Rollup merge of rust-lang#120521 - reitermarkus:generic-nonzero-constructors, r=dtolnay,oli-obk

Make `NonZero` constructors generic.

This makes `NonZero` constructors generic, so that `NonZero::new` can be used without turbofish syntax.

Tracking issue: rust-lang#120257

~~I cannot figure out how to make this work with `const` traits. Not sure if I'm using it wrong or whether there's a bug:~~

```rust
101 |         if n == T::ZERO {
    |            ^^^^^^^^^^^^ expected `host`, found `true`
    |
    = note: expected constant `host`
               found constant `true`
```

r? `@dtolnay`

Author: matthiaskrgr

library/core/src/num/nonzero.rs

@@ -35,7 +35,7 @@ mod private {
 pub trait ZeroablePrimitive: Sized + Copy + private::Sealed {}
 
 macro_rules! impl_zeroable_primitive {
-    ($NonZero:ident ( $primitive:ty )) => {
+    ($primitive:ty) => {
         #[unstable(
             feature = "nonzero_internals",
             reason = "implementation detail which may disappear or be replaced at any time",
@@ -52,18 +52,18 @@ macro_rules! impl_zeroable_primitive {
     };
 }
 
-impl_zeroable_primitive!(NonZeroU8(u8));
-impl_zeroable_primitive!(NonZeroU16(u16));
-impl_zeroable_primitive!(NonZeroU32(u32));
-impl_zeroable_primitive!(NonZeroU64(u64));
-impl_zeroable_primitive!(NonZeroU128(u128));
-impl_zeroable_primitive!(NonZeroUsize(usize));
-impl_zeroable_primitive!(NonZeroI8(i8));
-impl_zeroable_primitive!(NonZeroI16(i16));
-impl_zeroable_primitive!(NonZeroI32(i32));
-impl_zeroable_primitive!(NonZeroI64(i64));
-impl_zeroable_primitive!(NonZeroI128(i128));
-impl_zeroable_primitive!(NonZeroIsize(isize));
+impl_zeroable_primitive!(u8);
+impl_zeroable_primitive!(u16);
+impl_zeroable_primitive!(u32);
+impl_zeroable_primitive!(u64);
+impl_zeroable_primitive!(u128);
+impl_zeroable_primitive!(usize);
+impl_zeroable_primitive!(i8);
+impl_zeroable_primitive!(i16);
+impl_zeroable_primitive!(i32);
+impl_zeroable_primitive!(i64);
+impl_zeroable_primitive!(i128);
+impl_zeroable_primitive!(isize);
 
 /// A value that is known not to equal zero.
 ///
@@ -83,6 +83,90 @@ impl_zeroable_primitive!(NonZeroIsize(isize));
 #[rustc_diagnostic_item = "NonZero"]
 pub struct NonZero<T: ZeroablePrimitive>(T);
 
+impl<T> NonZero<T>
+where
+    T: ZeroablePrimitive,
+{
+    /// Creates a non-zero if the given value is not zero.
+    #[stable(feature = "nonzero", since = "1.28.0")]
+    #[rustc_const_stable(feature = "const_nonzero_int_methods", since = "1.47.0")]
+    #[rustc_allow_const_fn_unstable(const_refs_to_cell)]
+    #[must_use]
+    #[inline]
+    pub const fn new(n: T) -> Option<Self> {
+        // SAFETY: Memory layout optimization guarantees that `Option<NonZero<T>>` has
+        //         the same layout and size as `T`, with `0` representing `None`.
+        unsafe { crate::mem::transmute_copy(&n) }
+    }
+
+    /// Creates a non-zero without checking whether the value is non-zero.
+    /// This results in undefined behaviour if the value is zero.
+    ///
+    /// # Safety
+    ///
+    /// The value must not be zero.
+    #[stable(feature = "nonzero", since = "1.28.0")]
+    #[rustc_const_stable(feature = "nonzero", since = "1.28.0")]
+    #[must_use]
+    #[inline]
+    pub const unsafe fn new_unchecked(n: T) -> Self {
+        match Self::new(n) {
+            Some(n) => n,
+            None => {
+                // SAFETY: The caller guarantees that `n` is non-zero, so this is unreachable.
+                unsafe {
+                    crate::intrinsics::assert_unsafe_precondition!(
+                        "NonZero::new_unchecked requires the argument to be non-zero",
+                        () => false
+                    );
+
+                    crate::hint::unreachable_unchecked()
+                }
+            }
+        }
+    }
+
+    /// Converts a reference to a non-zero mutable reference
+    /// if the referenced value is not zero.
+    #[unstable(feature = "nonzero_from_mut", issue = "106290")]
+    #[must_use]
+    #[inline]
+    pub fn from_mut(n: &mut T) -> Option<&mut Self> {
+        // SAFETY: Memory layout optimization guarantees that `Option<NonZero<T>>` has
+        //         the same layout and size as `T`, with `0` representing `None`.
+        let opt_n = unsafe { &mut *(n as *mut T as *mut Option<Self>) };
+
+        opt_n.as_mut()
+    }
+
+    /// Converts a mutable reference to a non-zero mutable reference
+    /// without checking whether the referenced value is non-zero.
+    /// This results in undefined behavior if the referenced value is zero.
+    ///
+    /// # Safety
+    ///
+    /// The referenced value must not be zero.
+    #[unstable(feature = "nonzero_from_mut", issue = "106290")]
+    #[must_use]
+    #[inline]
+    pub unsafe fn from_mut_unchecked(n: &mut T) -> &mut Self {
+        match Self::from_mut(n) {
+            Some(n) => n,
+            None => {
+                // SAFETY: The caller guarantees that `n` references a value that is non-zero, so this is unreachable.
+                unsafe {
+                    crate::intrinsics::assert_unsafe_precondition!(
+                      "NonZero::from_mut_unchecked requires the argument to dereference as non-zero",
+                        () => false
+                    );
+
+                    crate::hint::unreachable_unchecked()
+                }
+            }
+        }
+    }
+}
+
 macro_rules! impl_nonzero_fmt {
     ( #[$stability: meta] ( $( $Trait: ident ),+ ) for $Ty: ident ) => {
         $(
@@ -100,7 +184,6 @@ macro_rules! impl_nonzero_fmt {
 macro_rules! nonzero_integer {
     (
         #[$stability:meta]
-        #[$const_new_unchecked_stability:meta]
         Self = $Ty:ident,
         Primitive = $signedness:ident $Int:ident,
         $(UnsignedNonZero = $UnsignedNonZero:ident,)?
@@ -143,74 +226,6 @@ macro_rules! nonzero_integer {
         pub type $Ty = NonZero<$Int>;
 
