proc: view: list available on error

proc/src/proc_view.rs

@@ -45,7 +45,16 @@ impl ToTokens for ViewDef {
     fn to_tokens (&self, out: &mut TokenStream2) {
         let Self(ViewMeta { output }, ViewImpl { block, exposed }) = self;
         let ident = &block.self_ty;
-        let exposed: Vec<_> = exposed.iter().map(|(k,v)|ViewArm(k.clone(), v.clone())).collect();
+        let mut available = vec![];
+        let exposed: Vec<_> = exposed.iter().map(|(k,v)|{
+            available.push(k.clone());
+            ViewArm(k.clone(), v.clone())
+        }).collect();
+        let available: String = available.join(", ");
+        let error_msg = LitStr::new(
+            &format!("expected Sym(content), got: {{value:?}}, available: {available}"),
+            Span::call_site()
+        );
         for token in quote! {
             #block
             /// Generated by [tengri_proc].
@@ -59,10 +68,7 @@ impl ToTokens for ViewDef {
                 fn get_content_sym <'source: 'state> (&'state self, value: &Value<'source>)
                     -> Option<RenderBox<'state, #output>>
                 {
-                    match value {
+                    match value { #(#exposed)* _ => panic!(#error_msg) }
-                        #(#exposed)*
-                        _ => panic!("expected Sym(content), got: {value:?}")
-                    }
                 }
             }
         } {