Require auto_index attribute to enable automatic index assigment

Automatically assigning indices based on the order of the fields is
error prone, for example when used with feature flags, when fields are
skipped or when the fields of the struct are changed or reordered.
With this patch, we require an explicit opt-in to this potentially
problematic behavior.  See also:
     #17

Author: robin-nitrokey

CHANGELOG.md

@@ -4,7 +4,8 @@
 
 [Unreleased]: https://github.com/trussed-dev/serde-indexed/compare/0.1.1...HEAD
 
--
+- Prefer explicit indexing over automatically assigned indices:
+  - Require `auto_index` attribute to enable automatic index assignment
 
 ## [v0.1.1][] (2024-04-03)
 

src/lib.rs

@@ -8,7 +8,7 @@ and a custom `offset` container attribute.
 use serde_indexed::{DeserializeIndexed, SerializeIndexed};
 
 #[derive(Clone, Debug, PartialEq, SerializeIndexed, DeserializeIndexed)]
-#[serde_indexed(offset = 1)]
+#[serde_indexed(auto_index, offset = 1)]
 pub struct SomeKeys {
     pub number: i32,
     #[serde(skip_serializing_if = "Option::is_none")]

src/parse.rs

@@ -12,6 +12,7 @@ pub struct Input {
 
 #[derive(Default)]
 pub struct StructAttrs {
+    pub auto_index: bool,
     pub offset: usize,
     // pub skip_nones: bool,
 }
@@ -24,7 +25,10 @@ pub struct Field {
 }
 
 fn parse_meta(attrs: &mut StructAttrs, meta: ParseNestedMeta) -> Result<()> {
-    if meta.path.is_ident("offset") {
+    if meta.path.is_ident("auto_index") {
+        attrs.auto_index = true;
+        Ok(())
+    } else if meta.path.is_ident("offset") {
         let value = meta.value()?;
         let offset: LitInt = value.parse()?;
         attrs.offset = offset.base10_parse()?;
@@ -75,7 +79,7 @@ impl Parse for Input {
             }
         };
 
-        let fields = fields_from_ast(&syn_fields.named)?;
+        let fields = fields_from_ast(&attrs, &syn_fields.named)?;
 
         //serde::internals::ast calls `fields_from_ast(cx, &fields.named, attrs, container_default)`
 
@@ -89,8 +93,16 @@ impl Parse for Input {
 }
 
 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",
+        ));
+    }
+
     // serde::internals::ast.rs:L183
     fields
         .iter()

tests/basics.rs

@@ -43,7 +43,7 @@ mod some_keys {
     use serde_bytes::Bytes;
 
     #[derive(Clone, Debug, PartialEq, SerializeIndexed, DeserializeIndexed)]
-    #[serde_indexed(offset = 1)]
+    #[serde_indexed(auto_index, offset = 1)]
     pub struct SomeKeys {
         pub number: i32,
         pub bytes: [u8; 7],
@@ -54,7 +54,7 @@ mod some_keys {
     }
 
     #[derive(Clone, Debug, PartialEq, SerializeIndexed, DeserializeIndexed)]
-    #[serde_indexed(offset = 1)]
+    #[serde_indexed(auto_index, offset = 1)]
     pub struct SomeRefKeys<'a, 'b, 'c> {
         pub number: i32,
         pub bytes: &'a ByteArray<7>,
@@ -66,6 +66,7 @@ mod some_keys {
 
     #[derive(Clone, Debug, PartialEq, SerializeIndexed, DeserializeIndexed)]
     // #[serde_indexed(offset = 1)]
+    #[serde_indexed(auto_index)]
     pub struct NakedOption {
         pub option: Option<SomeKeys>,
         pub num: usize,
@@ -74,6 +75,7 @@ mod some_keys {
 
     #[derive(Clone, Debug, PartialEq, SerializeIndexed, DeserializeIndexed)]
     // #[serde_indexed(offset = 1)]
+    #[serde_indexed(auto_index)]
     pub struct NakedRefOption<'a, 'b, 'c> {
         pub option: Option<SomeRefKeys<'a, 'b, 'c>>,
         pub num: usize,
@@ -82,6 +84,7 @@ mod some_keys {
 
     #[derive(Clone, Debug, PartialEq, SerializeIndexed, DeserializeIndexed)]
     // #[serde_indexed(offset = 1)]
+    #[serde_indexed(auto_index)]
     pub struct EmptyStruct {}
 
     fn an_example() -> (&'static [u8], SomeKeys) {
@@ -317,7 +320,7 @@ mod cow {
     use std::borrow::Cow;
 
     #[derive(PartialEq, Debug, SerializeIndexed, DeserializeIndexed)]
-    #[serde_indexed(offset = 1)]
+    #[serde_indexed(auto_index, offset = 1)]
     struct WithLifetimes<'a> {
         data: Cow<'a, [u8]>,
         #[serde(skip_serializing_if = "Option::is_none")]
@@ -381,7 +384,7 @@ mod generics {
     use serde_bytes::Bytes;
 
     #[derive(PartialEq, Debug, SerializeIndexed, DeserializeIndexed)]
-    #[serde_indexed(offset = 1)]
+    #[serde_indexed(auto_index, offset = 1)]
     struct WithGeneric<T> {
         data: T,
         #[serde(skip_serializing_if = "Option::is_none")]
@@ -396,7 +399,7 @@ mod generics {
     }
 
     #[derive(PartialEq, Debug, SerializeIndexed, DeserializeIndexed)]
-    #[serde_indexed(offset = 1)]
+    #[serde_indexed(auto_index, offset = 1)]
     struct WithConstGeneric<const N: usize> {
         data: ByteArray<N>,
         #[serde(skip_serializing_if = "Option::is_none")]
@@ -445,7 +448,7 @@ mod generics {
     }
 
     #[derive(PartialEq, Debug, SerializeIndexed, DeserializeIndexed)]
-    #[serde_indexed(offset = 1)]
+    #[serde_indexed(auto_index, offset = 1)]
     struct WithAllGenerics<'a, 'b, T, I, const N: usize, const Z: usize> {
         data1: heapless::Vec<T, N>,
         data2: heapless::Vec<I, Z>,