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

@@ -1,6 +1,7 @@
-use quote::quote;
 use proc_macro::TokenStream;
-use syn::{ Lit, Path, Type, Ident, DeriveInput, parse_macro_input };
+use quote::{ quote, quote_spanned };
+use syn::{ Lit, Path, Type, Ident, DeriveInput };
+use proc_macro2::{ TokenStream as TokenStream2 };
 use darling::{ ast, Error, Result, FromMeta, FromVariant, FromDeriveInput };
 
 // ---------------------------------------------------------------------------------------------- //
@@ -22,36 +23,18 @@ struct VariantReceiver {
 
 // ---------------------------------------------------------------------------------------------- //
 
+/// A boolean that must be written as `= true` or `= false`
 #[derive(Debug, Clone, Copy)]
 struct StrictBool(bool);
 
-/// - `descriptor = "..."` - prefix shown before all variant messages (default: enum name)
-/// - `exit = true|false`  - whether to call process::exit (default: false)
-#[derive(Debug, FromMeta)]
-struct EnumAttrs {
-    #[darling(default)]
-    descriptor: Option<String>,
-    #[darling(default)]
-    exit:       Option<StrictBool>,
-}
-
-/// - `message = "..."`   - error message (default: variant name as title case)
-/// - `colorizer = fn`    - optional fn(String) -> String for formatting
-/// - `exit = true|false` - overrides enum-level exit
-#[derive(Debug, FromMeta)]
-struct VariantAttrs {
-    #[darling(default)]
-    exit:      Option<StrictBool>,
-    #[darling(default)]
-    message:   Option<String>,
-    #[darling(default)]
-    colorizer: Option<Path>,
-}
-
-// ---------------------------------------------------------------------------------------------- //
-
 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 {
@@ -59,41 +42,81 @@ impl FromMeta for StrictBool
             _ => Err(Error::unexpected_lit_type(value)),
         }
     }
-
-    // intentionally NOT implementing from_word()
 }
 
-fn parse_pmp_attr<T: FromMeta>(attrs: &[syn::Attribute]) -> T
-where
-    T: Default,
+/// A string that must be written as `= "..."`
+#[derive(Debug, Clone)]
+struct StrictString(String);
+
+impl FromMeta for StrictString
 {
-    attrs
-        .iter()
-        .find(|a| a.path().is_ident("pmp_emitter"))
-        .map(|a| T::from_meta(&a.meta).expect("invalid pmp_emitter attribute"))
-        .unwrap_or_default()
-}
+    fn from_word() -> Result<Self> {
+        Err(Error::custom(
+            "expected `= \"...\"`, provide a string value"
+        ))
+    }
 
-impl Default for EnumAttrs {
-    fn default() -> Self {
-        Self { descriptor: None, exit: None }
+    fn from_value(value: &Lit) -> Result<Self> {
+        String::from_value(value).map(StrictString)
     }
 }
 
-impl Default for VariantAttrs {
-    fn default() -> Self {
-        Self { message: None, exit: None, colorizer: None }
+/// 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)
     }
 }
 
 // ---------------------------------------------------------------------------------------------- //
 
-fn pascal_to_title(s: &str) -> String 
+/// - `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<StrictPath>,
+    #[darling(default)]
+    descriptor: Option<StrictString>,
+}
+
+/// - `message = "..."`   - error message (default: variant name as title case)
+/// - `colorizer = fn`    - optional fn(String) -> String for formatting
+/// - `exit = true|false` - overrides enum-level exit
+#[derive(Debug, Default, FromMeta)]
+struct VariantAttrs {
+    #[darling(default)]
+    exit:      Option<StrictBool>,
+    #[darling(default)]
+    message:   Option<StrictString>,
+    #[darling(default)]
+    colorizer: Option<StrictPath>,
+}
+
+// ---------------------------------------------------------------------------------------------- //
+
+fn pascal_to_title(s: &str) -> String
 {
     let mut out = String::new();
 
-    for (i, ch) in s.chars().enumerate() 
-    {
+    for (i, ch) in s.chars().enumerate() {
         if ch.is_uppercase() && i > 0 { out.push(' '); }
         out.push(ch);
     }
@@ -101,78 +124,115 @@ fn pascal_to_title(s: &str) -> String
     out
 }
 
+/// Parse a `#[pmp_emitter(...)]` attribute from the given list into `T`.
+///
+/// Returns `Ok(T::default())` if no such attribute is present or
+/// a span-accurate `Err` if the attribute is present but malformed.
+fn parse_pmp_attr<T>(attrs: &[syn::Attribute]) -> syn::Result<T>
+where
+    T: FromMeta + Default,
+{
+    match attrs.iter().find(|a| a.path().is_ident("pmp_emitter")) {
+        None       => Ok(T::default()),
+        Some(attr) => T::from_meta(&attr.meta)
+            .map_err(|e| syn::Error::new_spanned(attr, e.to_string())),
+    }
+}
+
 // ---------------------------------------------------------------------------------------------- //
 
