Add dynamic loading support

Co-authored-by: Michael-F-Bryan <michaelfbryan@gmail.com>

src/codegen/mod.rs

@@ -182,6 +182,7 @@ impl From<DerivableTraits> for Vec<&'static str> {
     }
 }
 
+#[derive(Clone)]
 struct CodegenResult<'a> {
     items: Vec<proc_macro2::TokenStream>,
 
@@ -435,6 +436,15 @@ impl CodeGenerator for Item {
             return;
         }
 
+        if self.is_dynamic(ctx) {
+            debug!(
+                "Item is to be dynamically generated; ignoring it for now. \
+                 self = {:?}",
+                self
+            );
+            return;
+        }
+
         if self.is_blacklisted(ctx) || result.seen(self.id()) {
             debug!(
                 "<Item as CodeGenerator>::codegen: Ignoring hidden or seen: \
@@ -3913,6 +3923,126 @@ impl CodeGenerator for ObjCInterface {
     }
 }
 
+trait DynamicBindingGenerator {
+    /// Extra information from the caller.
+    type Extra;
+
+    fn dyngen<'a>(
+        &self,
+        ctx: &BindgenContext,
+        def_result: &mut CodegenResult<'a>,
+        impl_result: &mut CodegenResult<'a>,
+        extra: &Self::Extra,
+    );
+}
+
+impl DynamicBindingGenerator for Item {
+    type Extra = ();
+
+    fn dyngen<'a>(
+        &self,
+        ctx: &BindgenContext,
+        def_result: &mut CodegenResult<'a>,
+        impl_result: &mut CodegenResult<'a>,
+        _extra: &(),
+    ) {
+        assert!(self.is_dynamic(ctx));
+        if !self.is_enabled_for_codegen(ctx) {
+            return;
+        }
+
+        // If this item is blacklisted, or we've already seen it, nothing to do.
+        if self.is_blacklisted(ctx) ||
+            def_result.seen(self.id()) ||
+            impl_result.seen(self.id())
+        {
+            debug!(
+                "<Item as DynamicBindingGenerator>::codegen: Ignoring hidden or seen: \
+                 self = {:?}",
+                self
+            );
+            return;
+        }
+
+        debug!(
+            "<Item as DynamicBindingGenerator>::dyngen: self = {:?}",
+            self
+        );
+
+        if !ctx.codegen_items().contains(&self.id()) {
+            warn!("Found non-whitelisted item in dynamic binding generation: {:?}", self);
+        }
+
+        def_result.set_seen(self.id());
+        impl_result.set_seen(self.id());
+
+        match *self.kind() {
+            ItemKind::Function(ref fun) => {
+                assert!(fun.kind() == FunctionKind::Function);
+                fun.dyngen(ctx, def_result, impl_result, self);
+            }
+            _ => panic!(
+                "Unexpected item type when doing dynamic bindings generation."
+            ),
+        }
+    }
+}
+
+impl DynamicBindingGenerator for Function {
+    type Extra = Item;
+
+    fn dyngen<'a>(
+        &self,
+        ctx: &BindgenContext,
+        def_result: &mut CodegenResult<'a>,
+        impl_result: &mut CodegenResult<'a>,
+        item: &Item,
+    ) {
+        let signature_item = ctx.resolve_item(self.signature());
+        let signature = signature_item.kind().expect_type().canonical_type(ctx);
+        let signature = match *signature.kind() {
+            TypeKind::Function(ref sig) => sig,
+            _ => panic!("Signature kind is not a Function: {:?}", signature),
+        };
+
+        let canonical_name = item.canonical_name(ctx);
+        let abi = match signature.abi() {
+            Abi::ThisCall if !ctx.options().rust_features().thiscall_abi => {
+                warn!("Skipping function with thiscall ABI that isn't supported by the configured Rust target");
+                return;
+            }
+            Abi::Win64 if signature.is_variadic() => {
+                warn!("Skipping variadic function with Win64 ABI that isn't supported");
+                return;
+            }
+            Abi::Unknown(unknown_abi) => {
+                panic!(
+                    "Invalid or unknown abi {:?} for function {:?} ({:?})",
+                    unknown_abi, canonical_name, self
+                );
+            }
+            abi => abi,
+        };
+
+        let args = utils::fnsig_arguments(ctx, signature);
+        let args_identifiers =
+            utils::fnsig_argument_identifiers(ctx, signature);
+        let ret = utils::fnsig_return_ty(ctx, signature);
+
+        let ident = ctx.rust_ident(&canonical_name);
+
+        def_result.push(quote! {
+            #ident: unsafe extern #abi fn ( #( #args ),* ) #ret,
+        });
+
+        impl_result.push(quote! {
+        pub unsafe extern #abi fn #ident ( &self, #( #args ),* ) #ret {
+            (self.#ident)(#( #args_identifiers ),*)
+        }
+        });
+    }
+}
+
 pub(crate) fn codegen(
     context: BindgenContext,
 ) -> (Vec<proc_macro2::TokenStream>, BindgenOptions) {
@@ -3948,6 +4078,111 @@ pub(crate) fn codegen(
             &(),
         );
 
