rust: macros: support #[cfg] properly in #[vtable] macro.

Currently, we generate `HAS_` constants as long as the definition exists in
the source, regardless if it is cfg-ed out or not.

Currently, uses of `#[cfg]` present in both trait and impl, so it is not a
problem; however if only the impl side uses `#[cfg]` then `HAS_` constants
will incorrectly be true while it shouldnt't.

With `syn` support, we can now implement `#[cfg]` handling properly by
propagating the `#[cfg]` attributes to the constants.

Signed-off-by: Gary Guo <[email protected]>

Author: nbdd0121

rust/macros/helpers.rs

@@ -7,6 +7,7 @@ use syn::{
         Parse,
         ParseStream, //
     },
+    Attribute,
     Error,
     LitStr,
     Result, //
@@ -53,3 +54,11 @@ pub(crate) fn file() -> String {
         proc_macro::Span::call_site().file()
     }
 }
+
+/// Obtain all `#[cfg]` attributes.
+pub(crate) fn gather_cfg_attrs(attr: &[Attribute]) -> Vec<Attribute> {
+    attr.iter()
+        .filter(|a| a.path().is_ident("cfg"))
+        .cloned()
+        .collect()
+}

rust/macros/vtable.rs

@@ -23,7 +23,6 @@ use syn::{
 
 fn handle_trait(mut item: ItemTrait) -> Result<ItemTrait> {
     let mut gen_items = Vec::new();
-    let mut gen_consts = HashSet::new();
 
     gen_items.push(parse_quote! {
          /// A marker to prevent implementors from forgetting to use [`#[vtable]`](vtable)
@@ -38,22 +37,17 @@ fn handle_trait(mut item: ItemTrait) -> Result<ItemTrait> {
                 &format!("HAS_{}", name.to_string().to_uppercase()),
                 name.span(),
             );
-            // Skip if it's declared already -- this can happen if `#[cfg]` is used to selectively
-            // define functions.
-            // FIXME: `#[cfg]` should be copied and propagated to the generated consts.
-            if gen_consts.contains(&gen_const_name) {
-                continue;
-            }
 
             // We don't know on the implementation-site whether a method is required or provided
             // so we have to generate a const for all methods.
+            let cfg_attrs = crate::helpers::gather_cfg_attrs(&fn_item.attrs);
             let comment =
                 format!("Indicates if the `{name}` method is overridden by the implementor.");
             gen_items.push(parse_quote! {
+                #(#cfg_attrs)*
                 #[doc = #comment]
                 const #gen_const_name: bool = false;
             });
-            gen_consts.insert(gen_const_name);
         }
     }
 
@@ -87,10 +81,11 @@ fn handle_impl(mut item: ItemImpl) -> Result<ItemImpl> {
             if defined_consts.contains(&gen_const_name) {
                 continue;
             }
+            let cfg_attrs = crate::helpers::gather_cfg_attrs(&fn_item.attrs);
             gen_items.push(parse_quote! {
+                #(#cfg_attrs)*
                 const #gen_const_name: bool = true;
             });
-            defined_consts.insert(gen_const_name);
         }
     }