         impl $Ty {
-            /// Creates a non-zero without checking whether the value is non-zero.
-            /// This results in undefined behaviour if the value is zero.
-            ///
-            /// # Safety
-            ///
-            /// The value must not be zero.
-            #[$stability]
-            #[$const_new_unchecked_stability]
-            #[must_use]
-            #[inline]
-            pub const unsafe fn new_unchecked(n: $Int) -> Self {
-                crate::panic::debug_assert_nounwind!(
-                    n != 0,
-                    concat!(stringify!($Ty), "::new_unchecked requires a non-zero argument")
-                );
-                // SAFETY: this is guaranteed to be safe by the caller.
-                unsafe {
-                    Self(n)
-                }
-            }
-
-            /// Creates a non-zero if the given value is not zero.
-            #[$stability]
-            #[rustc_const_stable(feature = "const_nonzero_int_methods", since = "1.47.0")]
-            #[must_use]
-            #[inline]
-            pub const fn new(n: $Int) -> Option<Self> {
-                if n != 0 {
-                    // SAFETY: we just checked that there's no `0`
-                    Some(unsafe { Self(n) })
-                } else {
-                    None
-                }
-            }
-
-            /// 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 = "106290")]
-            #[must_use]
-            #[inline]
-            pub unsafe fn from_mut_unchecked(n: &mut $Int) -> &mut Self {
-                // SAFETY: Self is repr(transparent), and the value is assumed to be non-zero.
-                unsafe {
-                    let n_alias = &mut *n;
-                    core::intrinsics::assert_unsafe_precondition!(
-                        concat!(stringify!($Ty), "::from_mut_unchecked requires the argument to dereference as non-zero"),
-                        (n_alias: &mut $Int) => *n_alias != 0
-                    );
-                    &mut *(n as *mut $Int as *mut Self)
-                }
-            }
-
-            /// Converts a primitive mutable reference to a non-zero mutable reference
-            /// if the referenced integer is not zero.
-            #[unstable(feature = "nonzero_from_mut", issue = "106290")]
-            #[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 $Int as *mut Self) })
-            }
-
             /// Returns the value as a primitive type.
             #[$stability]
             #[inline]
@@ -724,7 +739,6 @@ macro_rules! nonzero_integer {
     (Self = $Ty:ident, Primitive = unsigned $Int:ident $(,)?) => {
         nonzero_integer! {
             #[stable(feature = "nonzero", since = "1.28.0")]
-            #[rustc_const_stable(feature = "nonzero", since = "1.28.0")]
             Self = $Ty,
             Primitive = unsigned $Int,
             UnsignedPrimitive = $Int,
@@ -735,7 +749,6 @@ macro_rules! nonzero_integer {
     (Self = $Ty:ident, Primitive = signed $Int:ident, $($rest:tt)*) => {
         nonzero_integer! {
             #[stable(feature = "signed_nonzero", since = "1.34.0")]
-            #[rustc_const_stable(feature = "signed_nonzero", since = "1.34.0")]
             Self = $Ty,
             Primitive = signed $Int,
             $($rest)*