[web] confirm safety of advance

After further testing and checks I'm fairy confident that the vectored
write code is safe.

Signed-off-by: Ben Aaron Goldberg <ben@benaaron.dev>

web/src/write_vectored_all.rs

@@ -77,7 +77,9 @@ fn advance<'a>(buf: &mut IoSlice<'a>, n: usize) {
 	if buf.len() < n {
 	    panic!("advancing IoSlice beyond its length");
 	}
-	// SAFTEY: hopefully
+	// This is just a hacky way of advancing the pointer inside the IoSlice
+	// SAFTEY: The newly constructed IoSlice has the same lifetime as the old and
+	//         this is guaranteed not to overflow the buffer due to the previous check
 	unsafe {
 		let mut ptr = buf.as_ptr() as *mut u8;
 		ptr = ptr.add(n);
@@ -87,3 +89,17 @@ fn advance<'a>(buf: &mut IoSlice<'a>, n: usize) {
 	}
 }
 
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn test_advance() {
+		let expected: Vec<_> = (10..100).collect();
+		let buf: Vec<_> = (0..100).collect();
+		let mut io_slice = IoSlice::new(&buf);
+		advance(&mut io_slice, 10);
+		assert_eq!(io_slice.len(), 90);
+		assert_eq!(&*io_slice, &expected);
+	}
+}