Added NonZeroXxx::from_mut(_unchecked)?

Author: SOF3

library/core/src/num/nonzero.rs

@@ -78,6 +78,33 @@ macro_rules! nonzero_integers {
                     }
                 }
 
+                /// Converts a primitive mutable reference to a non-zero mutable reference
+                /// if the referenced integer is not zero.
+                #[unstable(feature = "nonzero_from_mut", issue = "none")]
+                #[must_use]
+                #[inline]
+                pub fn from_mut(n: &mut $Int) -> Option<&mut Self> {
+                    // SAFETY: Self is repr(transparent), and the value is non-zero.
+                    // As long as the returned reference is alive,
+                    // the user cannot `*n = 0` directly.
+                    (*n != 0).then(|| unsafe { &mut *(n as *mut $Int as *mut Self) })
+                }
+
+                /// Converts a primitive mutable reference to a non-zero mutable reference
+                /// without checking whether the referenced value is non-zero.
+                /// This results in undefined behavior if `*n` is zero.
+                ///
+                /// # Safety
+                /// The referenced value must not be currently zero.
+                #[unstable(feature = "nonzero_from_mut", issue = "none")]
+                #[must_use]
+                #[inline]
+                pub unsafe fn from_mut_unchecked(n: &mut $Int) -> &mut Self {
+                    // SAFETY: Self is repr(transparent), and the value is assumed to be non-zero.
+                    unsafe { &mut *(n as *mut $Int as *mut Self) }
+                }
+
+
                 /// Returns the value as a primitive type.
                 #[$stability]
                 #[inline]