Add the SPDX license identifiers, since the initial patch added the
upstream sources with no modifications whatsoever.

Signed-off-by: Ariel Miculas <amiculas@cisco.com>
---
 rust/hex/error.rs | 2 ++
 rust/hex/lib.rs   | 2 ++
 rust/hex/serde.rs | 2 ++
 3 files changed, 6 insertions(+)

diff --git a/rust/hex/error.rs b/rust/hex/error.rs
index ff7a3b5c16bd..e553a046c9ed 100644
--- a/rust/hex/error.rs
+++ b/rust/hex/error.rs
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 use core::fmt;
 
 /// The error type for decoding a hex string into `Vec<u8>` or `[u8; N]`.
diff --git a/rust/hex/lib.rs b/rust/hex/lib.rs
index ec48961b9ffe..a23def3f80db 100644
--- a/rust/hex/lib.rs
+++ b/rust/hex/lib.rs
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 // Copyright (c) 2013-2014 The Rust Project Developers.
 // Copyright (c) 2015-2020 The rust-hex Developers.
 //
diff --git a/rust/hex/serde.rs b/rust/hex/serde.rs
index 335a15132a27..40ab1edb2bc8 100644
--- a/rust/hex/serde.rs
+++ b/rust/hex/serde.rs
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
 //! Hex encoding with `serde`.
 #[cfg_attr(
     all(feature = "alloc", feature = "serde"),
-- 
2.34.1