Fix packed structs with flexible array members

Wrap the FAM generic parameter in `ManuallyDrop` when the parent struct
is packed. This satisfies Rust's requirement that DST fields in packed
structs must not need `Drop` to be run.

The fix automatically detects packed structs and generates:
  pub field: ::std::mem::ManuallyDrop<FAM>
instead of:
  pub field: FAM

Fixes #2936

Author: jsgf

bindgen-tests/tests/expectations/tests/packed_flexarray.rs

@@ -0,0 +1,19 @@
+// bindgen-flags: --rust-target nightly --flexarray-dst --raw-line '#![cfg(feature = "nightly")]' --raw-line '#![feature(ptr_metadata, layout_for_ptr)]'
+
+#pragma pack(1)
+
+typedef struct PackedTest
+{
+    short int Head;
+    long int Tail[];
+}
+PackedTest;
+
+#pragma pack()
+
+typedef struct UnpackedTest
+{
+    short int Head;
+    long int Tail[];
+}
+UnpackedTest;

bindgen-tests/tests/headers/packed_flexarray.hpp

@@ -1515,8 +1515,20 @@ impl FieldCodegen<'_> for FieldData {
         } else if let Some(item) = field_ty.is_incomplete_array(ctx) {
             // Only FAM if its the last field
             if ctx.options().flexarray_dst && last_field {
+                // Check if parent struct is packed to determine if we need ManuallyDrop
+                let layout = parent_item.expect_type().layout(ctx);
+                let is_packed = parent.is_packed(ctx, layout.as_ref());
                 struct_layout.saw_flexible_array();
-                syn::parse_quote! { FAM }
+
+                // For packed structs, we need to wrap FAM in ManuallyDrop
+                // because Rust requires that DST fields in packed structs
+                // don't need Drop to be run.
+                if is_packed {
+                    let prefix = ctx.trait_prefix();
+                    syn::parse_quote! { ::#prefix::mem::ManuallyDrop<FAM> }
+                } else {
+                    syn::parse_quote! { FAM }
+                }
             } else {
                 result.saw_incomplete_array();