feat: error handle generation on validation fail

runtime/pavex_macros/src/error_handler.rs

@@ -52,7 +52,22 @@ impl CallableAnnotation for ErrorHandlerAnnotation {
         metadata: Self::InputSchema,
         item: Callable,
     ) -> Result<AnnotationCodegen, proc_macro::TokenStream> {
-        let error_ref_index = find_error_ref_index(&item).map_err(|e| e.write_errors())?;
+        let error_ref_index = match find_error_ref_index(&item) {
+            Ok(index) => index,
+            Err(e) => {
+                let properties = Properties::new(metadata, 0);
+                let result = emit(impl_, item, properties);
+
+                let error_tokens = e.write_errors();
+                let handle_def = result.id_def.unwrap_or_default();
+                let combined_error = quote::quote! {
+                    #error_tokens
+                    #handle_def
+                };
+                return Err(combined_error.into());
+            }
+        };
+
         let properties = Properties::new(metadata, error_ref_index);
         Ok(emit(impl_, item, properties))
     }

runtime/pavex_macros/src/methods.rs

@@ -17,6 +17,21 @@ use crate::{
 };
 
 pub fn methods(_metadata: TokenStream, input: TokenStream) -> Result<TokenStream, TokenStream> {
+    _methods_inner(_metadata, input.clone()).map_err(|error_tokens| {
+        let error_tokens = proc_macro2::TokenStream::from(error_tokens);
+        let input_ts: proc_macro2::TokenStream = input.into();
+        if let Ok(mut item) = syn::parse2::<syn::Item>(input_ts.clone()) {
+            crate::utils::PxStripper.visit_item_mut(&mut item);
+            return quote! { #error_tokens #item }.into();
+        }
+        quote! { #error_tokens #input_ts }.into()
+    })
+}
+
+fn _methods_inner(
+    _metadata: TokenStream,
+    input: TokenStream,
+) -> Result<TokenStream, proc_macro::TokenStream> {
     let mut impl_: syn::ItemImpl = syn::parse(input).map_err(|e| e.into_compile_error())?;
     let mut new_items: Vec<proc_macro2::TokenStream> = Vec::new();
 
@@ -78,15 +93,16 @@ pub fn methods(_metadata: TokenStream, input: TokenStream) -> Result<TokenStream
     }
 
     if no_pavex_method {
-        return Err(syn::Error::new(
+        let error_tokens = syn::Error::new(
             proc_macro2::Span::call_site(),
             "`#[pavex::methods]` is used to provide context for Pavex attributes on methods \
             (e.g., `#[pavex::get]`, `#[pavex::post]`, `#[pavex::error_handler]`, etc.).\n\
             This `impl` block contains no methods with Pavex attributes, so `#[pavex::methods]` \
             is unnecessary.",
         )
-        .into_compile_error()
-        .into());
+        .into_compile_error();
+
+        return Err(error_tokens.into());
     }
 
     PxStripper.visit_item_impl_mut(&mut impl_);

runtime/pavex_macros/src/utils/fn_like.rs

@@ -103,12 +103,7 @@ pub fn direct_entrypoint<M: CallableAnnotation>(
         Ok(t) => t.into(),
         Err(error_tokens) => {
             let error_tokens = proc_macro2::TokenStream::from(error_tokens);
-            let mut input_item: syn::Item = syn::parse2(input).expect("Input is not a valid syn::Item");
-            crate::utils::PxStripper.visit_item_mut(&mut input_item);
-            quote! {
-                #error_tokens
-                #input_item
-            }.into()
+            reemit_with_input(error_tokens, input)
         }
     }
 }
@@ -137,3 +132,22 @@ fn parse_metadata<T: darling::FromMeta>(metadata: TokenStream) -> Result<T, Toke
         darling::ast::NestedMeta::parse_meta_list(metadata).map_err(|e| e.to_compile_error())?;
     T::from_list(&attrs).map_err(|e| e.write_errors())
 }
+
+fn reemit_with_input(
+    error_tokens: proc_macro2::TokenStream,
+    input: proc_macro2::TokenStream,
+) -> proc_macro::TokenStream {
+    if let Ok(mut item) = syn::parse2::<syn::Item>(input.clone()) {
+        crate::utils::PxStripper.visit_item_mut(&mut item);
+        return quote! { #error_tokens #item }.into();
+    }
+    if let Ok(mut method) = syn::parse2::<syn::ImplItemFn>(input.clone()) {
+        crate::utils::PxStripper.visit_impl_item_fn_mut(&mut method);
+        return quote! { #error_tokens #method }.into();
+    }
+    if let Ok(mut trait_fn) = syn::parse2::<syn::TraitItemFn>(input.clone()) {
+        crate::utils::PxStripper.visit_trait_item_fn_mut(&mut trait_fn);
+        return quote! { #error_tokens #trait_fn }.into();
+    }
+    quote! { #error_tokens #input }.into()
+}

runtime/pavex_macros/src/utils/px_stripper.rs

@@ -1,6 +1,6 @@
 use syn::{
-    Attribute, Field, FieldValue, FnArg, ItemEnum, ItemImpl, ItemMod, ItemStruct, ItemTrait,
-    TraitItem, Variant,
+    Attribute, Field, FieldValue, FnArg, ImplItemFn, ItemEnum, ItemImpl, ItemMod,
+    ItemStruct, ItemTrait, TraitItem, TraitItemFn, Variant,
     visit_mut::{self, VisitMut},
 };
 
