feat: Change return type of NonZero::bit_width

Return `NonZero<u32>` instead of `u32`.

Author: sorairolake

library/core/src/num/nonzero.rs

@@ -1789,17 +1789,20 @@ macro_rules! nonzero_integer_signedness_dependent_methods {
         /// #
         /// # fn main() { test().unwrap(); }
         /// # fn test() -> Option<()> {
-        #[doc = concat!("assert_eq!(NonZero::<", stringify!($Int), ">::new(0b111)?.bit_width(), 3);")]
-        #[doc = concat!("assert_eq!(NonZero::<", stringify!($Int), ">::new(0b1110)?.bit_width(), 4);")]
+        #[doc = concat!("assert_eq!(NonZero::<", stringify!($Int), ">::MIN.bit_width(), NonZero::new(1)?);")]
+        #[doc = concat!("assert_eq!(NonZero::<", stringify!($Int), ">::new(0b111)?.bit_width(), NonZero::new(3)?);")]
+        #[doc = concat!("assert_eq!(NonZero::<", stringify!($Int), ">::new(0b1110)?.bit_width(), NonZero::new(4)?);")]
         /// # Some(())
         /// # }
         /// ```
         #[unstable(feature = "uint_bit_width", issue = "142326")]
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline(always)]
-        pub const fn bit_width(self) -> u32 {
-            Self::BITS - self.leading_zeros()
+        pub const fn bit_width(self) -> NonZero<u32> {
+            // SAFETY: Since `self.leading_zeros()` is always less than
+            // `Self::BITS`, this subtraction can never be zero.
+            unsafe { NonZero::new_unchecked(Self::BITS - self.leading_zeros()) }
         }
     };
 

library/coretests/tests/nonzero.rs

@@ -577,10 +577,10 @@ fn test_nonzero_bit_width() {
         ($($T:ty),+) => {
             $(
                 {
-                    assert_eq!(NonZero::<$T>::new(0b010_1100).unwrap().bit_width(), 6);
-                    assert_eq!(NonZero::<$T>::new(0b111_1001).unwrap().bit_width(), 7);
-                    assert_eq!(NonZero::<$T>::MIN.bit_width(), 1);
-                    assert_eq!(NonZero::<$T>::MAX.bit_width(), <$T>::BITS);
+                    assert_eq!(NonZero::<$T>::new(0b010_1100).unwrap().bit_width(), NonZero::new(6).unwrap());
+                    assert_eq!(NonZero::<$T>::new(0b111_1001).unwrap().bit_width(), NonZero::new(7).unwrap());
+                    assert_eq!(NonZero::<$T>::MIN.bit_width(), NonZero::new(1).unwrap());
+                    assert_eq!(NonZero::<$T>::MAX.bit_width(), NonZero::new(<$T>::BITS).unwrap());
                 }
             )+
         };