diff --git a/src/mem.rs b/src/mem.rs
index 90a5d6d40..618194a40 100644
--- a/src/mem.rs
+++ b/src/mem.rs
@@ -1,14 +1,109 @@
-/// SAFETY: the returned Vec must not be resized or reserverd
-pub unsafe fn aligned_u8_vec(cap: usize, align: usize) -> Vec<u8> {
-    use std::alloc::*;
+use std::{
+    alloc::{alloc, dealloc, handle_alloc_error, Layout},
+    ops::Deref,
+    ptr::NonNull,
+};
 
-    let layout =
-        Layout::from_size_align(cap, align).expect("invalid aligned value, must be power of 2");
-    unsafe {
-        let ptr = alloc(layout);
-        if ptr.is_null() {
-            panic!("failed to allocate {} bytes", cap);
+/// An owned byte buffer with a caller-specified allocation alignment.
+pub struct AlignedU8Vec {
+    ptr: NonNull<u8>,
+    len: usize,
+    capacity: usize,
+    layout: Layout,
+}
+
+// SAFETY: AlignedU8Vec uniquely owns its allocation and exposes mutation only
+// through exclusive access, so moving or sharing it is as safe as Vec<u8>.
+unsafe impl Send for AlignedU8Vec {}
+unsafe impl Sync for AlignedU8Vec {}
+
+impl AlignedU8Vec {
+    /// Appends bytes without exceeding the capacity requested at allocation.
+    pub fn extend_from_slice(&mut self, data: &[u8]) {
+        assert!(
+            data.len() <= self.capacity - self.len,
+            "extend beyond fixed capacity"
+        );
+        unsafe {
+            self.ptr
+                .as_ptr()
+                .add(self.len)
+                .copy_from_nonoverlapping(data.as_ptr(), data.len());
         }
-        Vec::from_raw_parts(ptr, 0, cap)
+        self.len += data.len();
+    }
+}
+
+impl Deref for AlignedU8Vec {
+    type Target = [u8];
+
+    fn deref(&self) -> &Self::Target {
+        unsafe { std::slice::from_raw_parts(self.ptr.as_ptr(), self.len) }
+    }
+}
+
+impl Drop for AlignedU8Vec {
+    fn drop(&mut self) {
+        unsafe {
+            dealloc(self.ptr.as_ptr(), self.layout);
+        }
+    }
+}
+
+/// Allocates an empty byte buffer with the requested capacity and alignment.
+pub fn aligned_u8_vec(cap: usize, align: usize) -> AlignedU8Vec {
+    let layout = Layout::from_size_align(cap.max(1), align)
+        .expect("invalid aligned value, must be power of 2");
+    unsafe {
+        let ptr = NonNull::new(alloc(layout)).unwrap_or_else(|| handle_alloc_error(layout));
+        AlignedU8Vec {
+            ptr,
+            len: 0,
+            capacity: cap,
+            layout,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn assert_send_sync<T: Send + Sync>() {}
+
+    #[test]
+    fn is_send_and_sync() {
+        assert_send_sync::<AlignedU8Vec>();
+    }
+
+    #[test]
+    fn preserves_alignment_and_contents() {
+        let mut data = aligned_u8_vec(10, 4096);
+        data.extend_from_slice(&[1, 2, 3, 4]);
+        assert_eq!(data.as_ptr() as usize % 4096, 0);
+        assert_eq!(&*data, &[1, 2, 3, 4]);
+    }
+
+    #[test]
+    #[should_panic(expected = "extend beyond fixed capacity")]
+    fn rejects_extension_when_capacity_is_zero() {
+        let mut data = aligned_u8_vec(0, 4);
+        data.extend_from_slice(&[1]);
+    }
+
+    #[test]
+    fn accumulates_across_multiple_extends() {
+        let mut data = aligned_u8_vec(8, 4);
+        data.extend_from_slice(&[1, 2, 3]);
+        data.extend_from_slice(&[4, 5]);
+        assert_eq!(&*data, &[1, 2, 3, 4, 5]);
+    }
+
+    #[test]
+    #[should_panic(expected = "extend beyond fixed capacity")]
+    fn rejects_extension_past_partial_fill() {
+        let mut data = aligned_u8_vec(4, 4);
+        data.extend_from_slice(&[1, 2, 3]);
+        data.extend_from_slice(&[4, 5]);
     }
 }