fix(derive): fix lifetimes for functions without args

Author: aqrln

example/example.js

@@ -2,5 +2,5 @@
 
 const addon = require('./build/Release/example.node');
 
-addon.hello(undefined);
+addon.hello();
 console.log(addon.add(1, 2));

example/src/lib.rs

@@ -6,12 +6,9 @@ extern crate napi_derive;
 use napi::{NapiArgs, NapiEnv, NapiNumber, NapiResult, NapiUndefined};
 
 #[derive(NapiArgs)]
-struct HelloArgs<'a>(NapiUndefined<'a>);
+struct HelloArgs;
 
-fn hello<'a>(
-    env: &'a NapiEnv,
-    _: HelloArgs<'a>,
-) -> NapiResult<NapiUndefined<'a>> {
+fn hello(env: &NapiEnv, _: HelloArgs) -> NapiResult<NapiUndefined> {
     println!("Hello from the Rust land!");
     NapiUndefined::new(env)
 }

napi-derive/src/lib.rs

@@ -74,10 +74,16 @@ fn impl_napi_args(
         quote! { #name }
     };
 
+    let (gen_lifetime, ref_lifetime) = if count > 0 {
+        (quote! { <'env> }, quote! { 'env })
+    } else {
+        (quote!{}, quote!{})
+    };
+
     Ok(quote! {
-        impl<'env> NapiArgs<'env> for #name<'env> {
+        impl<'env> NapiArgs<'env> for #name #gen_lifetime {
             fn from_cb_info(
-                env: &'env ::napi::NapiEnv,
+                env: & #ref_lifetime ::napi::NapiEnv,
                 cb_info: ::napi::sys::napi_callback_info,
             ) -> ::napi::NapiResult<Self> {
                 use ::napi::sys;