Reconstruct span information lost by compiler

impl/src/ast.rs

@@ -1,4 +1,5 @@
 use crate::attr::{self, Attrs};
+use proc_macro2::Span;
 use syn::{
     Data, DataEnum, DataStruct, DeriveInput, Error, Fields, Generics, Ident, Index, Member, Result,
     Type,
@@ -55,7 +56,8 @@ impl<'a> Input<'a> {
 impl<'a> Struct<'a> {
     fn from_syn(node: &'a DeriveInput, data: &'a DataStruct) -> Result<Self> {
         let mut attrs = attr::get(&node.attrs)?;
-        let fields = Field::multiple_from_syn(&data.fields)?;
+        let span = attrs.span().unwrap_or_else(Span::call_site);
+        let fields = Field::multiple_from_syn(&data.fields, span)?;
         if let Some(display) = &mut attrs.display {
             display.expand_shorthand(&fields);
         }
@@ -72,11 +74,12 @@ impl<'a> Struct<'a> {
 impl<'a> Enum<'a> {
     fn from_syn(node: &'a DeriveInput, data: &'a DataEnum) -> Result<Self> {
         let attrs = attr::get(&node.attrs)?;
+        let span = attrs.span().unwrap_or_else(Span::call_site);
         let variants = data
             .variants
             .iter()
             .map(|node| {
-                let mut variant = Variant::from_syn(node)?;
+                let mut variant = Variant::from_syn(node, span)?;
                 if let display @ None = &mut variant.attrs.display {
                     *display = attrs.display.clone();
                 }
@@ -99,35 +102,50 @@ impl<'a> Enum<'a> {
 }
 
 impl<'a> Variant<'a> {
-    fn from_syn(node: &'a syn::Variant) -> Result<Self> {
+    fn from_syn(node: &'a syn::Variant, span: Span) -> Result<Self> {
+        let attrs = attr::get(&node.attrs)?;
+        let span = attrs.span().unwrap_or(span);
         Ok(Variant {
             original: node,
-            attrs: attr::get(&node.attrs)?,
+            attrs,
             ident: node.ident.clone(),
-            fields: Field::multiple_from_syn(&node.fields)?,
+            fields: Field::multiple_from_syn(&node.fields, span)?,
         })
     }
 }
 
 impl<'a> Field<'a> {
-    fn multiple_from_syn(fields: &'a Fields) -> Result<Vec<Self>> {
+    fn multiple_from_syn(fields: &'a Fields, span: Span) -> Result<Vec<Self>> {
         fields
             .iter()
             .enumerate()
-            .map(|(i, field)| Field::from_syn(i, field))
+            .map(|(i, field)| Field::from_syn(i, field, span))
             .collect()
     }
 
-    fn from_syn(i: usize, node: &'a syn::Field) -> Result<Self> {
+    fn from_syn(i: usize, node: &'a syn::Field, span: Span) -> Result<Self> {
         Ok(Field {
             original: node,
             attrs: attr::get(&node.attrs)?,
-            member: node
-                .ident
-                .clone()
-                .map(Member::Named)
-                .unwrap_or_else(|| Member::Unnamed(Index::from(i))),
+            member: node.ident.clone().map(Member::Named).unwrap_or_else(|| {
+                Member::Unnamed(Index {
+                    index: i as u32,
+                    span,
+                })
+            }),
             ty: &node.ty,
         })
     }
 }
+
+impl Attrs<'_> {
+    pub fn span(&self) -> Option<Span> {
+        if let Some(display) = &self.display {
+            Some(display.fmt.span())
+        } else if let Some(transparent) = &self.transparent {
+            Some(transparent.span)
+        } else {
+            None
+        }
+    }
+}

impl/src/attr.rs

@@ -1,4 +1,4 @@
-use proc_macro2::{Delimiter, Group, TokenStream, TokenTree};
+use proc_macro2::{Delimiter, Group, Span, TokenStream, TokenTree};
 use quote::{format_ident, quote, ToTokens};
 use std::iter::FromIterator;
 use syn::parse::{Nothing, ParseStream};
@@ -12,7 +12,7 @@ pub struct Attrs<'a> {
     pub source: Option<&'a Attribute>,
     pub backtrace: Option<&'a Attribute>,
     pub from: Option<&'a Attribute>,
-    pub transparent: Option<&'a Attribute>,
+    pub transparent: Option<Transparent<'a>>,
 }
 
 #[derive(Clone)]
@@ -23,6 +23,12 @@ pub struct Display<'a> {
     pub has_bonus_display: bool,
 }
 
+#[derive(Copy, Clone)]
+pub struct Transparent<'a> {
+    pub original: &'a Attribute,
+    pub span: Span,
+}
+
 pub fn get(input: &[Attribute]) -> Result<Attrs> {
     let mut attrs = Attrs {
         display: None,
@@ -66,14 +72,17 @@ fn parse_error_attribute<'a>(attrs: &mut Attrs<'a>, attr: &'a Attribute) -> Resu
     syn::custom_keyword!(transparent);
 
     attr.parse_args_with(|input: ParseStream| {
-        if input.parse::<Option<transparent>>()?.is_some() {
+        if let Some(kw) = input.parse::<Option<transparent>>()? {
             if attrs.transparent.is_some() {
                 return Err(Error::new_spanned(
                     attr,
                     "duplicate #[error(transparent)] attribute",
                 ));
             }
-            attrs.transparent = Some(attr);
+            attrs.transparent = Some(Transparent {
+                original: attr,
+                span: kw.span,
+            });
             return Ok(());
         }
 

impl/src/valid.rs

@@ -19,7 +19,7 @@ impl Struct<'_> {
         if let Some(transparent) = self.attrs.transparent {
             if self.fields.len() != 1 {
                 return Err(Error::new_spanned(
-                    transparent,
+                    transparent.original,
                     "#[error(transparent)] requires exactly one field",
                 ));
             }
@@ -165,7 +165,7 @@ fn check_field_attrs(fields: &[Field]) -> Result<()> {
         }
         if let Some(transparent) = field.attrs.transparent {
             return Err(Error::new_spanned(
-                transparent,
+                transparent.original,
                 "#[error(transparent)] needs to go outside the enum or struct, not on an individual field",
             ));
         }

tests/test_display.rs

@@ -198,3 +198,29 @@ fn test_field() {
 
     assert("0", Error(Inner { data: 0 }));
 }
+
+#[test]
+fn test_macro_rules() {
+    // Regression test for https://github.com/dtolnay/thiserror/issues/86
+
+    macro_rules! decl_error {
+        ($variant:ident($value:ident)) => {
+            #[derive(Debug, Error)]
+            pub enum Error0 {
+                #[error("{0:?}")]
+                $variant($value),
+            }
+
+            #[derive(Debug, Error)]
+            #[error("{0:?}")]
+            pub enum Error1 {
+                $variant($value),
+            }
+        };
+    }
+
+    decl_error!(Repro(u8));
+
+    assert("0", Error0::Repro(0));
+    assert("0", Error1::Repro(0));
+}