@@ -75,6 +75,28 @@ impl VisitMut for PxStripper {
         attrs.retain(not_px_attr);
         visit_mut::visit_fn_arg_mut(self, node);
     }
+
+    fn visit_impl_item_fn_mut(&mut self, node: &mut ImplItemFn) {
+        strip_pavex_attrs(&mut node.attrs);
+        visit_mut::visit_impl_item_fn_mut(self, node);
+    }
+
+    fn visit_trait_item_fn_mut(&mut self, node: &mut TraitItemFn) {
+        strip_pavex_attrs(&mut node.attrs);
+        visit_mut::visit_trait_item_fn_mut(self, node);
+    }
+}
+
+fn strip_pavex_attrs(attrs: &mut Vec<Attribute>) {
+    attrs.retain(|a| !is_user_pavex_attr(a) && not_px_attr(a));
+}
+
+fn is_user_pavex_attr(a: &Attribute) -> bool {
+    a.path()
+        .segments
+        .first()
+        .map(|s| s.ident == "pavex")
+        .unwrap_or(false)
 }
 
 fn not_px_attr(a: &Attribute) -> bool {

runtime/pavex_macros/src/utils/type_like.rs

@@ -2,7 +2,7 @@
 //! Pavex components that accept type-like inputs.
 use convert_case::{Case, Casing as _};
 use proc_macro2::TokenStream;
-use quote::{quote, ToTokens, format_ident};
+use quote::{ToTokens, format_ident, quote};
 use syn::{parse_quote, visit_mut::VisitMut};
 
 use crate::utils::{AnnotationCodegen, deny_unreachable_pub_attr, validation::must_be_public};
@@ -218,12 +218,7 @@ pub fn entrypoint<M: TypeAnnotation>(
         Ok(t) => t.into(),
         Err(error_tokens) => {
             let error_tokens = proc_macro2::TokenStream::from(error_tokens);
-            let mut input_item: syn::Item = syn::parse2(input).expect("Input is not a valid syn::Item");
-            crate::utils::PxStripper.visit_item_mut(&mut input_item);
-            quote! {
-                #error_tokens
-                #input_item
-            }.into()
+            reemit_with_input(error_tokens, input)
         }
     }
 }
@@ -233,3 +228,14 @@ fn parse_metadata<T: darling::FromMeta>(metadata: TokenStream) -> Result<T, Toke
         darling::ast::NestedMeta::parse_meta_list(metadata).map_err(|e| e.to_compile_error())?;
     T::from_list(&attrs).map_err(|e| e.write_errors())
 }
+
+fn reemit_with_input(
+    error_tokens: proc_macro2::TokenStream,
+    input: proc_macro2::TokenStream,
+) -> proc_macro::TokenStream {
+    if let Ok(mut item) = syn::parse2::<syn::Item>(input.clone()) {
+        crate::utils::PxStripper.visit_item_mut(&mut item);
+        return quote! { #error_tokens #item }.into();
+    }
+    quote! { #error_tokens #input }.into()
+}

runtime/pavex_macros/tests/error_handler/fail/custom_id_generation.rs

@@ -0,0 +1,12 @@
+use pavex_macros::error_handler;
+
+#[error_handler(id = "CUSTOM")]  
+pub fn invalid_handler(_a: &str, _b: u64) -> pavex::Response {
+    todo!()
+}
+
+fn test_handle_exists() {
+    let _handle = CUSTOM;
+}
+
+fn main() {}

runtime/pavex_macros/tests/error_handler/fail/custom_id_generation.stderr

@@ -0,0 +1,6 @@
+error: Mark the error reference input with `#[px(error_ref)]`.
+       Pavex can't automatically identify it if your error handler has two or more input parameters.
+ --> tests/error_handler/fail/custom_id_generation.rs:4:23
+  |
+4 | pub fn invalid_handler(_a: &str, _b: u64) -> pavex::Response {
+  |                       ^^^^^^^^^^^^^^^^^^^

runtime/pavex_macros/tests/error_handler/fail/handle_generation_on_error.rs

@@ -0,0 +1,12 @@
+use pavex_macros::error_handler;
+
+#[error_handler]  
+pub fn invalid_handler(_a: &str, _b: u64) -> pavex::Response {
+    todo!()
+}
+
+fn test_handle_exists() {
+    let _handle = INVALID_HANDLER;
+}
+
+fn main() {}

runtime/pavex_macros/tests/error_handler/fail/handle_generation_on_error.stderr

@@ -0,0 +1,6 @@
+error: Mark the error reference input with `#[px(error_ref)]`.
+       Pavex can't automatically identify it if your error handler has two or more input parameters.
+ --> tests/error_handler/fail/handle_generation_on_error.rs:4:23
+  |
+4 | pub fn invalid_handler(_a: &str, _b: u64) -> pavex::Response {
+  |                       ^^^^^^^^^^^^^^^^^^^