Reflection for castables

1 files changed, 48 insertions, 0 deletions

diff --git a/macros/src/capable.rs b/macros/src/capable.rs
new file mode 100644
index 00000000..dcfdfc82
--- /dev/null
+++ b/macros/src/capable.rs
@@ -0,0 +1,48 @@
+use syn::parse::Parser;
+use syn::punctuated::Punctuated;
+use syn::Token;
+
+use super::*;
+
+/// Expand the `#[capability]` macro.
+pub fn capability(item: syn::ItemTrait) -> Result<TokenStream> {
+    let ident = &item.ident;
+    Ok(quote! {
+        #item
+        impl ::typst::model::Capability for dyn #ident {}
+    })
+}
+
+/// Expand the `#[capable(..)]` macro.
+pub fn capable(attr: TokenStream, item: syn::Item) -> Result<TokenStream> {
+    let (ident, generics) = match &item {
+        syn::Item::Struct(s) => (&s.ident, &s.generics),
+        syn::Item::Enum(s) => (&s.ident, &s.generics),
+        _ => bail!(item, "only structs and enums are supported"),
+    };
+
+    let (params, args, clause) = generics.split_for_impl();
+    let checks = Punctuated::<Ident, Token![,]>::parse_terminated
+        .parse2(attr)?
+        .into_iter()
+        .map(|capability| {
+            quote! {
+                if id == ::std::any::TypeId::of::<dyn #capability>() {
+                    return Some(unsafe {
+                        ::typst::util::fat::vtable(self as &dyn #capability)
+                    });
+                }
+            }
+        });
+
+    Ok(quote! {
+        #item
+
+        unsafe impl #params ::typst::model::Capable for #ident #args #clause {
+            fn vtable(&self, id: ::std::any::TypeId) -> ::std::option::Option<*const ()> {
+                #(#checks)*
+                None
+            }
+        }
+    })
+}