diff --git a/src/uu/factor/Cargo.toml b/src/uu/factor/Cargo.toml
index 1142560d9..f79afeecf 100644
--- a/src/uu/factor/Cargo.toml
+++ b/src/uu/factor/Cargo.toml
@@ -19,7 +19,7 @@ clap = { version = "2.33", features = ["wrap_help"] }
 coz = { version = "0.1.3", optional = true }
 num-traits = "0.2.13" # Needs at least version 0.2.13 for "OverflowingAdd"
 rand = { version = "0.7", features = ["small_rng"] }
-smallvec = "1.7"
+smallvec = "1.7"  # TODO(nicoo): Use `union` feature, requires Rust 1.49 or later.
 uucore = { version = ">=0.0.8", package = "uucore", path = "../../uucore" }
 uucore_procs = { version=">=0.0.7", package = "uucore_procs", path = "../../uucore_procs" }