Generalise compare_fields to work with iterators (#4823)

## Proposed Changes

Add `compare_fields(as_iter)` as a field attribute to `compare_fields_derive`. This allows any iterable type to be compared in the same as a slice (by index). 

This is forwards-compatible with tree-states types like `List` and `Vector` which can not be cast to slices.

common/compare_fields/Cargo.toml

@@ -4,6 +4,9 @@ version = "0.2.0"
 authors = ["Paul Hauner <[email protected]>"]
 edition = { workspace = true }
 
+[dependencies]
+itertools = { workspace = true }
+
 [dev-dependencies]
 compare_fields_derive = { workspace = true }
 

common/compare_fields/src/lib.rs

@@ -81,11 +81,8 @@
 //!     }
 //! ];
 //! assert_eq!(bar_a.compare_fields(&bar_b), bar_a_b);
-//!
-//!
-//!
-//! // TODO:
 //! ```
+use itertools::{EitherOrBoth, Itertools};
 use std::fmt::Debug;
 
 #[derive(Debug, PartialEq, Clone)]
@@ -112,13 +109,38 @@ impl Comparison {
     }
 
     pub fn from_slice<T: Debug + PartialEq<T>>(field_name: String, a: &[T], b: &[T]) -> Self {
+        Self::from_iter(field_name, a.iter(), b.iter())
+    }
+
+    pub fn from_into_iter<'a, T: Debug + PartialEq + 'a>(
+        field_name: String,
+        a: impl IntoIterator<Item = &'a T>,
+        b: impl IntoIterator<Item = &'a T>,
+    ) -> Self {
+        Self::from_iter(field_name, a.into_iter(), b.into_iter())
+    }
+
+    pub fn from_iter<'a, T: Debug + PartialEq + 'a>(
+        field_name: String,
+        a: impl Iterator<Item = &'a T>,
+        b: impl Iterator<Item = &'a T>,
+    ) -> Self {
         let mut children = vec![];
+        let mut all_equal = true;
 
-        for i in 0..std::cmp::max(a.len(), b.len()) {
-            children.push(FieldComparison::new(format!("{i}"), &a.get(i), &b.get(i)));
+        for (i, entry) in a.zip_longest(b).enumerate() {
+            let comparison = match entry {
+                EitherOrBoth::Both(x, y) => {
+                    FieldComparison::new(format!("{i}"), &Some(x), &Some(y))
+                }
+                EitherOrBoth::Left(x) => FieldComparison::new(format!("{i}"), &Some(x), &None),
+                EitherOrBoth::Right(y) => FieldComparison::new(format!("{i}"), &None, &Some(y)),
+            };
+            all_equal = all_equal && comparison.equal();
+            children.push(comparison);
         }
 
-        Self::parent(field_name, a == b, children)
+        Self::parent(field_name, all_equal, children)
     }
 
     pub fn retain_children<F>(&mut self, f: F)

common/compare_fields_derive/src/lib.rs

@@ -4,10 +4,11 @@ use proc_macro::TokenStream;
 use quote::quote;
 use syn::{parse_macro_input, DeriveInput};
 
-fn is_slice(field: &syn::Field) -> bool {
+fn is_iter(field: &syn::Field) -> bool {
     field.attrs.iter().any(|attr| {
         attr.path.is_ident("compare_fields")
-            && attr.tokens.to_string().replace(' ', "") == "(as_slice)"
+            && (attr.tokens.to_string().replace(' ', "") == "(as_slice)"
+                || attr.tokens.to_string().replace(' ', "") == "(as_iter)")
     })
 }
 
@@ -34,13 +35,13 @@ pub fn compare_fields_derive(input: TokenStream) -> TokenStream {
         let field_name = ident_a.to_string();
         let ident_b = ident_a.clone();
 
-        let quote = if is_slice(field) {
+        let quote = if is_iter(field) {
             quote! {
-                comparisons.push(compare_fields::Comparison::from_slice(
+                comparisons.push(compare_fields::Comparison::from_into_iter(
                         #field_name.to_string(),
                         &self.#ident_a,
-                        &b.#ident_b)
-                );
+                        &b.#ident_b
+                ));
             }
         } else {
             quote! {