Ensure the `KERN_*` macros are what we expect.
         assert!(prefix[0] == b'\x01');
@@ -97,7 +97,7 @@ pub mod format_strings {
 /// The format string must be one of the ones in [`format_strings`], and
 /// the module name must be null-terminated.
 ///
-/// [`_printk`]: ../../../../include/linux/_printk.h
+/// [`_printk`]: srctree/include/linux/_printk.h
 #[doc(hidden)]
 #[cfg_attr(not(CONFIG_PRINTK), allow(unused_variables))]
 pub unsafe fn call_printk(
@@ -120,7 +120,7 @@ pub unsafe fn call_printk(
 ///
 /// Public but hidden since it should only be used from public macros.
 ///
-/// [`_printk`]: ../../../../include/linux/printk.h
+/// [`_printk`]: srctree/include/linux/printk.h
 #[doc(hidden)]
 #[cfg_attr(not(CONFIG_PRINTK), allow(unused_variables))]
 pub fn call_printk_cont(args: fmt::Arguments<'_>) {

rust/kernel/sync/condvar.rs

@@ -69,7 +69,7 @@ macro_rules! new_condvar {
 /// }
 /// ```
 ///
-/// [`struct wait_queue_head`]: ../../../include/linux/wait.h
+/// [`struct wait_queue_head`]: srctree/include/linux/wait.h
 #[pin_data]
 pub struct CondVar {
     #[pin]

rust/kernel/sync/lock/mutex.rs

@@ -84,7 +84,7 @@ macro_rules! new_mutex {
 /// }
 /// ```
 ///
-/// [`struct mutex`]: ../../../../include/linux/mutex.h
+/// [`struct mutex`]: srctree/include/linux/mutex.h
 pub type Mutex<T> = super::Lock<T, MutexBackend>;
 
 /// A kernel `struct mutex` lock backend.

rust/kernel/sync/lock/spinlock.rs

@@ -82,7 +82,7 @@ macro_rules! new_spinlock {
 /// }
 /// ```
 ///
-/// [`spinlock_t`]: ../../../../include/linux/spinlock.h
+/// [`spinlock_t`]: srctree/include/linux/spinlock.h
 pub type SpinLock<T> = super::Lock<T, SpinLockBackend>;
 
 /// A kernel `spinlock_t` lock backend.

rust/kernel/task.rs

@@ -2,7 +2,7 @@
 
 //! Tasks (threads and processes).
 //!
-//! C header: [`include/linux/sched.h`](../../../../include/linux/sched.h).
+//! C header: [`include/linux/sched.h`](srctree/include/linux/sched.h).
 
 use crate::{bindings, types::Opaque};
 use core::{marker::PhantomData, ops::Deref, ptr};

rust/kernel/workqueue.rs

@@ -132,7 +132,7 @@
 //! }
 //! ```
 //!
-//! C header: [`include/linux/workqueue.h`](../../../../include/linux/workqueue.h)
+//! C header: [`include/linux/workqueue.h`](srctree/include/linux/workqueue.h)
 
 use crate::{bindings, prelude::*, sync::Arc, sync::LockClassKey, types::Opaque};
 use alloc::alloc::AllocError;

rust/macros/lib.rs

@@ -20,7 +20,7 @@ use proc_macro::TokenStream;
 /// The `type` argument should be a type which implements the [`Module`]
 /// trait. Also accepts various forms of kernel metadata.
 ///
-/// C header: [`include/linux/moduleparam.h`](../../../include/linux/moduleparam.h)
+/// C header: [`include/linux/moduleparam.h`](srctree/include/linux/moduleparam.h)
 ///
 /// [`Module`]: ../kernel/trait.Module.html
 ///