mlua_derive: Improve error reporting for #[lua] attributes

Author: khvzak

mlua_derive/src/userdata/attr.rs

@@ -1,3 +1,4 @@
+use proc_macro2::Span;
 use syn::meta::ParseNestedMeta;
 use syn::{Ident, LitStr, Result};
 
@@ -8,6 +9,7 @@ use syn::{Ident, LitStr, Result};
 /// - Impl methods: `getter`, `setter`, `field`, `meta`, `infallible`, `name`, `skip`
 #[derive(Default)]
 pub(crate) struct LuaAttr {
+    pub(crate) span: Option<Span>,
     pub(crate) name: Option<String>,
     pub(crate) infallible: bool,
     pub(crate) skip: bool,
@@ -64,6 +66,11 @@ impl LuaAttr {
         self.name.clone().unwrap_or_else(|| ident.to_string())
     }
 
+    /// Returns the span to use for error reporting.
+    pub(crate) fn span(&self) -> Span {
+        self.span.unwrap_or_else(Span::call_site)
+    }
+
     /// Returns the effective Lua metamethod name.
     ///
     /// If `name` is set via attribute, use it. Otherwise, if the function name

mlua_derive/src/userdata/mod.rs

@@ -2,7 +2,6 @@ mod attr;
 pub(crate) mod userdata_impl;
 
 use proc_macro::TokenStream;
-use proc_macro2::Span;
 use quote::{format_ident, quote};
 use syn::spanned::Spanned;
 use syn::{Attribute, Data, DeriveInput, Error, Fields, FieldsNamed, Meta, parse_macro_input};
@@ -32,8 +31,9 @@ fn parse_field_lua_attr(attrs: &[Attribute]) -> syn::Result<LuaAttr> {
         }
         match &attr.meta {
             Meta::List(_) => {
+                lua_attr.span = Some(attr.span());
                 attr.parse_nested_meta(|meta| lua_attr.parse_inner(meta))?;
-                validate_field_lua_attr(&lua_attr, attr.span())?;
+                validate_field_lua_attr(&lua_attr)?;
             }
             Meta::Path(_) => {}
             Meta::NameValue(_) => {
@@ -47,7 +47,7 @@ fn parse_field_lua_attr(attrs: &[Attribute]) -> syn::Result<LuaAttr> {
     Ok(lua_attr)
 }
 
-fn validate_field_lua_attr(attr: &LuaAttr, span: Span) -> syn::Result<()> {
+fn validate_field_lua_attr(attr: &LuaAttr) -> syn::Result<()> {
     for (set, name) in [
         (attr.getter, "getter"),
         (attr.setter, "setter"),
@@ -57,7 +57,7 @@ fn validate_field_lua_attr(attr: &LuaAttr, span: Span) -> syn::Result<()> {
     ] {
         if set {
             return Err(syn::Error::new(
-                span,
+                attr.span(),
                 format!("`{name}` is not valid for struct fields"),
             ));
         }

mlua_derive/src/userdata/userdata_impl.rs

@@ -1,7 +1,7 @@
 use std::sync::atomic::{AtomicUsize, Ordering};
 
 use proc_macro::TokenStream;
-use proc_macro2::{Span, TokenStream as TokenStream2};
+use proc_macro2::TokenStream as TokenStream2;
 use quote::{format_ident, quote};
 use syn::spanned::Spanned;
 use syn::{
@@ -183,8 +183,9 @@ fn parse_lua_attr(attrs: &[Attribute]) -> syn::Result<LuaAttr> {
         }
         match &attr.meta {
             Meta::List(_) => {
+                lua_attr.span = Some(attr.span());
                 attr.parse_nested_meta(|meta| lua_attr.parse_inner(meta))?;
-                validate_lua_attr(&lua_attr, attr.span())?;
+                validate_lua_attr(&lua_attr)?;
             }
             Meta::Path(_) => {}
             Meta::NameValue(_) => {
@@ -198,11 +199,11 @@ fn parse_lua_attr(attrs: &[Attribute]) -> syn::Result<LuaAttr> {
     Ok(lua_attr)
 }
 
-fn validate_lua_attr(attr: &LuaAttr, span: Span) -> syn::Result<()> {
+fn validate_lua_attr(attr: &LuaAttr) -> syn::Result<()> {
     for (set, name) in [(attr.get, "get"), (attr.set, "set")] {
         if set {
             return Err(syn::Error::new(
-                span,
+                attr.span(),
                 format!("`{name}` is not valid for methods"),
             ));
         }
@@ -250,8 +251,8 @@ pub fn userdata_impl(attr: TokenStream, item: TokenStream) -> TokenStream {
                     continue;
                 }
                 if lua_attr.getter || lua_attr.setter {
-                    return syn::Error::new_spanned(
-                        &const_item.ident,
+                    return syn::Error::new(
+                        lua_attr.span(),
                         "const items do not support `getter` or `setter`",
                     )
                     .to_compile_error()
@@ -285,15 +286,15 @@ pub fn userdata_impl(attr: TokenStream, item: TokenStream) -> TokenStream {
                 let primary = [lua_attr.getter, lua_attr.setter, lua_attr.field];
                 let primary_count = primary.iter().filter(|&&x| x).count();
                 if primary_count > 1 {
-                    return syn::Error::new_spanned(
-                        &method.sig,
+                    return syn::Error::new(
+                        lua_attr.span(),
                         "at most one of `getter`, `setter`, `field` can be specified",
                     )
                     .to_compile_error()
                     .into();
                 }
                 if lua_attr.meta && primary_count == 1 && !lua_attr.field {
-                    return syn::Error::new_spanned(&method.sig, "`meta` can only be combined with `field`")
+                    return syn::Error::new(lua_attr.span(), "`meta` can only be combined with `field`")
                         .to_compile_error()
                         .into();
                 }

tests/compile/userdata_const_getter.stderr

@@ -1,5 +1,5 @@
 error: const items do not support `getter` or `setter`
- --> tests/compile/userdata_const_getter.rs:7:11
+ --> tests/compile/userdata_const_getter.rs:6:5
   |
-7 |     const X: u32 = 42;
-  |           ^
+6 |     #[lua(getter)]
+  |     ^^^^^^^^^^^^^^

tests/compile/userdata_getter_and_meta.stderr

@@ -1,5 +1,5 @@
 error: `meta` can only be combined with `field`
- --> tests/compile/userdata_getter_and_meta.rs:7:5
+ --> tests/compile/userdata_getter_and_meta.rs:6:5
   |
-7 |     fn bar(&self) -> mlua::Result<u32> {
-  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+6 |     #[lua(getter, meta)]
+  |     ^^^^^^^^^^^^^^^^^^^^

tests/compile/userdata_getter_and_setter.stderr

@@ -1,5 +1,5 @@
 error: at most one of `getter`, `setter`, `field` can be specified
- --> tests/compile/userdata_getter_and_setter.rs:9:5
+ --> tests/compile/userdata_getter_and_setter.rs:8:5
   |
-9 |     fn x(&self) -> mlua::Result<u32> {
-  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+8 |     #[lua(getter, setter)]
+  |     ^^^^^^^^^^^^^^^^^^^^^^