diff --git a/compiler/rustc_macros/src/diagnostics/diagnostic.rs b/compiler/rustc_macros/src/diagnostics/diagnostic.rs
index d0c86527189..00f3951d9a0 100644
--- a/compiler/rustc_macros/src/diagnostics/diagnostic.rs
+++ b/compiler/rustc_macros/src/diagnostics/diagnostic.rs
@@ -5,8 +5,8 @@ use crate::diagnostics::error::{
     SessionDiagnosticDeriveError,
 };
 use crate::diagnostics::utils::{
-    report_error_if_not_applied_to_span, report_type_error, type_is_unit, type_matches_path,
-    Applicability, FieldInfo, FieldInnerTy, HasFieldMap, SetOnce,
+    build_field_mapping, report_error_if_not_applied_to_span, report_type_error, type_is_unit,
+    type_matches_path, Applicability, FieldInfo, FieldInnerTy, HasFieldMap, SetOnce,
 };
 use proc_macro2::{Ident, TokenStream};
 use quote::{format_ident, quote};
@@ -25,26 +25,11 @@ pub(crate) struct SessionDiagnosticDerive<'a> {
 
 impl<'a> SessionDiagnosticDerive<'a> {
     pub(crate) fn new(diag: syn::Ident, sess: syn::Ident, structure: Structure<'a>) -> Self {
-        // Build the mapping of field names to fields. This allows attributes to peek values from
-        // other fields.
-        let mut fields_map = HashMap::new();
-
-        // Convenience bindings.
-        let ast = structure.ast();
-
-        if let syn::Data::Struct(syn::DataStruct { fields, .. }) = &ast.data {
-            for field in fields.iter() {
-                if let Some(ident) = &field.ident {
-                    fields_map.insert(ident.to_string(), quote! { &self.#ident });
-                }
-            }
-        }
-
         Self {
             builder: SessionDiagnosticDeriveBuilder {
                 diag,
                 sess,
-                fields: fields_map,
+                fields: build_field_mapping(&structure),
                 kind: None,
                 code: None,
                 slug: None,
diff --git a/compiler/rustc_macros/src/diagnostics/utils.rs b/compiler/rustc_macros/src/diagnostics/utils.rs
index 636bcf1f7b1..24204556c98 100644
--- a/compiler/rustc_macros/src/diagnostics/utils.rs
+++ b/compiler/rustc_macros/src/diagnostics/utils.rs
@@ -2,10 +2,10 @@ use crate::diagnostics::error::{span_err, throw_span_err, SessionDiagnosticDeriv
 use proc_macro::Span;
 use proc_macro2::TokenStream;
 use quote::{format_ident, quote, ToTokens};
-use std::collections::BTreeSet;
+use std::collections::{BTreeSet, HashMap};
 use std::str::FromStr;
 use syn::{spanned::Spanned, Attribute, Meta, Type, TypeTuple};
-use synstructure::BindingInfo;
+use synstructure::{BindingInfo, Structure};
 
 /// Checks whether the type name of `ty` matches `name`.
 ///
@@ -325,3 +325,20 @@ impl quote::ToTokens for Applicability {
         });
     }
 }
+
+/// Build the mapping of field names to fields. This allows attributes to peek values from
+/// other fields.
+pub(crate) fn build_field_mapping<'a>(structure: &Structure<'a>) -> HashMap<String, TokenStream> {
+    let mut fields_map = HashMap::new();
+
+    let ast = structure.ast();
+    if let syn::Data::Struct(syn::DataStruct { fields, .. }) = &ast.data {
+        for field in fields.iter() {
+            if let Some(ident) = &field.ident {
+                fields_map.insert(ident.to_string(), quote! { &self.#ident });
+            }
+        }
+    }
+
+    fields_map
+}