+        // If the set of items to generate dynamic bindings for is nonempty...
+        if !context.dyngen_items().is_empty() {
+            let _t = context.timer("dyngen");
+            debug!("dyngen: {:?}", context.options());
+
+            // `def_result` tracks the tokens that will appears inside the library struct -- these
+            // are the definitions of the symbols inside the library struct, e.g.:
+            // ```
+            // struct Lib {
+            //    __library: ::libloading::Library,
+            //    x: unsafe extern ..., // <- tracks these!
+            //    ...
+            // }
+            // ```
+            //
+            // `impl_result` tracks the tokens that will appear inside the call implementation,
+            // e.g.:
+            //
+            // ```
+            // impl Lib {
+            //     ...
+            //     pub unsafe extern "C" fn foo(&self, ...) { // <- tracks these!
+            //         (self.foo)(...)
+            //     }
+            // }
+            // ```
+            //
+            // We clone the CodeGenerator object used for normal code generation, but set its items
+            // to the empty vector. We do this so that we can keep track of which items we have
+            // seen, etc.
+            let mut def_result = result.clone();
+            def_result.items = vec![];
+            let mut impl_result = result.clone();
+            impl_result.items = vec![];
+
+            // Run dynamic binding generation for each of the required items.
+            for item in context.dyngen_items() {
+                context.resolve_item(*item).dyngen(
+                    context,
+                    &mut def_result,
+                    &mut impl_result,
+                    &(),
+                );
+            }
+
+            let lib_ident = context.rust_ident(
+                context.options().dynamic_library_name.as_ref().unwrap(),
+            );
+
+            let struct_items = def_result.items;
+            result.push(quote! {
+                pub struct #lib_ident {
+                    __library: ::libloading::Library,
+                    #(#struct_items)*
+                }
+            });
+
+            let build_locals = context
+                .dyngen_items()
+                .iter()
+                .map(|&item| {
+                    let canonical_name = item.canonical_name(context);
+                    let ident = context.rust_ident(&canonical_name);
+
+                    quote! {
+                        let #ident = *__library.get(#canonical_name.as_bytes())?;
+                    }
+                })
+                .collect::<Vec<_>>();
+
+            let init_fields = context
+                .dyngen_items()
+                .iter()
+                .map(|&item| {
+                    let canonical_name = item.canonical_name(context);
+                    let ident = context.rust_ident(&canonical_name);
+
+                    quote! {
+                        #ident
+                    }
+                })
+                .collect::<Vec<_>>();
+
+            let impl_items = impl_result.items;
+            result.push(quote! {
+                impl #lib_ident {
+                    pub unsafe fn new<P>(
+                        path: P
+                    ) -> Result<Self, ::libloading::Error>
+                    where P: AsRef<::std::ffi::OsStr> {
+                        let __library = ::libloading::Library::new(path)?;
+                        #( #build_locals )*
+                        Ok(
+                            #lib_ident {
+                                __library: __library,
+                                #( #init_fields ),*
+                            }
+                        )
+                    }
+
+                    #( #impl_items )*
+                }
+            });
+        }
+
         result.items
     })
 }
@@ -4356,6 +4591,35 @@ mod utils {
         args
     }
 
+    pub fn fnsig_argument_identifiers(
+        ctx: &BindgenContext,
+        sig: &FunctionSig,
+    ) -> Vec<proc_macro2::TokenStream> {
+        let mut unnamed_arguments = 0;
+        let args = sig
+            .argument_types()
+            .iter()
+            .map(|&(ref name, _ty)| {
+                let arg_name = match *name {
+                    Some(ref name) => ctx.rust_mangle(name).into_owned(),
+                    None => {
+                        unnamed_arguments += 1;
+                        format!("arg{}", unnamed_arguments)
+                    }
+                };
+
+                assert!(!arg_name.is_empty());
+                let arg_name = ctx.rust_ident(arg_name);
+
+                quote! {
+                    #arg_name
+                }
+            })
+            .collect::<Vec<_>>();
+
+        args
+    }
+
     pub fn fnsig_block(
         ctx: &BindgenContext,
         sig: &FunctionSig,