Add tests for dynamic width specifiers exceeding u16::MAX

Rust PR rust-lang/rust#136932 (part of rust-lang/rust#99012) limited
format string width and precision to 16 bits, causing panics when
dynamic padding exceeds `u16::MAX`.

These tests validate handling excessively large widths discovered via
fuzzing in artichoke/strftime-ruby. They ensure correct, panic-free
behavior consistent with CRuby's `Time#strftime`.

Additionally add tests for width specifiers which exceed `INT_MAX` to
ensure they return the formatting string verbatim, which were among the
cases discussed in the upstream PR.

See:

- Upstream report: rust-lang/rust#136932 (comment)
- Proposed fix: rust-lang/rust#136932 (comment)

Author: lopopolo

src/tests/format.rs

@@ -1,57 +1,9 @@
 #![allow(clippy::should_panic_without_expect)]
 
 use crate::format::TimeFormatter;
-use crate::{Error, Time};
+use crate::Error;
 
-include!("../mock.rs.in");
-
-fn get_format_err(time: &MockTime<'_>, format: &str) -> Error {
-    TimeFormatter::new(time, format)
-        .fmt(&mut &mut [0u8; 100][..])
-        .unwrap_err()
-}
-
-fn check_format(time: &MockTime<'_>, format: &str, expected: &str) {
-    const SIZE: usize = 100;
-    let mut buf = [0u8; SIZE];
-    let mut cursor = &mut buf[..];
-
-    TimeFormatter::new(time, format).fmt(&mut cursor).unwrap();
-    let written = SIZE - cursor.len();
-    let data = core::str::from_utf8(&buf[..written]).unwrap();
-
-    assert_eq!(data, expected);
-}
-
-fn check_all(times: &[MockTime<'_>], format: &str, all_expected: &[&str]) {
-    assert_eq!(times.len(), all_expected.len());
-    for (time, expected) in times.iter().zip(all_expected) {
-        check_format(time, format, expected);
-    }
-}
-
-#[test]
-#[should_panic]
-#[rustfmt::skip]
-fn test_check_format_panics_on_error() {
-    let time = MockTime { year: 1111,  ..Default::default() };
-
-    check_format(&time, "'%Y'", "'1112'");
-}
-
-#[test]
-#[should_panic]
-#[rustfmt::skip]
-fn test_check_all_panics_on_error() {
-    let times = [
-        MockTime { year: -1111, ..Default::default() },
-        MockTime { year: -11,   ..Default::default() },
-        MockTime { year: 1,     ..Default::default() },
-        MockTime { year: 1111,  ..Default::default() },
-    ];
-
-    check_all(&times, "'%Y'", &["'-1111'", "'-0011'", "'0001'",  "'1112'"]);
-}
+use super::{check_all, check_format, get_format_err, MockTime};
 
 #[test]
 #[rustfmt::skip]

src/tests/mod.rs

@@ -1,2 +1,62 @@
 mod error;
 mod format;
+mod rust_fmt_argument_max_padding;
+
+use crate::format::TimeFormatter;
+use crate::{Error, Time};
+
+include!("../mock.rs.in");
+
+fn get_format_err(time: &MockTime<'_>, format: &str) -> Error {
+    TimeFormatter::new(time, format)
+        .fmt(&mut &mut [0u8; 100][..])
+        .unwrap_err()
+}
+
+fn get_format_err_bytes(time: &MockTime<'_>, format: &[u8]) -> Error {
+    TimeFormatter::new(time, format)
+        .fmt(&mut &mut [0u8; 100][..])
+        .unwrap_err()
+}
+
+fn check_format(time: &MockTime<'_>, format: &str, expected: &str) {
+    const SIZE: usize = 100;
+    let mut buf = [0u8; SIZE];
+    let mut cursor = &mut buf[..];
+
+    TimeFormatter::new(time, format).fmt(&mut cursor).unwrap();
+    let written = SIZE - cursor.len();
+    let data = core::str::from_utf8(&buf[..written]).unwrap();
+
+    assert_eq!(data, expected);
+}
+
+fn check_all(times: &[MockTime<'_>], format: &str, all_expected: &[&str]) {
+    assert_eq!(times.len(), all_expected.len());
+    for (time, expected) in times.iter().zip(all_expected) {
+        check_format(time, format, expected);
+    }
+}
+
+#[test]
+#[should_panic]
+#[rustfmt::skip]
+fn test_check_format_panics_on_error() {
+    let time = MockTime { year: 1111,  ..Default::default() };
+
+    check_format(&time, "'%Y'", "'1112'");
+}
+
+#[test]
+#[should_panic]
+#[rustfmt::skip]
+fn test_check_all_panics_on_error() {
+    let times = [
+        MockTime { year: -1111, ..Default::default() },
+        MockTime { year: -11,   ..Default::default() },
+        MockTime { year: 1,     ..Default::default() },
+        MockTime { year: 1111,  ..Default::default() },
+    ];
+
+    check_all(&times, "'%Y'", &["'-1111'", "'-0011'", "'0001'",  "'1112'"]);
+}