[PATCH v2 4/5] rust: Introduce file_from_location()

[Boqun Feng <boqun.feng@gmail.com>]

Most of kernel debugging facilities take a nul-terminated string for
file names for a callsite (generated from __FILE__), however the Rust
courterpart, Location, would return a Rust string (not nul-terminated)
from method .file(). And such a string cannot be passed to C debugging
function directly.

There is ongoing work to support a Location::file_with_nul() [1], which
returns a nul-terminated string from a Location. Since it's still
working in progress, and it will take some time before the feature
finally gets stabilized and the kernel's minimal rustc version might
also take a while to bump to a version that at least has that feature,
introduce a file_from_location() function, which returns a warning
string if Location::file_with_nul() is not available.

This should work in most cases because as for now the known usage of
Location::file_with_nul() is only in debugging code (e.g. might_sleep())
and there might be other information reported by the debugging code that
could help locate the problematic function, so missing the file name is
fine at the moment.

Link: https://github.com/rust-lang/rust/issues/141727 [1]
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250619151007.61767-2-boqun.feng@gmail.com
---
 init/Kconfig       |  3 +++
 rust/kernel/lib.rs | 48 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index af4c2f085455..6f4ec5633ffa 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -142,6 +142,9 @@ config RUSTC_HAS_SPAN_FILE
 config RUSTC_HAS_UNNECESSARY_TRANSMUTES
 	def_bool RUSTC_VERSION >= 108800
 
+config RUSTC_HAS_FILE_WITH_NUL
+	def_bool RUSTC_VERSION >= 108900
+
 config PAHOLE_VERSION
 	int
 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 6b4774b2b1c3..717a5b6160ca 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -40,6 +40,10 @@
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(coerce_unsized))]
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(dispatch_from_dyn))]
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(unsize))]
+//
+// `feature(file_with_nul)` is expected to become stable. Before Rust 1.89.0, it did not exist, so
+// enable it conditionally.
+#![cfg_attr(CONFIG_RUSTC_HAS_FILE_WITH_NUL, feature(file_with_nul))]
 
 // Ensure conditional compilation based on the kernel configuration works;
 // otherwise we may silently break things like initcall handling.
@@ -274,3 +278,47 @@ macro_rules! asm {
         ::core::arch::asm!( $($asm)*, $($rest)* )
     };
 }
+
+/// Gets the C string file name of a [`Location`].
+///
+/// If `file_with_nul()` is not available, returns a string that warns about it.
+///
+/// [`Location`]: core::panic::Location
+///
+/// # Examples
+///
+/// ```
+/// # use kernel::file_from_location;
+///
+/// #[track_caller]
+/// fn foo() {
+///     let caller = core::panic::Location::caller();
+///
+///     // Output:
+///     // - A path like "rust/kernel/example.rs" if file_with_nul() is available.
+///     // - "<Location::file_with_nul() not supported>" otherwise.
+///     let caller_file = file_from_location(caller);
+///
+///     // Prints out the message with caller's file name.
+///     pr_info!("foo() called in file {caller_file:?}\n");
+///
+///     # if cfg!(CONFIG_RUSTC_HAS_FILE_WITH_NUL) {
+///     #     assert_eq!(Ok(caller.file()), caller_file.to_str());
+///     # }
+/// }
+///
+/// # foo();
+/// ```
+#[inline]
+pub fn file_from_location<'a>(loc: &'a core::panic::Location<'a>) -> &'a core::ffi::CStr {
+    #[cfg(CONFIG_RUSTC_HAS_FILE_WITH_NUL)]
+    {
+        loc.file_with_nul()
+    }
+
+    #[cfg(not(CONFIG_RUSTC_HAS_FILE_WITH_NUL))]
+    {
+        let _ = loc;
+        c"<Location::file_with_nul() not supported>"
+    }
+}
-- 
2.39.5 (Apple Git-154)