[PATCH v9 1/4] rust: move `CStr`'s `Display` to helper struct

[Tamir Duberstein]

Remove `impl Display for CStr` in preparation for replacing `CStr` with
`core::ffi::CStr` which doesn't impl `Display`. Add `CStr::display`
returning a helper struct to replace the lost functionality; this
matches the APIs exposed by `std::ffi::OSstr` and `std::path::Path` for
printing non-Unicode data.

Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxx>
---
 rust/kernel/kunit.rs |  9 +++++---
 rust/kernel/str.rs   | 63 ++++++++++++++++++++++++++++++++++++++++++----------
 2 files changed, 57 insertions(+), 15 deletions(-)

diff --git a/rust/kernel/kunit.rs b/rust/kernel/kunit.rs
index 824da0e9738a..630b947c708c 100644
--- a/rust/kernel/kunit.rs
+++ b/rust/kernel/kunit.rs
@@ -56,6 +56,7 @@ macro_rules! kunit_assert {
                 break 'out;
             }
 
+            static NAME: &'static $crate::str::CStr = $crate::c_str!($name);
             static FILE: &'static $crate::str::CStr = $crate::c_str!($file);
             static LINE: i32 = core::line!() as i32 - $diff;
             static CONDITION: &'static $crate::str::CStr = $crate::c_str!(stringify!($condition));
@@ -71,11 +72,13 @@ macro_rules! kunit_assert {
                 //
                 // This mimics KUnit's failed assertion format.
                 $crate::kunit::err(format_args!(
-                    "    # {}: ASSERTION FAILED at {FILE}:{LINE}\n",
-                    $name
+                    "    # {NAME}: ASSERTION FAILED at {FILE}:{LINE}\n",
+                    NAME = NAME.display(),
+                    FILE = FILE.display(),
                 ));
                 $crate::kunit::err(format_args!(
-                    "    Expected {CONDITION} to be true, but is false\n"
+                    "    Expected {CONDITION} to be true, but is false\n",
+                    CONDITION = CONDITION.display(),
                 ));
                 $crate::kunit::err(format_args!(
                     "    Failure not reported to KUnit since this is a non-KUnit task\n"
diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index 28e2201604d6..50eb4266047a 100644
--- a/rust/kernel/str.rs
+++ b/rust/kernel/str.rs
@@ -376,27 +376,66 @@ pub fn to_ascii_uppercase(&self) -> Result<CString, AllocError> {
 
         Ok(s)
     }
-}
 
-impl fmt::Display for CStr {
-    /// Formats printable ASCII characters, escaping the rest.
+    /// Returns an object that implements [`Display`] for safely printing a [`CStr`] that may
+    /// contain non-Unicode data. If you would like an implementation which escapes the [`CStr`]
+    /// please use [`Debug`] instead.
+    ///
+    /// [`Display`]: fmt::Display
+    /// [`Debug`]: fmt::Debug
+    ///
+    /// # Examples
     ///
     /// ```
     /// # use kernel::c_str;
     /// # use kernel::fmt;
-    /// # use kernel::str::CStr;
     /// # use kernel::str::CString;
     /// let penguin = c_str!("🐧");
-    /// let s = CString::try_from_fmt(fmt!("{}", penguin))?;
+    /// let s = CString::try_from_fmt(fmt!("{}", penguin.display()))?;
     /// assert_eq!(s.as_bytes_with_nul(), "\\xf0\\x9f\\x90\\xa7\0".as_bytes());
     ///
     /// let ascii = c_str!("so \"cool\"");
-    /// let s = CString::try_from_fmt(fmt!("{}", ascii))?;
+    /// let s = CString::try_from_fmt(fmt!("{}", ascii.display()))?;
     /// assert_eq!(s.as_bytes_with_nul(), "so \"cool\"\0".as_bytes());
     /// # Ok::<(), kernel::error::Error>(())
     /// ```
+    #[inline]
+    pub fn display(&self) -> Display<'_> {
+        Display { inner: self }
+    }
+}
+
+/// Helper struct for safely printing a [`CStr`] with [`fmt!`] and `{}`.
+///
+/// A [`CStr`] might contain non-Unicode data. This `struct` implements the [`Display`] trait in a
+/// way that mitigates that. It is created by the [`display`](CStr::display) method on [`CStr`].
+///
+/// If you would like an implementation which escapes the string please use [`Debug`] instead.
+///
+/// # Examples
+///
+/// ```
+/// # use kernel::{fmt, c_str, str::CString};
+/// let ascii = c_str!("Hello, CStr!");
+/// let s = CString::try_from_fmt(fmt!("{}", ascii.display()))?;
+/// assert_eq!(s.as_bytes(), "Hello, CStr!".as_bytes());
+///
+/// let non_ascii = c_str!("🦀");
+/// let s = CString::try_from_fmt(fmt!("{}", non_ascii.display()))?;
+/// assert_eq!(s.as_bytes(), "\\xf0\\x9f\\xa6\\x80".as_bytes());
+/// # Ok::<(), kernel::error::Error>(())
+/// ```
+///
+/// [`fmt!`]: crate::fmt
+/// [`Debug`]: fmt::Debug
+/// [`Display`]: fmt::Display
+pub struct Display<'a> {
+    inner: &'a CStr,
+}
+
+impl fmt::Display for Display<'_> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        for &c in self.as_bytes() {
+        for &c in self.inner.as_bytes() {
             if (0x20..0x7f).contains(&c) {
                 // Printable character.
                 f.write_char(c as char)?;
@@ -595,13 +634,13 @@ fn test_cstr_as_str_unchecked() {
     #[test]
     fn test_cstr_display() {
         let hello_world = CStr::from_bytes_with_nul(b"hello, world!\0").unwrap();
-        assert_eq!(format!("{}", hello_world), "hello, world!");
+        assert_eq!(format!("{}", hello_world.display()), "hello, world!");
         let non_printables = CStr::from_bytes_with_nul(b"\x01\x09\x0a\0").unwrap();
-        assert_eq!(format!("{}", non_printables), "\\x01\\x09\\x0a");
+        assert_eq!(format!("{}", non_printables.display()), "\\x01\\x09\\x0a");
         let non_ascii = CStr::from_bytes_with_nul(b"d\xe9j\xe0 vu\0").unwrap();
-        assert_eq!(format!("{}", non_ascii), "d\\xe9j\\xe0 vu");
+        assert_eq!(format!("{}", non_ascii.display()), "d\\xe9j\\xe0 vu");
         let good_bytes = CStr::from_bytes_with_nul(b"\xf0\x9f\xa6\x80\0").unwrap();
-        assert_eq!(format!("{}", good_bytes), "\\xf0\\x9f\\xa6\\x80");
+        assert_eq!(format!("{}", good_bytes.display()), "\\xf0\\x9f\\xa6\\x80");
     }
 
     #[test]
@@ -612,7 +651,7 @@ fn test_cstr_display_all_bytes() {
             bytes[i as usize] = i.wrapping_add(1);
         }
         let cstr = CStr::from_bytes_with_nul(&bytes).unwrap();
-        assert_eq!(format!("{}", cstr), ALL_ASCII_CHARS);
+        assert_eq!(format!("{}", cstr.display()), ALL_ASCII_CHARS);
     }
 
     #[test]

-- 
2.48.1