(c2rust-analyze) Remove the slice rule for safe transmutability, as…

… it's only sound for non-empty slices, but we can't check that at compile-time.

c2rust-analyze/src/util.rs

@@ -373,9 +373,11 @@ impl<'a, T: ?Sized> PhantomLifetime<'a> for T {}
 /// * `A = B => A ~ B`
 /// * `A ~ B => *A ~ *B`
 /// * `uN ~ iN`, `iN ~ uN`, where `N` is an integer width
-/// * `A ~ B => [A] ~ B`
 /// * `A ~ B, N > 0 => [A; N] ~ B`, where `const N: usize`
 ///
+/// Note: `A ~ B => [A] ~ B` is not a rule because it would be unsound for zero-length slices,
+/// which we cannot check unlike for arrays, which we need for translated string literals.
+///
 /// Thus, [`true`] means it is definitely transmutable,
 /// while [`false`] means it may not be transmutable.
 pub fn is_transmutable_to<'tcx>(from: Ty<'tcx>, to: Ty<'tcx>) -> bool {
@@ -404,7 +406,6 @@ pub fn is_transmutable_to<'tcx>(from: Ty<'tcx>, to: Ty<'tcx>) -> bool {
                 !is_zero
             }
         }
-        ty::Slice(from) => is_transmutable_to(from, to),
         _ => false,
     };
 

c2rust-analyze/tests/analyze/string_casts.rs

@@ -6,10 +6,6 @@ pub fn deep_cast_ptr_to_ptr(x: *const *const u8) {
     x as *const *const i8;
 }
 
-pub fn cast_slice_ptr_to_ptr(s: *const [u8]) {
-    s as *const u8;
-}
-
 /// For the below disabled (`#[cfg(any())]`ed) tests, they currently crash in the rewriter
 /// due to it not being able to handle implicitly inserted `&raw` MIR statements yet.
 /// Thus, they also have `*_explicit` versions where