Merge pull request #76 from dtolnay/span

Improve span of missing Display impl error

impl/src/ast.rs

@@ -10,13 +10,15 @@ pub enum Input<'a> {
 }
 
 pub struct Struct<'a> {
+    pub original: &'a DeriveInput,
     pub attrs: Attrs<'a>,
     pub ident: Ident,
     pub generics: &'a Generics,
     pub fields: Vec<Field<'a>>,
 }
 
 pub struct Enum<'a> {
+    pub original: &'a DeriveInput,
     pub attrs: Attrs<'a>,
     pub ident: Ident,
     pub generics: &'a Generics,
@@ -58,6 +60,7 @@ impl<'a> Struct<'a> {
             display.expand_shorthand(&fields);
         }
         Ok(Struct {
+            original: node,
             attrs,
             ident: node.ident.clone(),
             generics: &node.generics,
@@ -86,6 +89,7 @@ impl<'a> Enum<'a> {
             })
             .collect::<Result<_>>()?;
         Ok(Enum {
+            original: node,
             attrs,
             ident: node.ident.clone(),
             generics: &node.generics,

impl/src/expand.rs

@@ -2,7 +2,7 @@ use crate::ast::{Enum, Field, Input, Struct};
 use proc_macro2::TokenStream;
 use quote::{format_ident, quote, quote_spanned, ToTokens};
 use syn::spanned::Spanned;
-use syn::{DeriveInput, Member, PathArguments, Result, Type};
+use syn::{Data, DeriveInput, Member, PathArguments, Result, Type, Visibility};
 
 pub fn derive(node: &DeriveInput) -> Result<TokenStream> {
     let input = Input::from_syn(node)?;
@@ -135,8 +135,10 @@ fn impl_struct(input: Struct) -> TokenStream {
         }
     });
 
+    let error_trait = spanned_error_trait(input.original);
+
     quote! {
-        impl #impl_generics std::error::Error for #ty #ty_generics #where_clause {
+        impl #impl_generics #error_trait for #ty #ty_generics #where_clause {
             #source_method
             #backtrace_method
         }
@@ -321,8 +323,10 @@ fn impl_enum(input: Enum) -> TokenStream {
         })
     });
 
+    let error_trait = spanned_error_trait(input.original);
+
     quote! {
-        impl #impl_generics std::error::Error for #ty #ty_generics #where_clause {
+        impl #impl_generics #error_trait for #ty #ty_generics #where_clause {
             #source_method
             #backtrace_method
         }
@@ -382,3 +386,22 @@ fn type_is_option(ty: &Type) -> bool {
         _ => false,
     }
 }
+
+fn spanned_error_trait(input: &DeriveInput) -> TokenStream {
+    let vis_span = match &input.vis {
+        Visibility::Public(vis) => Some(vis.pub_token.span()),
+        Visibility::Crate(vis) => Some(vis.crate_token.span()),
+        Visibility::Restricted(vis) => Some(vis.pub_token.span()),
+        Visibility::Inherited => None,
+    };
+    let data_span = match &input.data {
+        Data::Struct(data) => data.struct_token.span(),
+        Data::Enum(data) => data.enum_token.span(),
+        Data::Union(data) => data.union_token.span(),
+    };
+    let first_span = vis_span.unwrap_or(data_span);
+    let last_span = input.ident.span();
+    let path = quote_spanned!(first_span=> std::error::);
+    let error = quote_spanned!(last_span=> Error);
+    quote!(#path #error)
+}

tests/ui/missing-display.stderr

@@ -1,9 +1,8 @@
 error[E0277]: `MyError` doesn't implement `std::fmt::Display`
- --> $DIR/missing-display.rs:3:10
+ --> $DIR/missing-display.rs:4:1
   |
-3 | #[derive(Error, Debug)]
-  |          ^^^^^ `MyError` cannot be formatted with the default formatter
+4 | pub enum MyError {
+  | ^^^^^^^^^^^^^^^^ `MyError` cannot be formatted with the default formatter
   |
   = help: the trait `std::fmt::Display` is not implemented for `MyError`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
-  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)