feat(neon-macros): Export const and static

crates/neon-macros/Cargo.toml

@@ -11,5 +11,6 @@ edition = "2021"
 proc-macro = true
 
 [dependencies]
+proc-macro2 = "1.0.79"
 quote = "1.0.33"
-syn = "2.0.39"
+syn = { version = "2.0.57", features = ["full"] }

crates/neon-macros/src/export/global/meta.rs

@@ -0,0 +1,34 @@
+#[derive(Default)]
+pub(crate) struct Meta {
+    pub(super) name: Option<syn::LitStr>,
+    pub(super) json: bool,
+}
+
+pub(crate) struct Parser;
+
+impl syn::parse::Parser for Parser {
+    type Output = Meta;
+
+    fn parse2(self, tokens: proc_macro2::TokenStream) -> syn::Result<Self::Output> {
+        let mut attr = Meta::default();
+        let parser = syn::meta::parser(|meta| {
+            if meta.path.is_ident("name") {
+                attr.name = Some(meta.value()?.parse::<syn::LitStr>()?);
+
+                return Ok(());
+            }
+
+            if meta.path.is_ident("json") {
+                attr.json = true;
+
+                return Ok(());
+            }
+
+            Err(meta.error("unsupported property"))
+        });
+
+        parser.parse2(tokens)?;
+
+        Ok(attr)
+    }
+}

crates/neon-macros/src/export/global/mod.rs

@@ -0,0 +1,44 @@
+pub(crate) mod meta;
+
+// Create a new block expression for the RHS of an assignment
+pub(super) fn export(meta: meta::Meta, name: &syn::Ident, expr: Box<syn::Expr>) -> Box<syn::Expr> {
+    // Name for the registered create function
+    let create_name = quote::format_ident!("__NEON_EXPORT_CREATE__{name}");
+
+    // Default export name as identity unless a name is provided
+    let export_name = meta
+        .name
+        .map(|name| quote::quote!(#name))
+        .unwrap_or_else(|| quote::quote!(stringify!(#name)));
+
+    // If `json` is enabled, wrap the value in `Json` before `TryIntoJs` is called
+    let value = meta
+        .json
+        .then(|| quote::quote!(neon::types::extract::Json(&#name)))
+        .unwrap_or_else(|| quote::quote!(#name));
+
+    // Generate the function that is registered to create the global on addon initialization.
+    // Braces are included to prevent names from polluting user code.
+    let create_fn = quote::quote!({
+        #[doc(hidden)]
+        #[neon::macro_internal::linkme::distributed_slice(neon::macro_internal::EXPORTS)]
+        #[linkme(crate = neon::macro_internal::linkme)]
+        fn #create_name<'cx>(
+            cx: &mut neon::context::ModuleContext<'cx>,
+        ) -> neon::result::NeonResult<(&'static str, neon::handle::Handle<'cx, neon::types::JsValue>)> {
+            neon::types::extract::TryIntoJs::try_into_js(#value, cx).map(|v| (
+                #export_name,
+                neon::handle::Handle::upcast(&v),
+            ))
+        }
+    });
+
+    // Create a block to hold the original expression and the registered crate function
+    let expr = quote::quote!({
+        #create_fn
+        #expr
+    });
+
+    // Create an expression from the token stream
+    Box::new(syn::Expr::Verbatim(expr))
+}

crates/neon-macros/src/export/mod.rs

@@ -0,0 +1,43 @@
+mod global;
+
+// N.B.: Meta attribute parsing happens in this function because `syn::parse_macro_input!`
+// must be called from a function that returns `proc_macro::TokenStream`.
+pub(crate) fn export(
+    attr: proc_macro::TokenStream,
+    item: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+    // Parse item to determine the type of export
+    let item = syn::parse_macro_input!(item as syn::Item);
+
+    match item {
+        // Export a `const`
+        syn::Item::Const(mut item) => {
+            let meta = syn::parse_macro_input!(attr with global::meta::Parser);
+
+            item.expr = global::export(meta, &item.ident, item.expr);
+
+            quote::quote!(#item).into()
+        }
+
+        // Export a `static`
+        syn::Item::Static(mut item) => {
+            let meta = syn::parse_macro_input!(attr with global::meta::Parser);
+
+            item.expr = global::export(meta, &item.ident, item.expr);
+
+            quote::quote!(#item).into()
+        }
+
+        // Return an error span for all other types
+        _ => unsupported(item),
+    }
+}
+
+// Generate an error for unsupported item types
+fn unsupported(item: syn::Item) -> proc_macro::TokenStream {
+    let span = syn::spanned::Spanned::span(&item);
+    let msg = "`neon::export` can only be applied to consts, and statics.";
+    let err = syn::Error::new(span, msg);
+
+    err.into_compile_error().into()
+}

crates/neon-macros/src/lib.rs

@@ -1,5 +1,7 @@
 //! Procedural macros supporting [Neon](https://docs.rs/neon/latest/neon/)
 
+mod export;
+
 #[proc_macro_attribute]
 /// Marks a function as the main entry point for initialization in
 /// a Neon module.
@@ -58,3 +60,42 @@ pub fn main(
     )
     .into()
 }
+
+#[proc_macro_attribute]
+/// Register an item to be exported by the Neon addon
+///
+/// ## Exporting constants and statics
+///
+/// ```ignore
+/// #[neon::export]
+/// static GREETING: &str = "Hello, Neon!";
+///
+/// #[neon::export]
+/// const ANSWER: u8 = 42;
+/// ```
+///
+/// ### Renaming an export
+///
+/// By default, items will be exported with their Rust name. Exports may
+/// be renamed by providing the `name` attribute.
+///
+/// ```ignore
+/// #[neon::export(name = "myGreeting")]
+/// static GREETING: &str = "Hello, Neon!";
+/// ```
+///
+/// ### JSON exports
+///
+/// Complex values may be exported by automatically serializing to JSON and
+/// parsing in JavaScript. Any type that implements `serde::Serialize` may be used.
+///
+/// ```ignore
+/// #[neon::export]
+/// static MESSAGES: &[&str] = &["hello", "goodbye"];
+/// ```
+pub fn export(
+    attr: proc_macro::TokenStream,
+    item: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+    export::export(attr, item)
+}