Implement a procedural macro to derive the ContentHash trait for structs

This is a no-op in terms of function, but provides a nicer way to derive the
ContentHash trait for structs using the `#[derive(ContentHash)]` syntax used
for other traits such as `Debug`.

This commit only adds the macro. A subsequent commit will replace uses of
`content_hash!{}` with `#[derive(ContentHash)]`.

The new macro generates nice error messages, just like the old macro, although
they are slightly different.

Before:
```
error[E0277]: the trait bound `NotImplemented: content_hash::ContentHash` is not satisfied
   --> lib/src/content_hash.rs:242:30
    |
242 |             struct Broken{t: NotImplemented}
    |                              ^^^^^^^^^^^^^^ the trait `content_hash::ContentHash` is not implemented for `NotImplemented`
    |
    = help: the following other types implement trait `content_hash::ContentHash`:
              bool
              i32
              i64
              u8
              std::collections::HashMap<K, V>
              BTreeMap<K, V>
              std::collections::HashSet<K>
              content_hash::tests::test_struct_sanity::Foo
            and 38 others
```

After:
```
error[E0277]: the trait bound `NotImplemented: content_hash::ContentHash` is not satisfied
   --> lib/src/content_hash.rs:247:13
    |
247 |             t: NotImplemented,
    |             ^ the trait `content_hash::ContentHash` is not implemented for `NotImplemented`
    |
    = help: the following other types implement trait `content_hash::ContentHash`:
              bool
              i32
              i64
              u8
              std::collections::HashMap<K, V>
              BTreeMap<K, V>
              std::collections::HashSet<K>
              content_hash::tests::test_struct_sanity::Foo
            and 38 others

For more information about this error, try `rustc --explain E0277`.
error: could not compile `jj-lib` (lib test) due to 2 previous errors
```

This commit does two things to make proc macros re-exported by jj_lib useable
by deps:

1. jj_lib needs to be able refer to itself as `jj_lib` which it does
   by adding an `extern crate self as jj_lib` declaration.

2. jj_lib::content_hash needs to re-export the `digest::Update` type so that
   users of jj_lib can use the `#[derive(ContentHash)]` proc macro without
   directly depending on the digest crate. This is done by re-exporting it
   as `DigestUpdate`.


#3054

Cargo.toml

@@ -2,7 +2,7 @@ cargo-features = []
 
 [workspace]
 resolver = "2"
-members = ["cli", "lib", "lib/testutils", "lib/gen-protos"]
+members = ["cli", "lib", "lib/gen-protos", "lib/proc-macros", "lib/testutils"]
 
 [workspace.package]
 version = "0.14.0"
@@ -66,8 +66,10 @@ pest = "2.7.7"
 pest_derive = "2.7.7"
 pollster = "0.3.0"
 pretty_assertions = "1.4.0"
+proc-macro2 = "1.0.78"
 prost = "0.12.3"
 prost-build = "0.12.3"
+quote = "1.0.35"
 rand = "0.8.5"
 rand_chacha = "0.3.1"
 rayon = "1.8.1"
@@ -85,6 +87,7 @@ smallvec = { version = "1.13.0", features = [
     "union",
 ] }
 strsim = "0.11.0"
+syn = "2.0.48"
 tempfile = "3.10.0"
 test-case = "3.3.1"
 textwrap = "0.16.0"
@@ -110,6 +113,7 @@ zstd = "0.12.4"
 # their own (alphabetically sorted) block
 
 jj-lib = { path = "lib", version = "0.14.0" }
+jj-lib-proc-macros = { path = "lib/proc-macros", version = "0.14.0" }
 testutils = { path = "lib/testutils" }
 
 # Insta suggests compiling these packages in opt mode for faster testing.

lib/Cargo.toml

@@ -37,6 +37,7 @@ gix = { workspace = true }
 glob = { workspace = true }
 hex = { workspace = true }
 itertools = { workspace = true }
+jj-lib-proc-macros = { workspace = true }
 maplit = { workspace = true }
 once_cell = { workspace = true }
 pest = { workspace = true }

lib/proc-macros/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "jj-lib-proc-macros"
+publish = false
+
+version = { workspace = true }
+edition = { workspace = true }
+license = { workspace = true }
+
+[lib]
+proc-macro = true
+
+[dependencies]
+proc-macro2 = { workspace=true }
+quote = { workspace=true }
+syn = { workspace=true }

