Auto merge of #86467 - ChrisDenton:win-env-clear, r=JohnTitor

Windows: Fix `Command::env_clear` so it works if no variables are set

Previously, it would error unless at least one new environment variable was added. The missing null presumably meant that Windows was reading random memory in that case.

See: [CreateProcessW](https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-createprocessw) (scroll down to `lpEnvironment`). Essentially the environment block is a null terminated list of null terminated strings and an empty list is `\0\0` and not `\0`.

EDIT: Oh, [CreateEnvironmentBlock](https://docs.microsoft.com/en-gb/windows/win32/api/userenv/nf-userenv-createenvironmentblock) states this much more explicitly.

Fixes #31259

library/std/src/process/tests.rs

@@ -399,3 +399,12 @@ fn test_command_implements_send_sync() {
     fn take_send_sync_type<T: Send + Sync>(_: T) {}
     take_send_sync_type(Command::new(""))
 }
+
+// Ensure that starting a process with no environment variables works on Windows.
+// This will fail if the environment block is ill-formed.
+#[test]
+#[cfg(windows)]
+fn env_empty() {
+    let p = Command::new("cmd").args(&["/C", "exit 0"]).env_clear().spawn();
+    assert!(p.is_ok());
+}

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

@@ -530,6 +530,12 @@ fn make_envp(maybe_env: Option<BTreeMap<EnvKey, OsString>>) -> io::Result<(*mut
     if let Some(env) = maybe_env {
         let mut blk = Vec::new();
 
+        // If there are no environment variables to set then signal this by
+        // pushing a null.
+        if env.is_empty() {
+            blk.push(0);
+        }
+
         for (k, v) in env {
             blk.extend(ensure_no_nuls(k.0)?.encode_wide());
             blk.push('=' as u16);