WIP add option to use DST structs for flex arrays

bindgen-cli/options.rs

@@ -400,6 +400,9 @@ struct BindgenCommand {
     /// Wrap unsafe operations in unsafe blocks.
     #[arg(long)]
     wrap_unsafe_ops: bool,
+    /// Use DSTs to represent structures with flexible array members.
+    #[arg(long)]
+    flexarray_dst: bool,
     /// Derive custom traits on any kind of type. The CUSTOM value must be of the shape REGEX=DERIVE where DERIVE is a coma-separated list of derive macros.
     #[arg(long, value_name = "CUSTOM", value_parser = parse_custom_derive)]
     with_derive_custom: Vec<(Vec<String>, String)>,
@@ -554,6 +557,7 @@ where
         merge_extern_blocks,
         override_abi,
         wrap_unsafe_ops,
+        flexarray_dst,
         with_derive_custom,
         with_derive_custom_struct,
         with_derive_custom_enum,
@@ -1023,6 +1027,10 @@ where
         builder = builder.wrap_unsafe_ops(true);
     }
 
+    if flexarray_dst {
+        builder = builder.flexarray_dst(true);
+    }
+
     #[derive(Debug)]
     struct CustomDeriveCallback {
         derives: Vec<String>,

bindgen/codegen/mod.rs

@@ -1426,10 +1426,13 @@ impl<'a> FieldCodegen<'a> for FieldData {
             wrap_union_field_if_needed(ctx, struct_layout, ty, result)
         } else if let Some(item) = field_ty.is_incomplete_array(ctx) {
             result.saw_incomplete_array();
+            struct_layout.saw_incomplete_array();
 
             let inner = item.to_rust_ty_or_opaque(ctx, &());
 
-            if ctx.options().enable_cxx_namespaces {
+            if ctx.options().flexarray_dst {
+                syn::parse_quote! { [ #inner ] }
+            } else if ctx.options().enable_cxx_namespaces {
                 syn::parse_quote! { root::__IncompleteArrayField<#inner> }
             } else {
                 syn::parse_quote! { __IncompleteArrayField<#inner> }
@@ -2194,6 +2197,7 @@ impl CodeGenerator for CompInfo {
         let mut needs_default_impl = false;
         let mut needs_debug_impl = false;
         let mut needs_partialeq_impl = false;
+        let mut needs_flexarray_impl = false;
         if let Some(comment) = item.comment(ctx) {
             attributes.push(attributes::doc(comment));
         }
@@ -2228,6 +2232,10 @@ impl CodeGenerator for CompInfo {
             }
         }
 
+        if ctx.options().flexarray_dst {
+            needs_flexarray_impl = struct_layout.has_incomplete_array();
+        }
+
         let derivable_traits = derives_of_item(item, ctx, packed);
         if !derivable_traits.contains(DerivableTraits::DEBUG) {
             needs_debug_impl = ctx.options().derive_debug &&
@@ -2481,6 +2489,47 @@ impl CodeGenerator for CompInfo {
             });
         }
 
+        if needs_flexarray_impl {
+            let prefix = ctx.trait_prefix();
+
+            let layout = if ctx.options().rust_features().layout_for_ptr {
+                quote! {
+                    fn layout(len: usize) -> ::#prefix::alloc::Layout {
+                        // SAFETY: Null pointers are OK if we don't deref them
+                        unsafe {
+                            let p: *const Self = ::#prefix::ptr::from_raw_parts(ptr::null(), len);
+                            ::#prefix::alloc::Layout::for_value_raw(p)
+                        }
+                    }
+                }
+            } else {
+                quote!()
+            };
+
+            let from_ptr = if ctx.options().rust_features().ptr_metadata {
+                quote! {
+                    unsafe fn from_ptr<'a>(ptr: *const (), len: usize) -> &'a Self {
+                        let ptr: *const Self = ::#prefix::ptr::from_raw_parts(ptr, len);
+                        &*ptr
+                    }
+
+                    unsafe fn from_ptr_mut<'a>(ptr: *mut (), len: usize) -> ::#prefix::mem::MaybeUninit<&'a mut Self> {
+                        let ptr: *mut Self = ::#prefix::ptr::from_raw_parts_mut(ptr, len);
+                        ::#prefix::mem::MaybeUninit::new(&mut *ptr)
+                    }
+                }
+            } else {
+                quote!()
+            };
+
+            result.push(quote! {
+                impl #generics #ty_for_impl {
+                    #layout
+                    #from_ptr
+                }
+            });
+        }
+
         if needs_default_impl {
             let prefix = ctx.trait_prefix();
             let body = if ctx.options().rust_features().maybe_uninit {
@@ -5075,6 +5124,9 @@ pub(crate) mod utils {
         ctx: &BindgenContext,
         result: &mut Vec<proc_macro2::TokenStream>,
     ) {
+        if ctx.options().flexarray_dst {
+            return;
+        }
         let prefix = ctx.trait_prefix();
 
         // If the target supports `const fn`, declare eligible functions

bindgen/codegen/struct_layout.rs

@@ -27,6 +27,7 @@ pub(crate) struct StructLayoutTracker<'a> {
     latest_field_layout: Option<Layout>,
     max_field_align: usize,
     last_field_was_bitfield: bool,
+    last_field_was_incomplete_array: bool,
     visibility: FieldVisibilityKind,
 }
 
@@ -110,6 +111,7 @@ impl<'a> StructLayoutTracker<'a> {
             latest_field_layout: None,
             max_field_align: 0,
             last_field_was_bitfield: false,
+            last_field_was_incomplete_array: false,
         }
     }
 
@@ -121,6 +123,10 @@ impl<'a> StructLayoutTracker<'a> {
         self.is_rust_union
     }
 
+    pub(crate) fn has_incomplete_array(&self) -> bool {
+        self.last_field_was_incomplete_array
+    }
+
     pub(crate) fn saw_vtable(&mut self) {
         debug!("saw vtable for {}", self.name);
 
@@ -159,6 +165,10 @@ impl<'a> StructLayoutTracker<'a> {
         self.max_field_align = cmp::max(self.max_field_align, layout.align);
     }
 
+    pub(crate) fn saw_incomplete_array(&mut self) {
+        self.last_field_was_incomplete_array = true;
+    }
+
     /// Returns a padding field if necessary for a given new field _before_
     /// adding that field.
     pub(crate) fn saw_field(

bindgen/features.rs

@@ -96,6 +96,8 @@ macro_rules! define_rust_targets {
 define_rust_targets! {
     Nightly => {
         vectorcall_abi,
+        ptr_metadata: #81513,
+        layout_for_ptr: #69835,
     },
     Stable_1_73(73) => { thiscall_abi: #42202 },
     Stable_1_71(71) => { c_unwind_abi: #106075 },

bindgen/options/mod.rs

@@ -1980,6 +1980,19 @@ options! {
         },
         as_args: "--wrap-unsafe-ops",
     },
+    /// Use DSTs to represent structures with flexible array members.
+    flexarray_dst: bool {
+        methods: {
+            /// Use DSTs to represent structures with flexible array members.
+            ///
+            /// This option is disabled by default.
+            pub fn flexarray_dst(mut self, doit: bool) -> Self {
+                self.options.flexarray_dst = doit;
+                self
+            }
+        },
+        as_args: "--flexarray-dst",
+    },
     /// Patterns for functions whose ABI should be overriden.
     abi_overrides: HashMap<Abi, RegexSet> {
         methods: {