Provide option to get real virtual fn receiver.

For virtual functions, bindgen has traditionally emitted a void* pointer
because that's the least bad option for Rust code directly consuming the
bindings. For downstream postprocessors and code generators, this throws away
useful information about the actual type of the receiver. Provide a
command-line option to put that back.

Part of google/autocxx#124

bindgen-tests/tests/headers/specific_receiver.hpp

@@ -0,0 +1,7 @@
+// bindgen-flags: --use-specific-virtual-function-receiver --generate-inline-functions -- -x c++ -std=c++14
+
+class Fish {
+public:
+    virtual void swim() {
+    }
+};

bindgen/ir/function.rs

@@ -533,7 +533,10 @@ impl FunctionSig {
             let is_const = is_method && cursor.method_is_const();
             let is_virtual = is_method && cursor.method_is_virtual();
             let is_static = is_method && cursor.method_is_static();
-            if !is_static && !is_virtual {
+            if !is_static &&
+                (!is_virtual ||
+                    ctx.options().use_specific_virtual_function_receiver)
+            {
                 let parent = cursor.semantic_parent();
                 let class = Item::parse(parent, None, ctx)
                     .expect("Expected to parse the class");

bindgen/options/cli.rs

@@ -441,6 +441,9 @@ struct BindgenCommand {
     /// Always output explicit padding fields.
     #[arg(long)]
     explicit_padding: bool,
+    /// Always be specific about the 'receiver' of a virtual function.
+    #[arg(long)]
+    use_specific_virtual_function_receiver: bool,
     /// Use distinct char16_t
     #[arg(long)]
     use_distinct_char16_t: bool,
@@ -632,6 +635,7 @@ where
         translate_enum_integer_types,
         c_naming,
         explicit_padding,
+        use_specific_virtual_function_receiver,
         use_distinct_char16_t,
         vtable_generation,
         sort_semantically,
@@ -930,6 +934,7 @@ where
             translate_enum_integer_types,
             c_naming,
             explicit_padding,
+            use_specific_virtual_function_receiver,
             use_distinct_char16_t,
             vtable_generation,
             sort_semantically,

bindgen/options/mod.rs

@@ -153,6 +153,21 @@ macro_rules! options {
 }
 
 options! {
+    /// Whether to specify the type of a virtual function receiver
+    use_specific_virtual_function_receiver: bool {
+        methods: {
+            /// Normally, virtual functions have void* as their 'this' type.
+            /// If this flag is enabled, override that behavior to indicate a
+            /// pointer of the specific type.
+            /// Disabled by default.
+            pub fn use_specific_virtual_function_receiver(mut self, doit: bool) -> Builder {
+                self.options.use_specific_virtual_function_receiver = doit;
+                self
+            }
+        },
+        as_args: "--use-specific-virtual-function-receiver",
+    },
+
     /// Whether we should distinguish between C++'s 'char16_t' and 'u16'.
     /// The C++ type `char16_t` is its own special type; it's not a typedef
     /// of some other integer (this differs from C).