Add support for deriving ContentHash for Enums

This commit also updates the documentation regarding how ContentHash should be
implemented for enums to address the concerns raised in #3051.

lib/proc-macros/Cargo.toml

@@ -13,3 +13,4 @@ proc-macro = true
 proc-macro2 = { workspace=true }
 quote = { workspace=true }
 syn = { workspace=true }
+

lib/proc-macros/src/content_hash.rs

@@ -1,7 +1,7 @@
 use proc_macro2::TokenStream;
-use quote::{quote, quote_spanned};
+use quote::{format_ident, quote, quote_spanned};
 use syn::spanned::Spanned;
-use syn::{parse_quote, Data, Fields, GenericParam, Generics, Index};
+use syn::{parse_quote, Data, Field, Fields, GenericParam, Generics, Index, Variant};
 
 pub fn add_trait_bounds(mut generics: Generics) -> Generics {
     for param in &mut generics.params {
@@ -44,6 +44,53 @@ pub fn generate_hash_impl(data: &Data) -> TokenStream {
                 quote! {}
             }
         },
+        Data::Enum(ref data) => {
+            let match_hash_statements =
+                data.variants
+                    .iter()
+                    .enumerate()
+                    .map(|(i, v)| match &v.fields {
+                        Fields::Named(fields) => match_arm_with_fields(i, v, fields.named.iter()),
+                        Fields::Unnamed(fields) => {
+                            match_arm_with_fields(i, v, fields.unnamed.iter())
+                        }
+                        Fields::Unit => {
+                            let ix: u8 = 0;
+                            quote_spanned! {v.span() =>
+                                Self::#v => {state.update(&#ix.to_le_bytes());}
+                            }
+                        }
+                    });
+            quote! {
+                match self {
+                    #(#match_hash_statements)*
+                }
+            }
+        }
         _ => unimplemented!("ContentHash can only be derived for structs."),
     }
 }
+
+fn match_arm_with_fields<'a>(
+    index: usize,
+    variant: &Variant,
+    fields: impl IntoIterator<Item = &'a Field>,
+) -> TokenStream {
+    let bindings = fields
+        .into_iter()
+        .enumerate()
+        .map(|(i, f)| {
+            // If the field is named, use the name, otherwise generate a placeholder name.
+            f.ident.clone().unwrap_or(format_ident!("field_{}", i))
+        })
+        .collect::<Vec<_>>();
+    let variant_id = &variant.ident;
+    let ix = u8::try_from(index).unwrap();
+
+    quote_spanned! {variant.span() =>
+        Self::#variant_id( #(#bindings),* ) => {
+            state.update(&#ix.to_le_bytes());
+            #( crate::content_hash::ContentHash::hash(#bindings, state); )*
+        }
+    }
+}

lib/src/content_hash.rs

@@ -9,7 +9,7 @@ pub(crate) use jj_lib_proc_macros::ContentHash;
 /// Variable-length sequences should hash a 64-bit little-endian representation
 /// of their length, then their elements in order. Unordered containers should
 /// order their elements according to their `Ord` implementation. Enums should
-/// hash a 32-bit little-endian encoding of the ordinal number of the enum
+/// hash an 8-bit little-endian encoding of the ordinal number of the enum
 /// variant, then the variant's fields in lexical order.
 ///
 /// Structs can implement `ContentHash` by using `#[derive(ContentHash)]`.
@@ -234,6 +234,18 @@ mod tests {
         );
     }
 
+    // Test that the derived version of `ContentHash` matches the that's
+    // manually implemented for `std::Option`.
+    #[test]
+    fn derive_for_enum() {
+        #[derive(ContentHash)]
+        enum MyOption<T> {
+            None,
+            Some(T),
+        }
+        assert_eq!(hash(&Option::<i32>::None), hash(&MyOption::<i32>::None));
+        assert_eq!(hash(&Some(1)), hash(&MyOption::Some(1)));
+    }
     // This will be removed once all uses of content_hash! are replaced by the
     // derive version.
     #[test]