diff --git a/rust/chg/src/message.rs b/rust/chg/src/message.rs
--- a/rust/chg/src/message.rs
+++ b/rust/chg/src/message.rs
@@ -5,7 +5,7 @@
 
 //! Utility for parsing and building command-server messages.
 
-use bytes::Bytes;
+use bytes::{BufMut, Bytes, BytesMut};
 use std::error;
 use std::ffi::{OsStr, OsString};
 use std::io;
@@ -67,6 +67,45 @@
     }
 }
 
+// allocate large buffer as environment variables can be quite long
+const INITIAL_PACKED_ENV_VARS_CAPACITY: usize = 4096;
+
+/// Packs environment variables of platform encoding into bytes.
+///
+/// # Panics
+///
+/// Panics if key or value contains `\0` character, or key contains '='
+/// character.
+pub fn pack_env_vars_os<I, P>(vars: I) -> Bytes
+where
+    I: IntoIterator<Item = (P, P)>,
+    P: AsRef<OsStr>,
+{
+    let mut vars_iter = vars.into_iter();
+    if let Some((k, v)) = vars_iter.next() {
+        let mut dst = BytesMut::with_capacity(INITIAL_PACKED_ENV_VARS_CAPACITY);
+        pack_env_into(&mut dst, k.as_ref(), v.as_ref());
+        for (k, v) in vars_iter {
+            dst.reserve(1);
+            dst.put_u8(b'\0');
+            pack_env_into(&mut dst, k.as_ref(), v.as_ref());
+        }
+        dst.freeze()
+    } else {
+        Bytes::new()
+    }
+}
+
+fn pack_env_into(dst: &mut BytesMut, k: &OsStr, v: &OsStr) {
+    assert!(!k.as_bytes().contains(&0), "key shouldn't contain NUL");
+    assert!(!k.as_bytes().contains(&b'='), "key shouldn't contain '='");
+    assert!(!v.as_bytes().contains(&0), "value shouldn't contain NUL");
+    dst.reserve(k.as_bytes().len() + 1 + v.as_bytes().len());
+    dst.put_slice(k.as_bytes());
+    dst.put_u8(b'=');
+    dst.put_slice(v.as_bytes());
+}
+
 fn decode_latin1<S>(s: S) -> String
 where
     S: AsRef<[u8]>,
@@ -85,6 +124,7 @@
 mod tests {
     use super::*;
     use std::os::unix::ffi::OsStringExt;
+    use std::panic;
 
     #[test]
     fn parse_command_spec_good() {
@@ -127,7 +167,66 @@
         assert!(parse_command_spec(Bytes::from_static(b"paper\0less")).is_err());
     }
 
+    #[test]
+    fn pack_env_vars_os_good() {
+        assert_eq!(
+            pack_env_vars_os(vec![] as Vec<(OsString, OsString)>),
+            Bytes::new()
+        );
+        assert_eq!(
+            pack_env_vars_os(vec![os_string_pair_from(b"FOO", b"bar")]),
+            Bytes::from_static(b"FOO=bar")
+        );
+        assert_eq!(
+            pack_env_vars_os(vec![
+                os_string_pair_from(b"FOO", b""),
+                os_string_pair_from(b"BAR", b"baz")
+            ]),
+            Bytes::from_static(b"FOO=\0BAR=baz")
+        );
+    }
+
+    #[test]
+    fn pack_env_vars_os_large_key() {
+        let mut buf = vec![b'A'; INITIAL_PACKED_ENV_VARS_CAPACITY];
+        let envs = vec![os_string_pair_from(&buf, b"")];
+        buf.push(b'=');
+        assert_eq!(pack_env_vars_os(envs), Bytes::from(buf));
+    }
+
+    #[test]
+    fn pack_env_vars_os_large_value() {
+        let mut buf = vec![b'A', b'='];
+        buf.resize(INITIAL_PACKED_ENV_VARS_CAPACITY + 1, b'a');
+        let envs = vec![os_string_pair_from(&buf[..1], &buf[2..])];
+        assert_eq!(pack_env_vars_os(envs), Bytes::from(buf));
+    }
+
+    #[test]
+    fn pack_env_vars_os_nul_eq() {
+        assert!(panic::catch_unwind(|| {
+            pack_env_vars_os(vec![os_string_pair_from(b"\0", b"")])
+        })
+        .is_err());
+        assert!(panic::catch_unwind(|| {
+            pack_env_vars_os(vec![os_string_pair_from(b"FOO", b"\0bar")])
+        })
+        .is_err());
+        assert!(panic::catch_unwind(|| {
+            pack_env_vars_os(vec![os_string_pair_from(b"FO=", b"bar")])
+        })
+        .is_err());
+        assert_eq!(
+            pack_env_vars_os(vec![os_string_pair_from(b"FOO", b"=ba")]),
+            Bytes::from_static(b"FOO==ba")
+        );
+    }
+
     fn os_string_from(s: &[u8]) -> OsString {
         OsString::from_vec(s.to_vec())
     }
+
+    fn os_string_pair_from(k: &[u8], v: &[u8]) -> (OsString, OsString) {
+        (os_string_from(k), os_string_from(v))
+    }
 }