chore: use extism:user namespace by default for user-defined host fun…

…ctions

derive/src/lib.rs

@@ -106,9 +106,14 @@ pub fn plugin_fn(
 /// `host_fn` is used to define a host function that will be callable from within a plugin
 #[proc_macro_attribute]
 pub fn host_fn(
-    _attr: proc_macro::TokenStream,
+    attr: proc_macro::TokenStream,
     item: proc_macro::TokenStream,
 ) -> proc_macro::TokenStream {
+    let mut namespace = attr.to_string();
+    if namespace.is_empty() {
+        namespace = "extism:user".to_string();
+    }
+
     let item = parse_macro_input!(item as ItemForeignMod);
     if item.abi.name.is_none() || item.abi.name.unwrap().value() != "ExtismHost" {
         panic!("Expected `extern \"ExtismHost\"` block");
@@ -209,7 +214,7 @@ pub fn host_fn(
             let link_name = link_name.as_str();
 
             let impl_block = quote! {
-                #[link(wasm_import_module = "extism:env")]
+                #[link(wasm_import_module = #namespace)]
                 extern "C" {
                     #[link_name = #link_name]
                     fn #impl_name(#(#converted_inputs),*) -> #converted_output;