mktemp -t foo.XXXX should create in TMPDIR

src/uu/mktemp/src/mktemp.rs

@@ -191,7 +191,12 @@ impl Options {
                     (tmpdir, template.to_string())
                 }
                 Some(template) => {
-                    let tmpdir = matches.get_one::<String>(OPT_TMPDIR).map(String::from);
+                    let tmpdir = if matches.get_flag(OPT_T) {
+                        // mktemp -t foo.xxx should export in TMPDIR
+                        Some(env::temp_dir().display().to_string())
+                    } else {
+                        matches.get_one::<String>(OPT_TMPDIR).map(String::from)
+                    };
                     (tmpdir, template.to_string())
                 }
             }

tests/by-util/test_mktemp.rs

@@ -857,3 +857,18 @@ fn test_default_missing_value() {
     let scene = TestScenario::new(util_name!());
     scene.ucmd().arg("-d").arg("--tmpdir").succeeds();
 }
+
+#[test]
+fn test_default_issue_4821_t_tmpdir() {
+    let scene = TestScenario::new(util_name!());
+    let pathname = scene.fixtures.as_string();
+    let result = scene
+        .ucmd()
+        .env(TMPDIR, &pathname)
+        .arg("-t")
+        .arg("foo.XXXX")
+        .succeeds();
+    let stdout = result.stdout_str();
+    println!("stdout = {stdout}");
+    assert!(stdout.contains(&pathname));
+}