Build version-dependent examples only when supported

This apparently does not reduce noise in CI but at least allows to build
the examples with Rust 1.59.0.

Cargo.toml

@@ -47,6 +47,7 @@ assert_hex = "0.4.1"
 clap = { version = "3.1.6", features = ["derive"] }
 envconfig = "0.10.0"
 rstest = { version = "0.12.0", default-features = false }
+rustversion = "1.0.16"
 
 [features]
 default = ["libudev"]

examples/loopback.rs

@@ -193,6 +193,16 @@ fn loopback_standard<'a>(
     }
 }
 
+#[rustversion::before(1.63)]
+fn loopback_split<'a>(
+    _port: &mut Box<dyn SerialPort>,
+    _read_stats: &mut Stats<'a>,
+    _write_stats: &mut Stats<'a>,
+) {
+    unimplemented!("requires Rust 1.63 or later");
+}
+
+#[rustversion::since(1.63)]
 fn loopback_split<'a>(
     port: &mut Box<dyn SerialPort>,
     read_stats: &mut Stats<'a>,