Fix Windows Command search path bug

Author: ChrisDenton

library/std/src/sys/pal/windows/process.rs

@@ -260,9 +260,10 @@ impl Command {
         needs_stdin: bool,
         proc_thread_attribute_list: Option<&ProcThreadAttributeList<'_>>,
     ) -> io::Result<(Process, StdioPipes)> {
+        let env_saw_path = self.env.have_changed_path();
         let maybe_env = self.env.capture_if_changed();
 
-        let child_paths = if let Some(env) = maybe_env.as_ref() {
+        let child_paths = if env_saw_path && let Some(env) = maybe_env.as_ref() {
             env.get(&EnvKey::new("PATH")).map(|s| s.as_os_str())
         } else {
             None

tests/ui/process/win-command-child-path.rs

@@ -0,0 +1,72 @@
+//@ run-pass
+//@ only-windows
+//@ needs-subprocess
+//@ no-prefer-dynamic
+
+// Test Windows std::process::Command search path semantics when setting PATH on the child process.
+// NOTE: The exact semantics here are (possibly) subject to change.
+
+use std::process::Command;
+use std::{env, fs, path};
+
+fn main() {
+    if env::args().skip(1).any(|s| s == "--child") {
+        child();
+    } else if env::args().skip(1).any(|s| s == "--parent") {
+        parent();
+    } else {
+        setup();
+    }
+}
+
+// Set up the directories so that there are three app dirs:
+// app: Where the parent app is run from
+// parent: In the parent's PATH env var
+// child: In the child's PATH env var
+fn setup() {
+    let exe = env::current_exe().unwrap();
+
+    fs::create_dir_all("app").unwrap();
+    fs::copy(&exe, "app/myapp.exe").unwrap();
+    fs::create_dir_all("parent").unwrap();
+    fs::copy(&exe, "parent/myapp.exe").unwrap();
+    fs::create_dir_all("child").unwrap();
+    fs::copy(&exe, "child/myapp.exe").unwrap();
+
+    let parent_path = path::absolute("parent").unwrap();
+    let status =
+        Command::new("./app/myapp.exe").env("PATH", parent_path).arg("--parent").status().unwrap();
+    // print the status in case of abnormal exit
+    dbg!(status);
+    assert!(status.success());
+}
+
+// The child simply prints the name of its parent directory.
+fn child() {
+    let exe = env::current_exe().unwrap();
+    let parent = exe.parent().unwrap().file_name().unwrap();
+    println!("{}", parent.display());
+}
+
+fn parent() {
+    let exe = env::current_exe().unwrap();
+    let name = exe.file_name().unwrap();
+
+    // By default, the application dir will be search first for the exe
+    let output = Command::new(&name).arg("--child").output().unwrap();
+    assert_eq!(output.stdout, b"app\n");
+
+    // Setting an environment variable should not change the above.
+    let output = Command::new(&name).arg("--child").env("a", "b").output().unwrap();
+    assert_eq!(output.stdout, b"app\n");
+
+    // Setting a child path means that path will be searched first.
+    let child_path = path::absolute("child").unwrap();
+    let output = Command::new(&name).arg("--child").env("PATH", child_path).output().unwrap();
+    assert_eq!(output.stdout, b"child\n");
+
+    // Setting a child path that does not contain the exe (currently) means
+    // we fallback to searching the app dir.
+    let output = Command::new(&name).arg("--child").env("PATH", "").output().unwrap();
+    assert_eq!(output.stdout, b"app\n");
+}