Improve error message when custom arg decode fails

crates/pine-builtin-macro/src/lib.rs

@@ -779,6 +779,8 @@ fn generate_value_conversion(
     has_default: bool,
 ) -> proc_macro2::TokenStream {
     let type_str = quote! { #field_type }.to_string();
+    let field_name_str = field_name.to_string();
+    let field_name_str = field_name_str.trim_start_matches("r#");
 
     // Check if this is an Option type
     let is_option = type_str.contains("Option");
@@ -841,7 +843,7 @@ fn generate_value_conversion(
                 if matches!(arg_value, Value::Na) {
                     None
                 } else {
-                    Some(<#inner as ::pine_interpreter::FromArg<O>>::from_arg(&arg_value)?)
+                    Some(<#inner as ::pine_interpreter::FromArg<O>>::from_arg(#field_name_str, &arg_value)?)
                 }
             }
         }
@@ -866,7 +868,7 @@ fn generate_value_conversion(
     } else {
         // Any other type is converted from the argument — e.g. a string-constant
         // enum deserialized from a `Value::String`.
-        quote! { <#field_type as ::pine_interpreter::FromArg<O>>::from_arg(&arg_value)? }
+        quote! { <#field_type as ::pine_interpreter::FromArg<O>>::from_arg(#field_name_str, &arg_value)? }
     };
 
     if has_default {

crates/pine-interpreter/src/lib.rs

@@ -14,18 +14,25 @@ use thiserror::Error;
 
 pub use pine_core::LibraryLoader;
 
-/// Convert a call argument into a builtin parameter type. Blanket-implemented for
-/// any `serde`-deserializable type by deserializing the argument's string, so a
-/// string-constant enum (`LabelStyle`, …) can be a builtin parameter directly.
+/// Convert a builtin call argument into a custom parameter type — one that isn't
+/// a macro primitive (`String`, `f64`, `bool`, `Color`, `Value`, `Num`).
+///
+/// The rule: the type implements [`serde::Deserialize`] and is deserialized from
+/// the argument's `Value::String`, so a fieldless enum with
+/// `#[serde(rename_all = "snake_case")]` accepts the lowercase constants scripts
+/// write (`"close"`, `"signed"`, …). On mismatch the error names the argument and
+/// — for an enum — lists the accepted variants.
 pub trait FromArg<O: PineOutput>: Sized {
-    fn from_arg(value: &Value<O>) -> Result<Self, RuntimeError>;
+    fn from_arg(arg: &str, value: &Value<O>) -> Result<Self, RuntimeError>;
 }
 
 impl<O: PineOutput, T: serde::de::DeserializeOwned> FromArg<O> for T {
-    fn from_arg(value: &Value<O>) -> Result<Self, RuntimeError> {
-        let s = value.as_string()?;
+    fn from_arg(arg: &str, value: &Value<O>) -> Result<Self, RuntimeError> {
+        let s = value
+            .as_string()
+            .map_err(|_| RuntimeError::TypeError(format!("`{arg}` expects a string constant")))?;
         let de = serde::de::value::StrDeserializer::<serde::de::value::Error>::new(s.as_str());
-        T::deserialize(de).map_err(|e| RuntimeError::TypeError(format!("invalid value `{s}`: {e}")))
+        T::deserialize(de).map_err(|e| RuntimeError::TypeError(format!("`{arg}`: {e}")))
     }
 }