Write more tests for the proxy... (#1918)

And change a few more things in the process.

proxy/src/parse.rs

@@ -1,6 +1,5 @@
 //! Small parsing helpers.
 
-use std::convert::TryInto;
 use std::ffi::CStr;
 
 pub fn split_cstr(bytes: &[u8]) -> Option<(&CStr, &[u8])> {
@@ -10,9 +9,36 @@ pub fn split_cstr(bytes: &[u8]) -> Option<(&CStr, &[u8])> {
     Some((unsafe { CStr::from_bytes_with_nul_unchecked(cstr) }, other))
 }
 
+/// See <https://doc.rust-lang.org/std/primitive.slice.html#method.split_array_ref>.
 pub fn split_at_const<const N: usize>(bytes: &[u8]) -> Option<(&[u8; N], &[u8])> {
     (bytes.len() >= N).then(|| {
         let (head, tail) = bytes.split_at(N);
         (head.try_into().unwrap(), tail)
     })
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_split_cstr() {
+        assert!(split_cstr(b"").is_none());
+        assert!(split_cstr(b"foo").is_none());
+
+        let (cstr, rest) = split_cstr(b"\0").expect("uh-oh");
+        assert_eq!(cstr.to_bytes(), b"");
+        assert_eq!(rest, b"");
+
+        let (cstr, rest) = split_cstr(b"foo\0bar").expect("uh-oh");
+        assert_eq!(cstr.to_bytes(), b"foo");
+        assert_eq!(rest, b"bar");
+    }
+
+    #[test]
+    fn test_split_at_const() {
+        assert!(split_at_const::<0>(b"").is_some());
+        assert!(split_at_const::<1>(b"").is_none());
+        assert!(matches!(split_at_const::<1>(b"ok"), Some((b"o", b"k"))));
+    }
+}