feat(buf): make slice begin at uninit possible

compio-buf/src/io_buf.rs

@@ -64,16 +64,12 @@ pub unsafe trait IoBuf: 'static {
     {
         use std::ops::Bound;
 
-        let len = self.buf_len();
-
         let begin = match range.start_bound() {
             Bound::Included(&n) => n,
             Bound::Excluded(&n) => n + 1,
             Bound::Unbounded => 0,
         };
 
-        assert!(begin <= len);
-
         let end = match range.end_bound() {
             Bound::Included(&n) => Some(n.checked_add(1).expect("out of range")),
             Bound::Excluded(&n) => Some(n),

compio-buf/src/slice.rs

@@ -64,6 +64,9 @@ impl<T: IoBuf> Deref for Slice<T> {
 
     fn deref(&self) -> &Self::Target {
         let bytes = self.buffer.as_slice();
+        if self.begin >= bytes.len() {
+            return &[];
+        }
         let end = self.end.unwrap_or(bytes.len()).min(bytes.len());
         &bytes[self.begin..end]
     }
@@ -72,6 +75,9 @@ impl<T: IoBuf> Deref for Slice<T> {
 impl<T: IoBufMut> DerefMut for Slice<T> {
     fn deref_mut(&mut self) -> &mut Self::Target {
         let begin = self.begin();
+        if begin >= self.buffer.buf_len() {
+            return &mut [];
+        }
         let buf_len = self.buffer.buf_len();
         let end = self.end().unwrap_or(buf_len).min(buf_len);
         let ptr = unsafe { self.buffer.buffer() }.as_ptr();
@@ -299,3 +305,22 @@ impl<T: IoVectoredBufMut> IoVectoredBufMut for VectoredSliceMut<T> {
         self.buf.capacity_of(idx + self.idx)
     }
 }
+
+#[test]
+fn test_slice() {
+    let buf = b"hello world";
+    let slice = buf.slice(6..);
+    assert_eq!(slice.as_slice(), b"world");
+
+    let slice = buf.slice(..5);
+    assert_eq!(slice.as_slice(), b"hello");
+
+    let slice = buf.slice(3..8);
+    assert_eq!(slice.as_slice(), b"lo wo");
+
+    let slice = buf.slice(..);
+    assert_eq!(slice.as_slice(), b"hello world");
+
+    let slice = buf.slice(12..);
+    assert_eq!(slice.as_slice(), b"");
+}