1 files changed, 81 insertions, 0 deletions
diff --git a/src/func.rs b/src/func.rs
new file mode 100644
index 0000000..5557992
--- /dev/null
+++ b/src/func.rs
@@ -0,0 +1,81 @@
+use crate::SynResult;
+use crate::config::default_feature_name;
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{
+    ItemFn, LitStr,
+    parse::{Parse, ParseStream},
+    parse_macro_input,
+};
+
+#[doc = include_str!("../doc/args/func.md")]
+pub struct FuncArgs {
+    pub feature_name: String,
+}
+
+impl Default for FuncArgs {
+    fn default() -> Self {
+        FuncArgs {
+            feature_name: default_feature_name().to_string(),
+        }
+    }
+}
+
+impl Parse for FuncArgs {
+    fn parse(input: ParseStream) -> SynResult<Self> {
+        if input.is_empty() {
+            return Ok(FuncArgs::default());
+        }
+
+        // Parse: "feature_name"
+        let feat_lit: LitStr = input.parse()?;
+        let feature_name = feat_lit.value();
+
+        Ok(FuncArgs { feature_name })
+    }
+}
+
+pub(crate) fn func(attr: TokenStream, item: TokenStream) -> TokenStream {
+    let args = parse_macro_input!(attr as FuncArgs);
+    let feature_name = &args.feature_name;
+
+    let input_fn = parse_macro_input!(item as ItemFn);
+    let attrs = &input_fn.attrs;
+    let vis = &input_fn.vis;
+    let sig = &input_fn.sig;
+    let block = &input_fn.block;
+
+    let expanded = quote! {
+        #(#attrs)*
+        #[cfg(not(feature = #feature_name))]
+        #vis #sig #block
+
+        #(#attrs)*
+        #[cfg(feature = #feature_name)]
+        #vis async #sig #block
+    };
+
+    TokenStream::from(expanded)
+}
+
+/// Test module for `FuncArgs` parsing.
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn empty_input_defaults_to_async() {
+        // Input: empty → defaults to "foo_async"
+        let input = proc_macro2::TokenStream::new();
+        let args: FuncArgs = syn::parse2(input).unwrap();
+        assert_eq!(args.feature_name, "foo_async");
+    }
+
+    #[test]
+    fn custom_feature_name_parsed() {
+        // Input: "custom_name"
+        let input: proc_macro2::TokenStream = "\"custom_name\"".parse().unwrap();
+        let args: FuncArgs = syn::parse2(input).unwrap();
+        assert_eq!(args.feature_name, "custom_name");
+    }
+}