lib/proc-macros/src/content_hash.rs

@@ -0,0 +1,37 @@
+use proc_macro2::TokenStream;
+use quote::{quote, quote_spanned};
+use syn::spanned::Spanned;
+use syn::{Data, Fields, Index};
+
+pub fn generate_hash_impl(data: &Data) -> TokenStream {
+    match *data {
+        Data::Struct(ref data) => match data.fields {
+            Fields::Named(ref fields) => {
+                let hash_statements = fields.named.iter().map(|f| {
+                    let field_name = &f.ident;
+                    quote_spanned! {f.span()=>
+                        ::jj_lib::content_hash::ContentHash::hash(&self.#field_name, state);
+                    }
+                });
+                quote! {
+                    #(#hash_statements)*
+                }
+            }
+            Fields::Unnamed(ref fields) => {
+                let hash_statements = fields.unnamed.iter().enumerate().map(|(i, f)| {
+                    let index = Index::from(i);
+                    quote_spanned! {f.span() =>
+                        ::jj_lib::content_hash::ContentHash::hash(&self.#index, state);
+                    }
+                });
+                quote! {
+                    #(#hash_statements)*
+                }
+            }
+            Fields::Unit => {
+                quote! {}
+            }
+        },
+        _ => unimplemented!("ContentHash can only be derived for structs."),
+    }
+}

lib/proc-macros/src/lib.rs

@@ -0,0 +1,30 @@
+mod content_hash;
+
+extern crate proc_macro;
+
+use quote::quote;
+use syn::{parse_macro_input, DeriveInput};
+
+/// Derives the `ContentHash` trait for a struct by calling `ContentHash::hash`
+/// on each of the struct members in the order that they're declared. All
+/// members of the struct must implement the `ContentHash` trait.
+#[proc_macro_derive(ContentHash)]
+pub fn derive_content_hash(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+
+    // The name of the struct.
+    let name = &input.ident;
+
+    // Generate an expression to hash each of the fields in the struct.
+    let hash_impl = content_hash::generate_hash_impl(&input.data);
+
+    let expanded = quote! {
+        #[automatically_derived]
+        impl ::jj_lib::content_hash::ContentHash for #name {
+            fn hash(&self, state: &mut impl ::jj_lib::content_hash::DigestUpdate) {
+                #hash_impl
+            }
+        }
+    };
+    expanded.into()
+}

lib/src/backend.rs

@@ -25,7 +25,7 @@ use std::vec::Vec;
 use async_trait::async_trait;
 use thiserror::Error;
 
-use crate::content_hash::ContentHash;
+use crate::content_hash::{ContentHash, DigestUpdate};
 use crate::index::Index;
 use crate::merge::Merge;
 use crate::object_id::{id_type, ObjectId};
@@ -111,7 +111,7 @@ impl PartialEq for MergedTreeId {
 impl Eq for MergedTreeId {}
 
 impl ContentHash for MergedTreeId {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         match self {
             MergedTreeId::Legacy(tree_id) => {
                 state.update(&0u32.to_le_bytes());
@@ -247,7 +247,7 @@ impl TreeValue {
 }
 
 impl ContentHash for TreeValue {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         use TreeValue::*;
         match self {
             File { id, executable } => {

lib/src/content_hash.rs

@@ -1,7 +1,11 @@
 //! Portable, stable hashing suitable for identifying values
 
 use blake2::Blake2b512;
+// Re-export DigestUpdate so that the ContentHash proc macro can be used in
+// external crates without directly depending on the digest crate.
+pub use digest::Update as DigestUpdate;
 use itertools::Itertools as _;
+pub use jj_lib_proc_macros::ContentHash;
 
 /// Portable, stable hashing suitable for identifying values
 ///
@@ -10,9 +14,11 @@ use itertools::Itertools as _;
 /// order their elements according to their `Ord` implementation. Enums should
 /// hash a 32-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)]`.
 pub trait ContentHash {
     /// Update the hasher state with this object's content
-    fn hash(&self, state: &mut impl digest::Update);
+    fn hash(&self, state: &mut impl DigestUpdate);
 }
 
