date: don't ignore width prefix in %N format specifier (#12010)

* do not ignore %N if specified width is smaller than default width

* add test for issue #12001

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* cargo clippy

* truncate instead of popping in a loop

* Update format_modifiers.rs

* skip truncation if width is 0

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/uu/date/src/format_modifiers.rs

@@ -474,6 +474,10 @@ fn apply_modifiers(value: &str, parsed: &ParsedSpec<'_>) -> Result<String, Forma
             padded.push_str(&result);
             result = padded;
         }
+    } else if specifier.ends_with('N') {
+        if effective_width <= get_default_width(specifier) && effective_width != 0 {
+            result.truncate(effective_width);
+        }
     }
 
     Ok(result)

tests/by-util/test_date.rs

@@ -3007,3 +3007,12 @@ fn test_korean_time_zone() {
         .succeeds()
         .stdout_is("2026-01-15 01:00:00 UTC\n");
 }
+
+// https://github.com/uutils/coreutils/issues/12001
+// date: width prefix in %N format specifier is ignored ( %3N, %6N always output full 9 nanosecond digits) #12001
+#[test]
+fn test_nanoseconds_width_prefix_ignored_issue12001() {
+    let result = new_ucmd!().arg("+%3N").succeeds();
+    // compare to 4 because of \n
+    assert_eq!(result.stdout().len(), 4);
+}