make const align_offset useful

Author: Lukas Markeffsky

core/src/ptr/const_ptr.rs

@@ -1322,6 +1322,21 @@ impl<T: ?Sized> *const T {
     /// ```
     #[stable(feature = "align_offset", since = "1.36.0")]
     #[rustc_const_unstable(feature = "const_align_offset", issue = "90962")]
+    #[cfg(not(bootstrap))]
+    pub const fn align_offset(self, align: usize) -> usize
+    where
+        T: Sized,
+    {
+        assert!(align.is_power_of_two(), "align_offset: align is not a power-of-two");
+
+        // SAFETY: `align` has been checked to be a power of 2 above
+        unsafe { align_offset(self, align) }
+    }
+
+    #[stable(feature = "align_offset", since = "1.36.0")]
+    #[rustc_const_unstable(feature = "const_align_offset", issue = "90962")]
+    #[allow(missing_docs)]
+    #[cfg(bootstrap)]
     pub const fn align_offset(self, align: usize) -> usize
     where
         T: Sized,

core/src/ptr/mod.rs

@@ -1574,10 +1574,14 @@ pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
 
 /// Align pointer `p`.
 ///
-/// Calculate offset (in terms of elements of `stride` stride) that has to be applied
+/// Calculate offset (in terms of elements of `size_of::<T>()` stride) that has to be applied
 /// to pointer `p` so that pointer `p` would get aligned to `a`.
 ///
-/// Note: This implementation has been carefully tailored to not panic. It is UB for this to panic.
+/// # Safety
+/// `a` must be a power of two.
+///
+/// # Notes
+/// This implementation has been carefully tailored to not panic. It is UB for this to panic.
 /// The only real change that can be made here is change of `INV_TABLE_MOD_16` and associated
 /// constants.
 ///
@@ -1586,8 +1590,10 @@ pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
 /// than trying to adapt this to accommodate that change.
 ///
 /// Any questions go to @nagisa.
+// #[cfg(not(bootstrap))] -- Calling this function in a const context from the bootstrap
+// compiler will always cause an error.
 #[lang = "align_offset"]
-pub(crate) unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
+pub(crate) const unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
     // FIXME(#75598): Direct use of these intrinsics improves codegen significantly at opt-level <=
     // 1, where the method versions of these operations are not inlined.
     use intrinsics::{
@@ -1604,7 +1610,7 @@ pub(crate) unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
     ///
     /// Implementation of this function shall not panic. Ever.
     #[inline]
-    unsafe fn mod_inv(x: usize, m: usize) -> usize {
+    const unsafe fn mod_inv(x: usize, m: usize) -> usize {
         /// Multiplicative modular inverse table modulo 2⁴ = 16.
         ///
         /// Note, that this table does not contain values where inverse does not exist (i.e., for
@@ -1646,8 +1652,13 @@ pub(crate) unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
         inverse & m_minus_one
     }
 
-    let addr = p.addr();
     let stride = mem::size_of::<T>();
+
+    // SAFETY: At runtime transmuting a pointer to `usize` is always safe, because they have the
+    // same layout. During const eval we hook this function to ensure that the pointer always has
+    // an address (only the standard library can do this).
+    let addr = unsafe { mem::transmute(p) };
+
     // SAFETY: `a` is a power-of-two, therefore non-zero.
     let a_minus_one = unsafe { unchecked_sub(a, 1) };
 

core/src/ptr/mut_ptr.rs

@@ -1590,6 +1590,21 @@ impl<T: ?Sized> *mut T {
     /// ```
     #[stable(feature = "align_offset", since = "1.36.0")]
     #[rustc_const_unstable(feature = "const_align_offset", issue = "90962")]
+    #[cfg(not(bootstrap))]
+    pub const fn align_offset(self, align: usize) -> usize
+    where
+        T: Sized,
+    {
+        assert!(align.is_power_of_two(), "align_offset: align is not a power-of-two");
+
+        // SAFETY: `align` has been checked to be a power of 2 above
+        unsafe { align_offset(self, align) }
+    }
+
+    #[stable(feature = "align_offset", since = "1.36.0")]
+    #[rustc_const_unstable(feature = "const_align_offset", issue = "90962")]
+    #[allow(missing_docs)]
+    #[cfg(bootstrap)]
     pub const fn align_offset(self, align: usize) -> usize
     where
         T: Sized,