[#224] Add crucible support for core::slice::as_chunks

This commit moves the allocate family of functions into core::crucible::alloc so that as_chunks_unchecked can call them.

Author: glguy

libs/Patches.md

@@ -291,6 +291,13 @@ into the main commit for that patch, and then the *Update* line can be removed.
   * We use the Wasm configuration for the internal `guard::enable` function,
     which simply leaks everything.
 
+* Avoid unsupported pointer-cast in `std::slice::as_chunks_unchecked` (last applied Feb 26, 2026)
+
+  Due to limitations of the current memory model we need to allocate a new
+  array of arrays and then shallow-copy the original elements of the slice
+  into those arrays. We note that this is unsound in the face of interior
+  mutability and it leaks memory.
+
 # Notes
 
 This section contains more detailed notes about why certain patches are written

libs/core/src/crucible/alloc.rs

@@ -0,0 +1,22 @@
+#![expect(unused_variables)]
+
+// These functions have been moved temporarily in support
+// of core::slice::as_chunks_unchecked. They should be
+// returned to crucible::alloc once they are no longer needed.
+
+/// Allocate an array of `len` elements of type `T`.  The array begins uninitialized.
+pub fn allocate<T>(len: usize) -> *mut T {
+    unimplemented!("allocate")
+}
+
+/// Allocate an array of `len` elements of type `T`.  The array initially contains all zeros.  This
+/// fails if `crux-mir` doesn't know how to zero-initialize `T`.
+pub fn allocate_zeroed<T>(len: usize) -> *mut T {
+    unimplemented!("allocate_zeroed")
+}
+
+/// Reallocate the array at `*ptr` to contain `new_len` elements.  This reallocation always happens
+/// in-place and never fails, so there is no need to return a new pointer.
+pub fn reallocate<T>(ptr: *mut T, new_len: usize) {
+    unimplemented!("reallocate")
+}

libs/core/src/crucible/mod.rs

@@ -8,3 +8,6 @@ pub mod ptr;
 
 #[unstable(feature = "crucible_intrinsics", issue = "none")]
 pub mod concurrency;
+
+#[unstable(feature = "crucible_intrinsics", issue = "none")]
+pub mod alloc;

libs/core/src/slice/mod.rs

@@ -7,7 +7,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::cmp::Ordering::{self, Equal, Greater, Less};
-use crate::intrinsics::{exact_div, unchecked_sub};
+use crate::intrinsics::{const_eval_select, exact_div, unchecked_sub};
 use crate::mem::{self, MaybeUninit, SizedTypeProperties};
 use crate::num::NonZero;
 use crate::ops::{OneSidedRange, OneSidedRangeBound, Range, RangeBounds, RangeInclusive};
@@ -1332,6 +1332,7 @@ impl<T> [T] {
     /// ```
     #[stable(feature = "slice_as_chunks", since = "1.88.0")]
     #[rustc_const_stable(feature = "slice_as_chunks", since = "1.88.0")]
+    #[rustc_allow_const_fn_unstable(const_eval_select)]
     #[inline]
     #[must_use]
     #[track_caller]
@@ -1341,11 +1342,31 @@ impl<T> [T] {
             "slice::as_chunks_unchecked requires `N != 0` and the slice to split exactly into `N`-element chunks",
             (n: usize = N, len: usize = self.len()) => n != 0 && len.is_multiple_of(n),
         );
-        // SAFETY: Caller must guarantee that `N` is nonzero and exactly divides the slice length
-        let new_len = unsafe { exact_div(self.len(), N) };
-        // SAFETY: We cast a slice of `new_len * N` elements into
-        // a slice of `new_len` many `N` elements chunks.
-        unsafe { from_raw_parts(self.as_ptr().cast(), new_len) }
+        
+        const fn const_case<T, const N: usize>(xs: &[T]) -> &[[T; N]] {
+            // SAFETY: Caller must guarantee that `N` is nonzero and exactly divides the slice length
+            let new_len = unsafe { exact_div(xs.len(), N) };
+
+            // SAFETY: We cast a slice of `new_len * N` elements into
+            // a slice of `new_len` many `N` elements chunks.
+            unsafe { from_raw_parts(xs.as_ptr().cast(), new_len) }
+        }
+
+        // The non-const case needs to be able to call allocate which is not const fn
+        fn mut_case<T, const N: usize>(xs: &[T]) -> &[[T; N]] {
+            // SAFETY: Caller must guarantee that `N` is nonzero and exactly divides the slice length
+            let new_len = unsafe { exact_div(xs.len(), N) };
+
+            let ptr = crate::crucible::alloc::allocate::<[T; N]>(new_len);
+            for i in 0..new_len {
+                unsafe {
+                    ptr.add(i).write(crate::array::from_fn(|j| xs.as_ptr().add(N*i+j).read()));
+                }
+            }
+            unsafe { from_raw_parts(ptr, new_len) }
+        }
+
+        const_eval_select((self,), const_case, mut_case)
     }
 
     /// Splits the slice into a slice of `N`-element arrays,

libs/crucible/alloc.rs

@@ -5,22 +5,21 @@ use core::ptr::NonNull;
 
 /// Allocate an array of `len` elements of type `T`.  The array begins uninitialized.
 pub fn allocate<T>(len: usize) -> *mut T {
-    unimplemented!("allocate")
+    core::crucible::alloc::allocate(len)
 }
 
 /// Allocate an array of `len` elements of type `T`.  The array initially contains all zeros.  This
 /// fails if `crux-mir` doesn't know how to zero-initialize `T`.
 pub fn allocate_zeroed<T>(len: usize) -> *mut T {
-    unimplemented!("allocate_zeroed")
+    core::crucible::alloc::allocate_zeroed(len)
 }
 
 /// Reallocate the array at `*ptr` to contain `new_len` elements.  This reallocation always happens
 /// in-place and never fails, so there is no need to return a new pointer.
 pub fn reallocate<T>(ptr: *mut T, new_len: usize) {
-    unimplemented!("reallocate")
+    core::crucible::alloc::reallocate(ptr, new_len)
 }
 
-
 pub struct TypedAllocator<T>(pub PhantomData<T>);
 
 impl<T> TypedAllocator<T> {