Improve error messages on failure to derive

rustler_codegen/src/ex_struct.rs

@@ -1,6 +1,6 @@
 use proc_macro2::{Span, TokenStream};
 
-use syn::{self, Field, Ident};
+use syn::{self, spanned::Spanned, Field, Ident};
 
 use super::context::Context;
 use super::RustlerAttr;
@@ -70,7 +70,7 @@ fn gen_decoder(ctx: &Context, fields: &[&Field], atoms_module_name: &Ident) -> T
             let atom_fun = Context::field_to_atom_fun(field);
             let variable = Context::escape_ident_with_index(&ident.to_string(), index, "struct");
 
-            let assignment = quote! {
+            let assignment = quote_spanned! { field.span() =>
                 let #variable = try_decode_field(env, term, #atom_fun())?;
             };
 
@@ -131,7 +131,7 @@ fn gen_encoder(ctx: &Context, fields: &[&Field], atoms_module_name: &Ident) -> T
         .map(|field| {
             let field_ident = field.ident.as_ref().unwrap();
             let atom_fun = Context::field_to_atom_fun(field);
-            quote! {
+            quote_spanned! { field.span() =>
                 map = map.map_put(#atom_fun().encode(env), self.#field_ident.encode(env)).unwrap();
             }
         })

rustler_codegen/src/map.rs

@@ -1,6 +1,6 @@
 use proc_macro2::{Span, TokenStream};
 
-use syn::{self, Field, Ident};
+use syn::{self, spanned::Spanned, Field, Ident};
 
 use super::context::Context;
 
@@ -64,7 +64,7 @@ fn gen_decoder(ctx: &Context, fields: &[&Field], atoms_module_name: &Ident) -> T
             let atom_fun = Context::field_to_atom_fun(field);
             let variable = Context::escape_ident_with_index(&ident.to_string(), index, "map");
 
-            let assignment = quote! {
+            let assignment = quote_spanned! { field.span() =>
             let #variable = try_decode_field(env, term, #atom_fun())?;
             };
 
@@ -119,7 +119,7 @@ fn gen_encoder(ctx: &Context, fields: &[&Field], atoms_module_name: &Ident) -> T
             let field_ident = field.ident.as_ref().unwrap();
             let atom_fun = Context::field_to_atom_fun(field);
 
-            quote! {
+            quote_spanned! { field.span() =>
                 map = map.map_put(#atom_fun().encode(env), self.#field_ident.encode(env)).unwrap();
             }
         })

rustler_codegen/src/record.rs

@@ -1,6 +1,6 @@
 use proc_macro2::{Span, TokenStream};
 
-use syn::{self, Field, Ident, Index};
+use syn::{self, spanned::Spanned, Field, Ident, Index};
 
 use super::context::Context;
 use super::RustlerAttr;
@@ -66,7 +66,7 @@ fn gen_decoder(ctx: &Context, fields: &[&Field], atoms_module_name: &Ident) -> T
 
             let variable = Context::escape_ident(&pos_in_struct, "record");
 
-            let assignment = quote! {
+            let assignment = quote_spanned! { field.span() =>
                 let #variable = try_decode_index(&terms, #pos_in_struct, #actual_index)?;
             };
 
@@ -150,7 +150,7 @@ fn gen_encoder(ctx: &Context, fields: &[&Field], atoms_module_name: &Ident) -> T
                 Some(ident) => quote! { self.#ident },
             };
 
-            quote! { #field_source.encode(env) }
+            quote_spanned! { field.span() => #field_source.encode(env) }
         })
         .collect();
 

rustler_codegen/src/tuple.rs

@@ -1,6 +1,6 @@
 use proc_macro2::TokenStream;
 
-use syn::{self, Field, Index};
+use syn::{self, spanned::Spanned, Field, Index};
 
 use super::context::Context;
 
@@ -51,7 +51,7 @@ fn gen_decoder(ctx: &Context, fields: &[&Field]) -> TokenStream {
 
             let variable = Context::escape_ident(&pos_in_struct, "struct");
 
-            let assignment = quote! {
+            let assignment = quote_spanned! { field.span() =>
                 let #variable = try_decode_index(&terms, #pos_in_struct, #index)?;
             };
 
@@ -120,7 +120,7 @@ fn gen_encoder(ctx: &Context, fields: &[&Field]) -> TokenStream {
                 Some(ident) => quote! { self.#ident },
             };
 
-            quote! { #field_source.encode(env) }
+            quote_spanned! { field.span() => #field_source.encode(env) }
         })
         .collect();
 

rustler_codegen/src/unit_enum.rs

@@ -1,7 +1,7 @@
 use proc_macro2::{Span, TokenStream};
 
 use heck::SnakeCase;
-use syn::{self, Fields, Ident, Variant};
+use syn::{self, spanned::Spanned, Fields, Ident, Variant};
 
 use super::context::Context;
 
@@ -16,7 +16,9 @@ pub fn transcoder_decorator(ast: &syn::DeriveInput) -> TokenStream {
     for variant in variants {
         if let Fields::Unit = variant.fields {
         } else {
-            panic!("NifUnitEnum can only be used with enums that contain all unit variants.");
+            return quote_spanned! { variant.span() =>
+                compile_error!("NifUnitEnum can only be used with enums containing unit variants.");
+            };
         }
     }
 

rustler_codegen/src/untagged_enum.rs

@@ -1,6 +1,6 @@
 use proc_macro2::TokenStream;
 
-use syn::{self, Fields, Variant};
+use syn::{self, spanned::Spanned, Fields, Variant};
 
 use super::context::Context;
 
@@ -15,12 +15,14 @@ pub fn transcoder_decorator(ast: &syn::DeriveInput) -> TokenStream {
     for variant in variants {
         if let Fields::Unnamed(_) = variant.fields {
             if variant.fields.iter().count() != 1 {
-                panic!("NifUntaggedEnum can only be used with enums that contain all NewType variants.");
+                return quote_spanned! { variant.span() =>
+                    compile_error!("NifUntaggedEnum can only be used with enums that contain all NewType variants.");
+                };
             }
         } else {
-            panic!(
-                "NifUntaggedEnum can only be used with enums that contain all NewType variants."
-            );
+            return quote_spanned! { variant.span() =>
+                compile_error!("NifUntaggedEnum can only be used with enums that contain all NewType variants.");
+            };
         }
     }