From 07586dba9e03bf20bd17b7d6413ecb1a4c6253ab Mon Sep 17 00:00:00 2001
From: Robin Krahl <robin@nitrokey.com>
Date: Sun, 1 Jun 2025 23:16:44 +0200
Subject: [PATCH] Add index attribute for explicit index assignment

Fixes: https://github.com/trussed-dev/serde-indexed/issues/17
---
 CHANGELOG.md    |  1 +
 src/lib.rs      |  5 +++--
 src/parse.rs    | 45 ++++++++++++++++++++++++++++++++++++---------
 tests/basics.rs | 39 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 79 insertions(+), 11 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0be1c29..d34e643 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@
 - No longer fails deserialising maps with unknown fields ([#19][])
 - Prefer explicit indexing over automatically assigned indices ([#17][]):
   - Require `auto_index` attribute to enable automatic index assignment
+  - Add `index` attribute for explicit index assignment
 
 [#2]: https://github.com/trussed-dev/serde-indexed/issues/2
 [#11]: https://github.com/trussed-dev/serde-indexed/pull/11
diff --git a/src/lib.rs b/src/lib.rs
index 9f5dfc3..4b7c211 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -8,11 +8,12 @@ and a custom `offset` container attribute.
 use serde_indexed::{DeserializeIndexed, SerializeIndexed};
 
 #[derive(Clone, Debug, PartialEq, SerializeIndexed, DeserializeIndexed)]
-#[serde_indexed(auto_index, offset = 1)]
 pub struct SomeKeys {
+    #[serde(index = 1)]
     pub number: i32,
-    #[serde(skip_serializing_if = "Option::is_none")]
+    #[serde(index = 2, skip_serializing_if = "Option::is_none")]
     pub option: Option<u8>,
+    #[serde(index = 3)]
     pub bytes: [u8; 7],
 }
 ```
diff --git a/src/parse.rs b/src/parse.rs
index 7324de1..412aeef 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -113,7 +113,12 @@ impl Parse for Input {
     }
 }
 
-fn parse_field(index: usize, field: &syn::Field) -> Result<Field> {
+fn parse_field(
+    attrs: &StructAttrs,
+    auto_index: usize,
+    field: &syn::Field,
+    indices: &mut Vec<usize>,
+) -> Result<Field> {
     let ident = field
         .ident
         .as_ref()
@@ -123,6 +128,7 @@ fn parse_field(index: usize, field: &syn::Field) -> Result<Field> {
     let mut deserialize_with = None;
     let mut serialize_with = None;
     let mut no_increment = false;
+    let mut explicit_index = None;
 
     for attr in &field.attrs {
         if attr.path().is_ident("serde") {
@@ -191,6 +197,22 @@ fn parse_field(index: usize, field: &syn::Field) -> Result<Field> {
                     serialize_with = Some(syn::parse2(serialize_tokens)?);
                     deserialize_with = Some(syn::parse2(deserialize_tokens)?);
 
+                    Ok(())
+                } else if meta.path.is_ident("index") {
+                    if explicit_index.is_some() {
+                        return Err(meta.error("Multiple attributes for index"));
+                    }
+                    if attrs.auto_index {
+                        return Err(meta.error(
+                            "The index attribute cannot be combined with the auto_index attribute",
+                        ));
+                    }
+                    let litint: LitInt = meta.value()?.parse()?;
+                    let int = litint.base10_parse()?;
+                    if indices.contains(&int) {
+                        return Err(meta.error("This index has already been assigned"));
+                    }
+                    explicit_index = Some(int);
                     Ok(())
                 } else {
                     return Err(meta.error("Unkown field attribute"));
@@ -199,6 +221,17 @@ fn parse_field(index: usize, field: &syn::Field) -> Result<Field> {
         }
     }
 
+    let index = if attrs.auto_index {
+        auto_index
+    } else if let Some(index) = explicit_index {
+        indices.push(index);
+        index
+    } else {
+        return Err(Error::new_spanned(
+            field,
+            "Field without index attribute and `#[serde(auto_index)]` is not enabled on the struct",
+        ));
+    };
     Ok(Field {
         label: ident.to_string(),
         member: syn::Member::Named(ident.clone()),
@@ -216,18 +249,12 @@ fn fields_from_ast(
     attrs: &StructAttrs,
     fields: &syn::punctuated::Punctuated<syn::Field, Token![,]>,
 ) -> Result<Vec<Field>> {
-    if !attrs.auto_index {
-        return Err(Error::new_spanned(
-            fields,
-            "auto_index attribute must be set",
-        ));
-    }
-
+    let mut indices = Vec::new();
     let mut index = 0;
     fields
         .iter()
         .map(|field| {
-            let field = parse_field(index, field)?;
+            let field = parse_field(attrs, index, field, &mut indices)?;
             if !field.no_increment {
                 index += 1;
             }
diff --git a/tests/basics.rs b/tests/basics.rs
index 64a3c07..eb1d149 100644
--- a/tests/basics.rs
+++ b/tests/basics.rs
@@ -617,3 +617,42 @@ mod generics {
         )
     }
 }
+
+mod index {
+    use super::*;
+
+    #[derive(PartialEq, Debug, SerializeIndexed, DeserializeIndexed)]
+    struct WithIndices {
+        #[serde(index = 9)]
+        test1: usize,
+        #[serde(index = 2)]
+        test2: usize,
+        #[serde(index = 0x5A)]
+        test3: usize,
+    }
+
+    fn indices_example() -> WithIndices {
+        WithIndices {
+            test1: 42,
+            test2: 1,
+            test3: 99,
+        }
+    }
+
+    #[test]
+    fn tokens() {
+        assert_tokens(
+            &indices_example(),
+            &[
+                Token::Map { len: Some(3) },
+                Token::U64(9),
+                Token::U64(42),
+                Token::U64(2),
+                Token::U64(1),
+                Token::U64(0x5A),
+                Token::U64(99),
+                Token::MapEnd,
+            ],
+        );
+    }
+}