Reapply "update &[T] -> &[T; N] TryFrom impl"

Originally from commit e645cee

crux-mir/lib/Patches.md

@@ -83,3 +83,7 @@ identify all of the code that was changed in each patch.
  type. Currently, `crucible-mir` does not support Rust unions, so we instead
  implement slice `len` in terms of our own `crucible_slice_len_hook` function
  that we override.
+
+* Use `crucible_array_from_slice_hook` in `&[T] -> &[T; N]` `TryFrom` impl (last applied: May 26, 2023)
+
+ The actual implementation uses a pointer cast that Crucible can't handle.

crux-mir/lib/core/src/array/mod.rs

@@ -262,13 +262,20 @@ impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
  type Error = TryFromSliceError;
 
  fn try_from(slice: &[T]) -> Result<&[T; N], TryFromSliceError> {
- if slice.len() == N {
- let ptr = slice.as_ptr() as *const [T; N];
- // SAFETY: ok because we just checked that the length fits
- unsafe { Ok(&*ptr) }
- } else {
- Err(TryFromSliceError(()))
+ fn crucible_array_from_slice_hook<T, const N: usize>(
+ slice: &[T],
+ len: usize,
+ ) -> Option<&[T; N]> {
+ if slice.len() == N {
+ let ptr = slice.as_ptr() as *const [T; N];
+ // SAFETY: ok because we just checked that the length fits
+ unsafe { Some(&*ptr) }
+ } else {
+ None
+ }
  }
+ crucible_array_from_slice_hook(slice, N)
+ .ok_or(TryFromSliceError(()))
  }
 }
 
@@ -289,6 +296,7 @@ impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
  type Error = TryFromSliceError;
 
  fn try_from(slice: &mut [T]) -> Result<&mut [T; N], TryFromSliceError> {
+ // Mut version is not supported by crucible - see comment in TransCustom.array_from_slice
  if slice.len() == N {
  let ptr = slice.as_mut_ptr() as *mut [T; N];
  // SAFETY: ok because we just checked that the length fits