Add custom error message for use of --native on unsupported platfor…

…ms (#700)

src/config.rs

@@ -160,12 +160,16 @@ impl Config {
             .help("PID of a running python program to spy on")
             .takes_value(true);
 
-        #[cfg(feature = "unwind")]
-        let native = Arg::new("native")
+        let mut native = Arg::new("native")
             .short('n')
             .long("native")
             .help("Collect stack traces from native extensions written in Cython, C or C++");
 
+        // Only show `--native` on platforms where it's supported
+        if !cfg!(feature = "unwind") {
+            native = native.hide(true);
+        }
+
         #[cfg(not(target_os="freebsd"))]
         let nonblocking = Arg::new("nonblocking")
                     .long("nonblocking")
@@ -327,12 +331,8 @@ impl Config {
                     .help("Shell type"),
             );
 
-        // add native unwinding if appropriate
-        #[cfg(feature = "unwind")]
         let record = record.arg(native.clone());
-        #[cfg(feature = "unwind")]
         let top = top.arg(native.clone());
-        #[cfg(feature = "unwind")]
         let dump = dump.arg(native.clone());
 
         // Nonblocking isn't an option for freebsd, remove
@@ -361,6 +361,14 @@ impl Config {
 
         let (subcommand, matches) = matches.subcommand().unwrap();
 
+        // Check if `--native` was used on an unsupported platform
+        if !cfg!(feature = "unwind") && matches.contains_id("native") {
+            eprintln!(
+                "Collecting stack traces from native extensions (`--native`) is not supported on your platform."
+            );
+            std::process::exit(1);
+        }
+
         match subcommand {
             "record" => {
                 config.sampling_rate = matches.value_of_t("rate")?;