Refactor attribute parsing in pmp-emitter-core: enforce strict types, add StrictBool, StrictString, and StrictPath wrappers, and improve error reporting

crates/pmp-emitter/crates/pmp-emitter-core/src/lib.rs

@@ -29,6 +29,12 @@ struct StrictBool(bool);
 
 impl FromMeta for StrictBool
 {
+    fn from_word() -> Result<Self> {
+        Err(Error::custom(
+            "expected `= true/false`, provide a boolean value"
+        ))
+    }
+
     fn from_value(value: &Lit) -> Result<Self>
     {
         match value {
@@ -36,22 +42,59 @@ impl FromMeta for StrictBool
             _ => Err(Error::unexpected_lit_type(value)),
         }
     }
+}
 
-    // intentionally NOT implementing from_word() - bare `exit` is rejected
+/// A string that must be written as `= "..."`
+#[derive(Debug, Clone)]
+struct StrictString(String);
+
+impl FromMeta for StrictString
+{
+    fn from_word() -> Result<Self> {
+        Err(Error::custom(
+            "expected `= \"...\"`, provide a string value"
+        ))
+    }
+
+    fn from_value(value: &Lit) -> Result<Self> {
+        String::from_value(value).map(StrictString)
+    }
+}
+
+/// A path that must be written as `= my_fn`
+#[derive(Debug, Clone)]
+struct StrictPath(Path);
+
+impl FromMeta for StrictPath
+{
+    fn from_word() -> Result<Self> {
+        Err(Error::custom(
+            "expected `= my_fn`, provide a function path"
+        ))
+    }
+
+    fn from_value(value: &Lit) -> Result<Self> {
+        Path::from_value(value).map(StrictPath)
+    }
+
+    fn from_expr(expr: &syn::Expr) -> Result<Self> {
+        Path::from_expr(expr).map(StrictPath)
+    }
 }
 
 // ---------------------------------------------------------------------------------------------- //
 
 /// - `descriptor = "..."` - prefix shown before all variant messages (default: enum name)
 /// - `exit = true|false`  - whether to call process::exit (default: false)
+/// - `colorizer = fn`     - fn(String) -> String applied to all variants unless overridden
 #[derive(Debug, Default, FromMeta)]
 struct EnumAttrs {
     #[darling(default)]
     exit:       Option<StrictBool>,
     #[darling(default)]
-    colorizer:  Option<Path>,
+    colorizer:  Option<StrictPath>,
     #[darling(default)]
-    descriptor: Option<String>,
+    descriptor: Option<StrictString>,
 }
 
 /// - `message = "..."`   - error message (default: variant name as title case)
@@ -62,9 +105,9 @@ struct VariantAttrs {
     #[darling(default)]
     exit:      Option<StrictBool>,
     #[darling(default)]
-    message:   Option<String>,
+    message:   Option<StrictString>,
     #[darling(default)]
-    colorizer: Option<Path>,
+    colorizer: Option<StrictPath>,
 }
 
 // ---------------------------------------------------------------------------------------------- //
@@ -106,7 +149,9 @@ fn expand(shape: EmitterInput, raw_input: &DeriveInput) -> syn::Result<TokenStre
 
     let enum_ident  = &shape.ident;
     let descriptor  = enum_attrs.descriptor
+        .map(|s| s.0)
         .unwrap_or_else(|| pascal_to_title(&enum_ident.to_string()));
+
     let global_exit      = enum_attrs.exit.map(|b| b.0).unwrap_or(false);
     let global_colorizer = enum_attrs.colorizer;
 
@@ -128,7 +173,8 @@ fn expand(shape: EmitterInput, raw_input: &DeriveInput) -> syn::Result<TokenStre
         let v_attrs: VariantAttrs = match parse_pmp_attr(&v.attrs)
         {
             Ok(a)  => a,
-            Err(e) => {
+            Err(e) => 
+            {
                 match &mut errors {
                     None           => errors = Some(e),
                     Some(existing) => existing.combine(e),
@@ -137,7 +183,7 @@ fn expand(shape: EmitterInput, raw_input: &DeriveInput) -> syn::Result<TokenStre
             }
         };
 
-        let message     = v_attrs.message.unwrap_or_else(|| title.clone());
+        let message     = v_attrs.message.map(|s| s.0).unwrap_or_else(|| title.clone());
         let should_exit = v_attrs.exit.map(|b| b.0).unwrap_or(global_exit);
 
         // ┌─ Output format ───────────────────────────────┐
@@ -148,7 +194,7 @@ fn expand(shape: EmitterInput, raw_input: &DeriveInput) -> syn::Result<TokenStre
         let header = format!("{}: {} (\"{}\")", descriptor, title, message);
 
         let formatted_header = match v_attrs.colorizer.as_ref().or(global_colorizer.as_ref()) {
-            Some(path) => quote! { #path(#header) },
+            Some(StrictPath(path)) => quote! { #path(#header) },
             None                   => quote! { #header.to_string() },
         };
 
@@ -158,14 +204,21 @@ fn expand(shape: EmitterInput, raw_input: &DeriveInput) -> syn::Result<TokenStre
         // source location instead of pointing at generated code.
         let arm = match v.fields.style
         {
-            ast::Style::Tuple => quote_spanned! { var_ident.span() =>
-                #enum_ident::#var_ident(trace) => {
+            ast::Style::Tuple =>
+            {
+                let pattern = if v.fields.len() == 1 { quote! { (trace) } } else { quote! { (trace, ..) } };
+
+                quote_spanned! { var_ident.span() =>
+                    #enum_ident::#var_ident #pattern =>
+                    {
                         println!("{}\n\n{}", #formatted_header, trace);
                         #exit_tokens
                     }
+                }
             },
             _ => quote_spanned! { var_ident.span() =>
-                #enum_ident::#var_ident => {
+                #enum_ident::#var_ident =>
+                {
                     println!("{}", #formatted_header);
                     #exit_tokens
                 }