Only enable error_arc_ref past Rust 1.52

Requires `impl Error for Arc<T>`, which only exists on recent versions.

Cargo.toml

@@ -69,14 +69,15 @@ release_max_level_trace = []
 erased-serde = { version = "0.3", optional = true }
 serde = { version = "1", optional = true }
 serde_derive = { version = "1", optional = true }
+
+[dev-dependencies]
 # NOTE: This is just a macro (not a runtime dependency)
 #
 # It is used to conditionally enable use of newer rust language
 # features depending on the compiler features.
-# Currently, this is not needed because our MSRV is sufficient
-# for all the features we use.
-# However, in the future we may need to re-add it.
-# rustversion = "1"
+#
+# For the time being, this is only needed for tests.
+rustversion = "1"
 
 [[example]]
 name = "singlethread"

src/tests.rs

@@ -148,6 +148,8 @@ mod std_only {
         info!(logger, "foo"; "error" => #&error);
     }
 
+    // requires `impl Error for Arc<T>` - since 1.52
+    #[rustversion::since(1.52)]
     #[test]
     fn error_arc_ref() {
         let error = TestError::new("foo");