Simplify shared alignment assertions (#841)

Replace array-underflow checks with const assert expressions.
Add diagnostics that explain the pointer-tagging alignment invariant.

Signed-off-by: bestgopher <[email protected]>

src/bytes.rs

@@ -1348,7 +1348,12 @@ impl Drop for Shared {
 // This is a necessary invariant since we depend on allocating `Shared` a
 // shared object to implicitly carry the `KIND_ARC` flag in its pointer.
 // This flag is set when the LSB is 0.
-const _: [(); 0 - mem::align_of::<Shared>() % 2] = []; // Assert that the alignment of `Shared` is divisible by 2.
+const _: () = {
+    assert!(
+        mem::align_of::<Shared>() % 2 == 0,
+        "Shared alignment must be divisible by 2 for pointer tagging"
+    );
+};
 
 static SHARED_VTABLE: Vtable = Vtable {
     clone: shared_clone,

src/bytes_mut.rs

@@ -92,7 +92,12 @@ impl Shared {
 // This is a necessary invariant since we depend on allocating `Shared` a
 // shared object to implicitly carry the `KIND_ARC` flag in its pointer.
 // This flag is set when the LSB is 0.
-const _: [(); 0 - mem::align_of::<Shared>() % 2] = []; // Assert that the alignment of `Shared` is divisible by 2.
+const _: () = {
+    assert!(
+        mem::align_of::<Shared>() % 2 == 0,
+        "Shared alignment must be divisible by 2 for pointer tagging"
+    );
+};
 
 // Buffer storage strategy flags.
 const KIND_ARC: usize = 0b0;