1 files changed, 28 insertions, 0 deletions

diff --git a/rsnix/src/strbuf.rs b/rsnix/src/strbuf.rs
index 262720d..0e3ea56 100644
--- a/rsnix/src/strbuf.rs
+++ b/rsnix/src/strbuf.rs
@@ -69,3 +69,31 @@ impl<const N: usize> fmt::Debug for String<N> {
         write!(f, "{:?}", self.as_str())
     }
 }
+
+/// Small `fmt::Write` over a byte slice — no allocation, bounded.
+/// Shared by the panic handler (task-panic message) and the scheduler
+/// (page-fault abort message) so they don't each reinvent it.
+pub struct FmtBuf<'a> {
+    b: &'a mut [u8],
+    n: usize,
+}
+
+impl<'a> FmtBuf<'a> {
+    pub fn new(b: &'a mut [u8]) -> Self {
+        FmtBuf { b, n: 0 }
+    }
+
+    /// Bytes written so far (clamped at the buffer length).
+    pub fn len(&self) -> usize {
+        self.n
+    }
+}
+
+impl fmt::Write for FmtBuf<'_> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        let m = s.len().min(self.b.len() - self.n);
+        self.b[self.n..self.n + m].copy_from_slice(&s.as_bytes()[..m]);
+        self.n += m;
+        Ok(())
+    }
+}