-#[proc_macro_derive(PmpEmitter, attributes(pmp_emitter))]
-pub fn pmp_emitter(item: TokenStream) -> TokenStream
+/// All real expansion logic lives here, separate from the proc-macro entry point,
+/// so it can be called and tested without the proc-macro machinery.
+fn expand(shape: EmitterInput, raw_input: &DeriveInput) -> syn::Result<TokenStream2>
 {
-    let input = parse_macro_input!(item as DeriveInput);
-
-    // parse enum shape
-    let shape = match EmitterInput::from_derive_input(&input) {
-        Ok(a)  => a,
-        Err(e) => return e.write_errors().into(),
-    };
-
-    // parse enum-level #[pmp_emitter(...)] directly from the raw attrs
-    let enum_attrs: EnumAttrs = parse_pmp_attr(&input.attrs);
+    let enum_attrs: EnumAttrs = parse_pmp_attr(&raw_input.attrs)?;
 
     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 variants = shape
-        .data
-        .take_enum()
-        .expect("PmpEmitter can only be derived on enums");
+    let global_exit      = enum_attrs.exit.map(|b| b.0).unwrap_or(false);
+    let global_colorizer = enum_attrs.colorizer;
 
-    let match_arms = variants.iter().map(|v|
+    // Reject non-enums with a proper span pointing at the type name.
+    let variants = shape.data.take_enum().ok_or_else(|| {
+        syn::Error::new_spanned(enum_ident, "PmpEmitter can only be derived on enums")
+    })?;
+
+    // Accumulate all per-variant attribute errors before failing, so the user
+    // sees every problem in one compilation instead of one-at-a-time.
+    let mut errors: Option<syn::Error>    = None;
+    let mut match_arms: Vec<TokenStream2> = Vec::with_capacity(variants.len());
+
+    for v in &variants
+    {
+        let var_ident = &v.ident;
+        let title     = pascal_to_title(&var_ident.to_string());
+
+        let v_attrs: VariantAttrs = match parse_pmp_attr(&v.attrs)
         {
-            let var_ident = &v.ident;
-            let title     = pascal_to_title(&var_ident.to_string());
-
-            // parse variant-level #[pmp_emitter(...)] from the variant's raw attrs
-            let vattrs: VariantAttrs = parse_pmp_attr(&v.attrs);
-
-            let message     = vattrs.message.unwrap_or_else(|| title.clone());
-            let should_exit = vattrs.exit.map(|b| b.0).unwrap_or(global_exit);
-
-            // ┌─ Output format ───────────────────────────────┐
-            // │  {descriptor}: {Variant Title} ("{message}")  │
-            // │                                               │
-            // │  {stacktrace}                                 │
-            // └───────────────────────────────────────────────┘
-            let header = format!("{}: {} (\"{}\")", descriptor, title, message);
-
-            let formatted_header = match &vattrs.colorizer
+            Ok(a)  => a,
+            Err(e) => 
             {
-                Some(path) => quote! { #path(#header) },
-                None       => quote! { #header.to_string() },
-            };
+                match &mut errors {
+                    None           => errors = Some(e),
+                    Some(existing) => existing.combine(e),
+                }
+                continue; // keep parsing remaining variants
+            }
+        };
 
-            let exit_tokens = if should_exit { quote! { std::process::exit(1); } } else { quote! {} };
+        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);
 
-            match v.fields.style
+        // ┌─ Output format ───────────────────────────────┐
+        // │  {descriptor}: {Variant Title} ("{message}")  │
+        // │                                               │
+        // │  {stacktrace}                                 │
+        // └───────────────────────────────────────────────┘
+        let header = format!("{}: {} (\"{}\")", descriptor, title, message);
+
+        let formatted_header = match v_attrs.colorizer.as_ref().or(global_colorizer.as_ref()) {
+            Some(StrictPath(path)) => quote! { #path(#header) },
+            None                   => quote! { #header.to_string() },
+        };
+
+        let exit_tokens = if should_exit { quote! { std::process::exit(1); } } else { quote! {} };
+
+        // quote_spanned! ties any downstream type errors back to the variant's
+        // source location instead of pointing at generated code.
+        let arm = match v.fields.style
+        {
+            ast::Style::Tuple =>
             {
-                ast::Style::Tuple => quote! {
-                #enum_ident::#var_ident(trace) =>
-                {
-                    println!("{}\n\n{}", #formatted_header, trace);
-                    #exit_tokens
+                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! {
+            _ => quote_spanned! { var_ident.span() =>
                 #enum_ident::#var_ident =>
                 {
                     println!("{}", #formatted_header);
                     #exit_tokens
                 }
             },
-            }
-        });
+        };
 
-    quote! {
+        match_arms.push(arm);
+    }
+
+    // Surface all accumulated variant errors at once.
+    if let Some(e) = errors { return Err(e); }
+
+    Ok(quote!
+    {
         impl #enum_ident
         {
             pub fn emit(&self) {
@@ -186,5 +246,23 @@ pub fn pmp_emitter(item: TokenStream) -> TokenStream
                 self.emit()
             }
         }
-    }.into()
+    })
+}
+
+// ---------------------------------------------------------------------------------------------- //
+
+/// Thin entry point: parse -> delegate -> convert any error to `compile_error!`.
+#[proc_macro_derive(PmpEmitter, attributes(pmp_emitter))]
+pub fn pmp_emitter(item: TokenStream) -> TokenStream
+{
+    let input = syn::parse_macro_input!(item as DeriveInput);
+
+    let shape = match EmitterInput::from_derive_input(&input) {
+        Ok(s)  => s,
+        Err(e) => return e.write_errors().into(),
+    };
+
+    expand(shape, &input)
+        .unwrap_or_else(syn::Error::into_compile_error)
+        .into()
 }