rust: str: add `CString` type

Add the `CString` type, which is an owned string that is guaranteed
to have exactly one `NUL` byte at the end, i.e. the owned equivalent
to `CStr` introduced earlier.

It is used for interoperability with kernel APIs that take C strings.

In order to do so, implement the `RawFormatter::new()` constructor
and the `RawFormatter::bytes_written()` method as well.

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/str.rs

@@ -2,6 +2,7 @@
 
 //! String representations.
 
+use alloc::vec::Vec;
 use core::fmt::{self, Write};
 use core::ops::{self, Deref, Index};
 
@@ -384,13 +385,22 @@ mod tests {
 /// is less than `end`.
 pub(crate) struct RawFormatter {
     // Use `usize` to use `saturating_*` functions.
-    #[allow(dead_code)]
     beg: usize,
     pos: usize,
     end: usize,
 }
 
 impl RawFormatter {
+    /// Creates a new instance of [`RawFormatter`] with an empty buffer.
+    fn new() -> Self {
+        // INVARIANT: The buffer is empty, so the region that needs to be writable is empty.
+        Self {
+            beg: 0,
+            pos: 0,
+            end: 0,
+        }
+    }
+
     /// Creates a new instance of [`RawFormatter`] with the given buffer pointers.
     ///
     /// # Safety
@@ -429,6 +439,11 @@ impl RawFormatter {
     pub(crate) fn pos(&self) -> *mut u8 {
         self.pos as _
     }
+
+    /// Return the number of bytes written to the formatter.
+    pub(crate) fn bytes_written(&self) -> usize {
+        self.pos - self.beg
+    }
 }
 
 impl fmt::Write for RawFormatter {
@@ -469,7 +484,6 @@ impl Formatter {
     ///
     /// The memory region starting at `buf` and extending for `len` bytes must be valid for writes
     /// for the lifetime of the returned [`Formatter`].
-    #[allow(dead_code)]
     pub(crate) unsafe fn from_buffer(buf: *mut u8, len: usize) -> Self {
         // SAFETY: The safety requirements of this function satisfy those of the callee.
         Self(unsafe { RawFormatter::from_buffer(buf, len) })
@@ -496,3 +510,76 @@ impl fmt::Write for Formatter {
         }
     }
 }
+
+/// An owned string that is guaranteed to have exactly one `NUL` byte, which is at the end.
+///
+/// Used for interoperability with kernel APIs that take C strings.
+///
+/// # Invariants
+///
+/// The string is always `NUL`-terminated and contains no other `NUL` bytes.
+///
+/// # Examples
+///
+/// ```
+/// use kernel::str::CString;
+///
+/// let s = CString::try_from_fmt(fmt!("{}{}{}", "abc", 10, 20)).unwrap();
+/// assert_eq!(s.as_bytes_with_nul(), "abc1020\0".as_bytes());
+///
+/// let tmp = "testing";
+/// let s = CString::try_from_fmt(fmt!("{tmp}{}", 123)).unwrap();
+/// assert_eq!(s.as_bytes_with_nul(), "testing123\0".as_bytes());
+///
+/// // This fails because it has an embedded `NUL` byte.
+/// let s = CString::try_from_fmt(fmt!("a\0b{}", 123));
+/// assert_eq!(s.is_ok(), false);
+/// ```
+pub struct CString {
+    buf: Vec<u8>,
+}
+
+impl CString {
+    /// Creates an instance of [`CString`] from the given formatted arguments.
+    pub fn try_from_fmt(args: fmt::Arguments<'_>) -> Result<Self, Error> {
+        // Calculate the size needed (formatted string plus `NUL` terminator).
+        let mut f = RawFormatter::new();
+        f.write_fmt(args)?;
+        f.write_str("\0")?;
+        let size = f.bytes_written();
+
+        // Allocate a vector with the required number of bytes, and write to it.
+        let mut buf = Vec::try_with_capacity(size)?;
+        // SAFETY: The buffer stored in `buf` is at least of size `size` and is valid for writes.
+        let mut f = unsafe { Formatter::from_buffer(buf.as_mut_ptr(), size) };
+        f.write_fmt(args)?;
+        f.write_str("\0")?;
+
+        // SAFETY: The number of bytes that can be written to `f` is bounded by `size`, which is
+        // `buf`'s capacity. The contents of the buffer have been initialised by writes to `f`.
+        unsafe { buf.set_len(f.bytes_written()) };
+
+        // Check that there are no `NUL` bytes before the end.
+        // SAFETY: The buffer is valid for read because `f.bytes_written()` is bounded by `size`
+        // (which the minimum buffer size) and is non-zero (we wrote at least the `NUL` terminator)
+        // so `f.bytes_written() - 1` doesn't underflow.
+        let ptr = unsafe { bindings::memchr(buf.as_ptr().cast(), 0, (f.bytes_written() - 1) as _) };
+        if !ptr.is_null() {
+            return Err(EINVAL);
+        }
+
+        // INVARIANT: We wrote the `NUL` terminator and checked above that no other `NUL` bytes
+        // exist in the buffer.
+        Ok(Self { buf })
+    }
+}
+
+impl Deref for CString {
+    type Target = CStr;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: The type invariants guarantee that the string is `NUL`-terminated and that no
+        // other `NUL` bytes exist.
+        unsafe { CStr::from_bytes_with_nul_unchecked(self.buf.as_slice()) }
+    }
+}