Use 0: for named-fields debug output of tuples

Change the output when the user requests debug printing of a tuple
struct in named fields syntax.

Previously we output `_0: ` (for example), which isn't really right:
Rust permits `0: `, and the field is actually called `0`.

This is a behavioural change, but I don't think Debug output is
regarded as semver-relevant, so this shouldn't be a breaking change.

src/trait_handlers/debug/debug_enum.rs

@@ -3,6 +3,7 @@ use syn::{Data, DeriveInput, Fields, Meta, Type};
 
 use super::models::{FieldAttributeBuilder, FieldName, TypeAttributeBuilder, TypeName};
 use crate::{common::path::path_to_string, supported_traits::Trait, trait_handlers::TraitHandler};
+use crate::common::ident_index::IdentOrIndex;
 
 pub(crate) struct DebugEnumHandler;
 
@@ -219,9 +220,9 @@ impl TraitHandler for DebugEnumHandler {
 
                                 let field_name_var = format_ident!("_{}", index);
 
-                                let key = match field_attribute.name {
-                                    FieldName::Custom(name) => name,
-                                    FieldName::Default => field_name_var.clone(),
+                                let key: IdentOrIndex = match field_attribute.name {
+                                    FieldName::Custom(name) => name.into(),
+                                    FieldName::Default => index.into(),
                                 };
 
                                 pattern_token_stream.extend(quote!(#field_name_var,));

src/trait_handlers/debug/debug_struct.rs

@@ -1,4 +1,4 @@
-use quote::{format_ident, quote};
+use quote::quote;
 use syn::{Data, DeriveInput, Fields, Meta, Type};
 
 use super::{
@@ -64,17 +64,9 @@ impl TraitHandler for DebugStructHandler {
                     }
 
                     let field_name = IdentOrIndex::from_ident_with_index(field.ident.as_ref(), index);
-                    let key = match field_attribute.name {
-                        FieldName::Custom(name) => {
-                            name
-                        },
-                        FieldName::Default => {
-                            if let Some(ident) = field.ident.as_ref() {
-                                ident.clone()
-                            } else {
-                                format_ident!("_{}", index)
-                            }
-                        },
+                    let key: IdentOrIndex = match field_attribute.name {
+                        FieldName::Custom(name) => name.into(),
+                        FieldName::Default => field_name.clone(),
                     };
 
                     let ty = &field.ty;

tests/debug_enum.rs

@@ -253,7 +253,7 @@ fn named_field_1() {
             f1: 1
         })
     );
-    assert_eq!("Tuple { _0: 1 }", format!("{:?}", Enum::Tuple(1)));
+    assert_eq!("Tuple { 0: 1 }", format!("{:?}", Enum::Tuple(1)));
 }
 
 #[test]
@@ -273,7 +273,7 @@ fn named_field_2() {
             f1: 1
         })
     );
-    assert_eq!("Tuple { _0: 1 }", format!("{:?}", Enum::Tuple(1)));
+    assert_eq!("Tuple { 0: 1 }", format!("{:?}", Enum::Tuple(1)));
 }
 
 #[test]
@@ -302,7 +302,7 @@ fn named_field_3() {
             f1: 1
         })
     );
-    assert_eq!("Tuple { _0: Hi }", format!("{:?}", Enum::Tuple(1)));
+    assert_eq!("Tuple { 0: Hi }", format!("{:?}", Enum::Tuple(1)));
 }
 
 #[test]

tests/debug_struct.rs

@@ -245,7 +245,7 @@ fn named_field_1() {
     #[educe(Debug(named_field = true))]
     struct Tuple(u8);
 
-    assert_eq!("Tuple { _0: 1 }", format!("{:?}", Tuple(1)));
+    assert_eq!("Tuple { 0: 1 }", format!("{:?}", Tuple(1)));
 }
 
 #[test]
@@ -267,7 +267,7 @@ fn named_field_2() {
     #[educe(Debug(named_field(true)))]
     struct Tuple(u8);
 
-    assert_eq!("Tuple { _0: 1 }", format!("{:?}", Tuple(1)));
+    assert_eq!("Tuple { 0: 1 }", format!("{:?}", Tuple(1)));
 }
 
 #[test]