 /// The 512-bit BLAKE2b content hash
@@ -24,48 +30,48 @@ pub fn blake2b_hash(x: &(impl ContentHash + ?Sized)) -> digest::Output<Blake2b51
 }
 
 impl ContentHash for () {
-    fn hash(&self, _: &mut impl digest::Update) {}
+    fn hash(&self, _: &mut impl DigestUpdate) {}
 }
 
 impl ContentHash for bool {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         u8::from(*self).hash(state);
     }
 }
 
 impl ContentHash for u8 {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         state.update(&[*self]);
     }
 }
 
 impl ContentHash for u32 {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         state.update(&self.to_le_bytes());
     }
 }
 
 impl ContentHash for i32 {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         state.update(&self.to_le_bytes());
     }
 }
 
 impl ContentHash for u64 {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         state.update(&self.to_le_bytes());
     }
 }
 
 impl ContentHash for i64 {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         state.update(&self.to_le_bytes());
     }
 }
 
 // TODO: Specialize for [u8] once specialization exists
 impl<T: ContentHash> ContentHash for [T] {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         state.update(&(self.len() as u64).to_le_bytes());
         for x in self {
             x.hash(state);
@@ -74,19 +80,19 @@ impl<T: ContentHash> ContentHash for [T] {
 }
 
 impl<T: ContentHash> ContentHash for Vec<T> {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         self.as_slice().hash(state)
     }
 }
 
 impl ContentHash for String {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         self.as_bytes().hash(state);
     }
 }
 
 impl<T: ContentHash> ContentHash for Option<T> {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         match self {
             None => state.update(&0u32.to_le_bytes()),
             Some(x) => {
@@ -102,7 +108,7 @@ where
     K: ContentHash + Ord,
     V: ContentHash,
 {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         state.update(&(self.len() as u64).to_le_bytes());
         let mut kv = self.iter().collect_vec();
         kv.sort_unstable_by_key(|&(k, _)| k);
@@ -117,7 +123,7 @@ impl<K> ContentHash for std::collections::HashSet<K>
 where
     K: ContentHash + Ord,
 {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         state.update(&(self.len() as u64).to_le_bytes());
         for k in self.iter().sorted() {
             k.hash(state);
@@ -130,7 +136,7 @@ where
     K: ContentHash,
     V: ContentHash,
 {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         state.update(&(self.len() as u64).to_le_bytes());
         for (k, v) in self.iter() {
             k.hash(state);
@@ -243,6 +249,31 @@ mod tests {
         );
     }
 
+    // This will be removed once all uses of content_hash! are replaced by the
+    // derive version.
+    #[test]
+    fn derive_is_equivalent_to_macro() {
+        content_hash! {
+            struct FooMacro { x: Vec<Option<i32>>, y: i64}
+        }
+
+        #[derive(ContentHash)]
+        struct FooDerive {
+            x: Vec<Option<i32>>,
+            y: i64,
+        }
+
+        let foo_macro = FooMacro {
+            x: vec![None, Some(42)],
+            y: 17,
+        };
+        let foo_derive = FooDerive {
+            x: vec![None, Some(42)],
+            y: 17,
+        };
+        assert_eq!(hash(&foo_macro), hash(&foo_derive));
+    }
+
     fn hash(x: &(impl ContentHash + ?Sized)) -> digest::Output<Blake2b512> {
         blake2b_hash(x)
     }

lib/src/lib.rs

@@ -18,6 +18,13 @@
 #![deny(unused_must_use)]
 #![forbid(unsafe_code)]
 
+// Needed so that proc macros can be used inside jj_lib and by external crates
+// that depend on it.
+// See:
+// - https://github.com/rust-lang/rust/issues/54647#issuecomment-432015102
+// - https://github.com/rust-lang/rust/issues/54363
+extern crate self as jj_lib;
+
 #[macro_use]
 pub mod content_hash;
 

lib/src/merge.rs

@@ -29,7 +29,7 @@ use smallvec::{smallvec_inline, SmallVec};
 
 use crate::backend;
 use crate::backend::{BackendError, FileId, TreeId, TreeValue};
-use crate::content_hash::ContentHash;
+use crate::content_hash::{ContentHash, DigestUpdate};
 use crate::object_id::ObjectId;
 use crate::repo_path::RepoPath;
 use crate::store::Store;
@@ -457,7 +457,7 @@ impl<T> Merge<Merge<T>> {
 }
 
 impl<T: ContentHash> ContentHash for Merge<T> {
-    fn hash(&self, state: &mut impl digest::Update) {
+    fn hash(&self, state: &mut impl DigestUpdate) {
         self.values.hash(state)
     }
 }