Add test for Command::current_dir behavior.

Author: ehuss

Diff for: src/test/ui/command/command-current-dir.rs

@@ -0,0 +1,43 @@
+// run-pass
+// ignore-emscripten no processes
+// ignore-sgx no processes
+
+use std::env;
+use std::fs;
+use std::path::Path;
+use std::process::Command;
+
+fn main() {
+    // Checks the behavior of current_dir when used with a relative exe path.
+    let me = env::current_exe().unwrap();
+    if matches!(env::args().skip(1).next().as_deref(), Some("current-dir")) {
+        let cwd = env::current_dir().unwrap();
+        assert_eq!(cwd.file_name().unwrap(), "bar");
+        std::process::exit(0);
+    }
+    let exe = me.file_name().unwrap();
+    let cwd = std::env::current_dir().unwrap();
+    eprintln!("cwd={:?}", cwd);
+    let foo = cwd.join("foo");
+    let bar = cwd.join("bar");
+    fs::create_dir_all(&foo).unwrap();
+    fs::create_dir_all(&bar).unwrap();
+    fs::copy(&me, foo.join(exe)).unwrap();
+
+    // Unfortunately this is inconsistent based on the platform, see
+    // https://github.com/rust-lang/rust/issues/37868. On Windows,
+    // it is relative *before* changing the directory, and on Unix
+    // it is *after* changing the directory.
+    let relative_exe = if cfg!(windows) {
+        Path::new("foo").join(exe)
+    } else {
+        Path::new("../foo").join(exe)
+    };
+
+    let status = Command::new(relative_exe)
+        .arg("current-dir")
+        .current_dir("bar")
+        .status()
+        .unwrap();
+    assert!(status.success());
+}