Use real exec on cfg(unix) targets

Author: saethlin

cargo-miri/bin.rs

@@ -239,9 +239,22 @@ fn xargo_check() -> Command {
 /// Execute the command. If it fails, fail this process with the same exit code.
 /// Otherwise, continue.
 fn exec(mut cmd: Command) {
-    let exit_status = cmd.status().expect("failed to run command");
-    if exit_status.success().not() {
-        std::process::exit(exit_status.code().unwrap_or(-1))
+    // On non-Unix imitate POSIX exec as closely as we can
+    #[cfg(not(unix))]
+    {
+        let exit_status = cmd.status().expect("failed to run command");
+        if exit_status.success().not() {
+            std::process::exit(exit_status.code().unwrap_or(-1))
+        }
+    }
+    // On Unix targets, actually exec
+    // If exec returns, process setup has failed. This is the same error condition as the expect in
+    // the non-Unix case.
+    #[cfg(unix)]
+    {
+        use std::os::unix::process::CommandExt;
+        let error = cmd.exec();
+        Err(error).expect("failed to run command")
     }
 }