date: address review comments (docs and tests)

- Add documentation for distribute_flag helper.
- Add unit tests for flag distribution and locale expansion.
- Add integration test for locale expansion with uppercase flag.

Author: CrazyRoka

src/uu/date/src/locale.rs

@@ -135,6 +135,11 @@ cfg_langinfo! {
         }
     }
 
+    /// Distributes the given flag to all format specifiers in the string.
+    ///
+    /// For example, if the format string is "%a %b" and the flag is '^',
+    /// the result will be "%^a %^b". Literal percent signs ("%%") are skipped
+    /// and left as is.
     fn distribute_flag(fmt: &str, flag: char) -> String {
         let mut res = String::with_capacity(fmt.len() * 2);
         let mut chars = fmt.chars().peekable();
@@ -367,4 +372,43 @@ mod tests {
             );
         }
     }
+
+    #[test]
+    #[cfg(any(
+        target_os = "linux",
+        target_vendor = "apple",
+        target_os = "freebsd",
+        target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "dragonfly"
+    ))]
+    fn test_distribute_flag() {
+        // Standard distribution
+        assert_eq!(distribute_flag("%a %b", '^'), "%^a %^b");
+        // Ignore literals
+        assert_eq!(distribute_flag("foo %a bar", '_'), "foo %_a bar");
+        // Skip escaped percent signs
+        assert_eq!(distribute_flag("%% %a", '^'), "%% %^a");
+        // Handle flags that might already exist
+        assert_eq!(distribute_flag("%_a", '^'), "%^_a");
+    }
+
+    #[test]
+    #[cfg(any(
+        target_os = "linux",
+        target_vendor = "apple",
+        target_os = "freebsd",
+        target_os = "netbsd",
+        target_os = "openbsd",
+        target_os = "dragonfly"
+    ))]
+    fn test_expand_locale_format_basic() {
+        let format = "%^x";
+        let expanded = expand_locale_format(format).to_string().to_lowercase();
+
+        assert_ne!(expanded, "%^x", "Should have expanded %^x");
+        assert!(expanded.contains("%^d"), "Should contain %^d");
+        assert!(expanded.contains("%^m"), "Should contain %^m");
+        assert!(expanded.contains("%^y"), "Should contain %^y");
+    }
 }

tests/by-util/test_date.rs

@@ -1521,3 +1521,31 @@ fn test_format_upper_c_locale_expansion() {
             out.contains("MON") && out.contains("JAN")
         });
 }
+
+#[test]
+#[cfg(any(
+    target_os = "linux",
+    target_vendor = "apple",
+    target_os = "freebsd",
+    target_os = "netbsd",
+    target_os = "openbsd",
+    target_os = "dragonfly"
+))]
+fn test_format_upper_c_locale_expansion_french() {
+    let result = new_ucmd!()
+        .env("LC_ALL", "fr_FR.UTF-8")
+        .arg("-d")
+        .arg("2024-01-01T13:00:00")
+        .arg("+%^c")
+        .run();
+
+    let stdout = result.stdout_str();
+
+    // Should have 13:00 (not 1:00)
+    assert!(
+        stdout.contains("13:00"),
+        "French locale should show 13:00 for 1 PM, got: {stdout}"
+    );
+
+    assert_eq!(stdout.to_uppercase(), stdout, "Output should be uppercase");
+}