style: Add support for deriving ToShmem.

Differential Revision: https://phabricator.services.mozilla.com/D17189

components/to_shmem_derive/to_shmem.rs

@@ -0,0 +1,74 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use derive_common::cg;
+use proc_macro2::TokenStream;
+use syn;
+use synstructure::{BindStyle, Structure};
+
+pub fn derive(mut input: syn::DeriveInput) -> TokenStream {
+    let mut where_clause = input.generics.where_clause.take();
+    let attrs = cg::parse_input_attrs::<ShmemInputAttrs>(&input);
+    if !attrs.no_bounds {
+        for param in input.generics.type_params() {
+            cg::add_predicate(
+                &mut where_clause,
+                parse_quote!(#param: ::to_shmem::ToShmem),
+            );
+        }
+    }
+    for variant in Structure::new(&input).variants() {
+        for binding in variant.bindings() {
+            let attrs = cg::parse_field_attrs::<ShmemFieldAttrs>(&binding.ast());
+            if attrs.field_bound {
+                let ty = &binding.ast().ty;
+                cg::add_predicate(
+                    &mut where_clause,
+                    parse_quote!(#ty: ::to_shmem::ToShmem),
+                )
+            }
+        }
+    }
+
+    input.generics.where_clause = where_clause;
+
+    let match_body = cg::fmap_match(&input, BindStyle::Ref, |binding| {
+        quote! {
+            ::std::mem::ManuallyDrop::into_inner(
+                ::to_shmem::ToShmem::to_shmem(#binding, builder)
+            )
+        }
+    });
+
+    let name = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    quote! {
+        impl #impl_generics ::to_shmem::ToShmem for #name #ty_generics #where_clause {
+            #[allow(unused_variables)]
+            fn to_shmem(
+                &self,
+                builder: &mut ::to_shmem::SharedMemoryBuilder,
+            ) -> ::std::mem::ManuallyDrop<Self> {
+                ::std::mem::ManuallyDrop::new(
+                    match *self {
+                        #match_body
+                    }
+                )
+            }
+        }
+    }
+}
+
+#[darling(attributes(shmem), default)]
+#[derive(Default, FromDeriveInput)]
+pub struct ShmemInputAttrs {
+    pub no_bounds: bool,
+}
+
+#[darling(attributes(shmem), default)]
+#[derive(Default, FromField)]
+pub struct ShmemFieldAttrs {
+    pub field_bound: bool,
+}