fix(ast_macros): do not panic in macro (#13102)

Previously if you edited an AST struct, adding or removing fields, `#[ast]` macro would panic because the fields of the struct don't match what it's expecting from the `STRUCTS` metadata which is generated by `oxc_ast_tools`.

It would cover the whole struct with red squiggles, with a cryptic error message. This was really annoying.

Instead, make the macro output a `compile_error!` macro invocation. This makes IDE and `cargo build` show an error saying that you need to run `just ast` to re-run the codegen. Also, because the macro doesn't crash any more, you don't have to re-start Rust Analyser once you've fixed the problem.

This is less annoying for humans, and also the more helpful error message will hopefully point AI in the right direction.

crates/oxc_ast_macros/src/ast.rs

@@ -1,8 +1,8 @@
 use proc_macro2::{TokenStream, TokenTree};
-use quote::quote;
+use quote::{quote, quote_spanned};
 use syn::{
     Attribute, Fields, FieldsNamed, Ident, Item, ItemEnum, ItemStruct, parse_quote,
-    punctuated::Punctuated, token::Comma,
+    punctuated::Punctuated, spanned::Spanned, token::Comma,
 };
 
 use crate::generated::{derived_traits::get_trait_crate_and_generics, structs::STRUCTS};
@@ -47,12 +47,14 @@ pub struct StructDetails {
 fn modify_struct(item: &mut ItemStruct, args: TokenStream) -> TokenStream {
     let assertions = assert_generated_derives(&item.attrs);
 
-    reorder_struct_fields(item, args);
+    let reorder_result = reorder_struct_fields(item, args);
+    let error = reorder_result.err().map(|message| compile_error(&item.ident, message));
 
     quote! {
         #[repr(C)]
         #[derive(::oxc_ast_macros::Ast)]
         #item
+        #error
         #assertions
     }
 }
@@ -60,28 +62,33 @@ fn modify_struct(item: &mut ItemStruct, args: TokenStream) -> TokenStream {
 /// Re-order struct fields, depending on instructions in `STRUCTS` (which is codegen-ed).
 ///
 /// Mutates `item` in place, re-ordering its fields.
-fn reorder_struct_fields(item: &mut ItemStruct, args: TokenStream) {
+fn reorder_struct_fields(item: &mut ItemStruct, args: TokenStream) -> Result<(), &'static str> {
     // Skip foreign types
     if let Some(TokenTree::Ident(ident)) = args.into_iter().next() {
         if ident == "foreign" {
-            return;
+            return Ok(());
         }
     }
 
-    // Get struct data. Exit if no fields need re-ordering.
+    // Get struct data
     let struct_name = item.ident.to_string();
-    let Some(field_order) = STRUCTS[&struct_name].field_order else {
-        return;
+    let Some(struct_details) = STRUCTS.get(&struct_name) else {
+        return Err("Struct is unknown. Run `just ast` to re-run the codegen.");
+    };
+
+    // Exit if fields don't need re-ordering
+    let Some(field_order) = struct_details.field_order else {
+        return Ok(());
     };
 
     // Re-order fields.
     // `field_order` contains indexes of fields in the order they should be.
-    let Fields::Named(FieldsNamed { named, .. }) = &mut item.fields else { unreachable!() };
-
-    assert!(
-        named.len() == field_order.len(),
-        "Wrong number of fields for `{struct_name}` in `STRUCTS`"
-    );
+    let named = match &mut item.fields {
+        Fields::Named(FieldsNamed { named, .. }) if named.len() == field_order.len() => named,
+        _ => {
+            return Err("Struct has been altered. Run `just ast` to re-run the codegen.");
+        }
+    };
 
     // Create 2 sets of fields.
     // 1st set are the fields in original order, each prefixed with `#[cfg(doc)]`.
@@ -98,6 +105,8 @@ fn reorder_struct_fields(item: &mut ItemStruct, args: TokenStream) {
         pair.value_mut().attrs.insert(0, parse_quote!( #[cfg(not(doc))]));
         pair
     }));
+
+    Ok(())
 }
 
 /// Generate assertions that traits used in `#[generate_derive]` are in scope.
@@ -115,31 +124,40 @@ fn reorder_struct_fields(item: &mut ItemStruct, args: TokenStream) {
 ///
 /// If `GetSpan` is not in scope, or it is not the correct `oxc_span::GetSpan`,
 /// this will raise a compilation error.
+///
+/// If any errors e.g. cannot parse `#[generate_derive]`, or unknown traits, just skip them.
+/// It is the responsibility of `oxc_ast_tools` to raise errors for those.
 fn assert_generated_derives(attrs: &[Attribute]) -> TokenStream {
-    // We don't care here if a trait is derived multiple times.
-    // It is the responsibility of `oxc_ast_tools` to raise errors for those.
-    let assertions = attrs
-        .iter()
-        .filter(|attr| attr.path().is_ident("generate_derive"))
-        .flat_map(parse_attr)
-        .map(|trait_ident| {
+    let mut assertions = quote!();
+    for attr in attrs {
+        if !attr.path().is_ident("generate_derive") {
+            continue;
+        }
+
+        let Ok(parsed) = attr.parse_args_with(Punctuated::<Ident, Comma>::parse_terminated) else {
+            continue;
+        };
+
+        for trait_ident in parsed {
             let trait_name = trait_ident.to_string();
             let Some((trait_path, generics)) = get_trait_crate_and_generics(&trait_name) else {
-                panic!("Invalid derive trait(generate_derive): {trait_name}");
+                continue;
             };
 
             // These are wrapped in a scope to avoid the need for unique identifiers
-            quote! {{
+            assertions.extend(quote! {{
                 trait AssertionTrait: #trait_path #generics {}
                 impl<T: #trait_ident #generics> AssertionTrait for T {}
-            }}
-        });
-    quote!( const _: () = { #(#assertions)* }; )
+            }});
+        }
+    }
+
+    quote! {
+        const _: () = { #assertions };
+    }
 }
 
-#[inline]
-fn parse_attr(attr: &Attribute) -> impl Iterator<Item = Ident> + use<> {
-    attr.parse_args_with(Punctuated::<Ident, Comma>::parse_terminated)
-        .expect("`#[generate_derive]` only accepts traits as single segment paths. Found an invalid argument.")
-        .into_iter()
+/// Generate a `compile_error!` macro invocation with the given message, and the span of `spanned`.
+fn compile_error<S: Spanned>(spanned: &S, message: &str) -> TokenStream {
+    quote_spanned! { spanned.span() => compile_error!(#message); }
 }