1 files changed, 72 insertions, 0 deletions
diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
new file mode 100644
index 000000000..2c4b4bac2
--- /dev/null
+++ b/rust/kernel/str.rs
@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! String representations.
+
+use core::fmt;
+
+/// Allows formatting of [`fmt::Arguments`] into a raw buffer.
+///
+/// It does not fail if callers write past the end of the buffer so that they can calculate the
+/// size required to fit everything.
+///
+/// # Invariants
+///
+/// The memory region between `pos` (inclusive) and `end` (exclusive) is valid for writes if `pos`
+/// 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 the given buffer pointers.
+    ///
+    /// # Safety
+    ///
+    /// If `pos` is less than `end`, then the region between `pos` (inclusive) and `end`
+    /// (exclusive) must be valid for writes for the lifetime of the returned [`RawFormatter`].
+    pub(crate) unsafe fn from_ptrs(pos: *mut u8, end: *mut u8) -> Self {
+        // INVARIANT: The safety requirements guarantee the type invariants.
+        Self {
+            beg: pos as _,
+            pos: pos as _,
+            end: end as _,
+        }
+    }
+
+    /// Returns the current insert position.
+    ///
+    /// N.B. It may point to invalid memory.
+    pub(crate) fn pos(&self) -> *mut u8 {
+        self.pos as _
+    }
+}
+
+impl fmt::Write for RawFormatter {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        // `pos` value after writing `len` bytes. This does not have to be bounded by `end`, but we
+        // don't want it to wrap around to 0.
+        let pos_new = self.pos.saturating_add(s.len());
+
+        // Amount that we can copy. `saturating_sub` ensures we get 0 if `pos` goes past `end`.
+        let len_to_copy = core::cmp::min(pos_new, self.end).saturating_sub(self.pos);
+
+        if len_to_copy > 0 {
+            // SAFETY: If `len_to_copy` is non-zero, then we know `pos` has not gone past `end`
+            // yet, so it is valid for write per the type invariants.
+            unsafe {
+                core::ptr::copy_nonoverlapping(
+                    s.as_bytes().as_ptr(),
+                    self.pos as *mut u8,
+                    len_to_copy,
+                )
+            };
+        }
+
+        self.pos = pos_new;
+        Ok(())
+    }
+}