From 44fdb98764e13faecbe2c307798bbe97348bddde Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Thu, 16 Dec 2021 18:35:15 -0500
Subject: [PATCH] Use field span in `rustc_macros` when emitting decode call

This will cause backtraces to point to the location of
the field in the struct/enum, rather than the derive macro.

This makes it clear which field was being decoded when the
backtrace was captured (which is especially useful if
there are multiple fields with the same type).
---
 compiler/rustc_macros/src/serialize.rs | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/compiler/rustc_macros/src/serialize.rs b/compiler/rustc_macros/src/serialize.rs
index 66e6b571beb..3351564299c 100644
--- a/compiler/rustc_macros/src/serialize.rs
+++ b/compiler/rustc_macros/src/serialize.rs
@@ -1,6 +1,7 @@
 use proc_macro2::TokenStream;
-use quote::quote;
+use quote::{quote, quote_spanned};
 use syn::parse_quote;
+use syn::spanned::Spanned;
 
 pub fn type_decodable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
     let decoder_ty = quote! { __D };
@@ -104,6 +105,8 @@ fn decodable_body(
 }
 
 fn decode_field(field: &syn::Field, index: usize, is_struct: bool) -> proc_macro2::TokenStream {
+    let field_span = field.ident.as_ref().map_or(field.ty.span(), |ident| ident.span());
+
     let decode_inner_method = if let syn::Type::Reference(_) = field.ty {
         quote! { ::rustc_middle::ty::codec::RefDecodable::decode }
     } else {
@@ -111,20 +114,21 @@ fn decode_field(field: &syn::Field, index: usize, is_struct: bool) -> proc_macro
     };
     let (decode_method, opt_field_name) = if is_struct {
         let field_name = field.ident.as_ref().map_or_else(|| index.to_string(), |i| i.to_string());
-        (
-            proc_macro2::Ident::new("read_struct_field", proc_macro2::Span::call_site()),
-            quote! { #field_name, },
-        )
+        (proc_macro2::Ident::new("read_struct_field", field_span), quote! { #field_name, })
     } else {
-        (
-            proc_macro2::Ident::new("read_enum_variant_arg", proc_macro2::Span::call_site()),
-            quote! {},
-        )
+        (proc_macro2::Ident::new("read_enum_variant_arg", field_span), quote! {})
+    };
+
+    let __decoder = quote! { __decoder };
+    // Use the span of the field for the method call, so
+    // that backtraces will point to the field.
+    let decode_call = quote_spanned! {field_span=>
+        ::rustc_serialize::Decoder::#decode_method(
+                #__decoder, #opt_field_name #decode_inner_method)
     };
 
     quote! {
-        match ::rustc_serialize::Decoder::#decode_method(
-            __decoder, #opt_field_name #decode_inner_method) {
+        match #decode_call  {
             ::std::result::Result::Ok(__res) => __res,
             ::std::result::Result::Err(__err) => return ::std::result::Result::Err(__err),
         }