From f425864692614cb6385a9fd23fc23818c81b8151 Mon Sep 17 00:00:00 2001
From: Philip Metzger <philipmetzger@bluewin.ch>
Date: Fri, 1 May 2026 16:54:33 +0200
Subject: [PATCH 1/5] crates: Add the `jj-core` crate

This will be common base for building upon `jj` if you only want the
internals which make the other systems work, like the `Backend` or
`WorkingCopy` trait. It should be of utmost importance to make the
crate as low dependency as possible so it'ss not in the critical path
during compilation.

Part of #6284
---
 Cargo.lock      |  4 ++++
 Cargo.toml      |  9 ++++++++-
 core/Cargo.toml | 17 +++++++++++++++++
 core/src/lib.rs | 20 ++++++++++++++++++++
 4 files changed, 49 insertions(+), 1 deletion(-)
 create mode 100644 core/Cargo.toml
 create mode 100644 core/src/lib.rs

diff --git a/Cargo.lock b/Cargo.lock
index 1b4a6f10fc5..13e38360b94 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2635,6 +2635,10 @@ dependencies = [
  "whoami",
 ]
 
+[[package]]
+name = "jj-core"
+version = "0.44.0"
+
 [[package]]
 name = "jj-lib"
 version = "0.44.0"
diff --git a/Cargo.toml b/Cargo.toml
index ad90a9fba29..bfeb2bc2124 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,7 +2,14 @@ cargo-features = []
 
 [workspace]
 resolver = "3"
-members = ["cli", "lib", "lib/gen-protos", "lib/proc-macros", "lib/testutils"]
+members = [
+    "cli",
+    "core",
+    "lib",
+    "lib/gen-protos",
+    "lib/proc-macros",
+    "lib/testutils",
+]
 
 [workspace.package]
 version = "0.44.0"
diff --git a/core/Cargo.toml b/core/Cargo.toml
new file mode 100644
index 00000000000..ff3a82422fe
--- /dev/null
+++ b/core/Cargo.toml
@@ -0,0 +1,17 @@
+[package]
+name = "jj-core"
+version.workspace = true
+license.workspace = true
+rust-version.workspace = true
+edition.workspace = true
+readme.workspace = true
+homepage.workspace = true
+repository.workspace = true
+documentation.workspace = true
+categories.workspace = true
+keywords.workspace = true
+
+[dependencies]
+
+[lints]
+workspace = true
diff --git a/core/src/lib.rs b/core/src/lib.rs
new file mode 100644
index 00000000000..02eb60d1a21
--- /dev/null
+++ b/core/src/lib.rs
@@ -0,0 +1,20 @@
+// Copyright 2026 The Jujutsu Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! The core library powering the Jujutsu Version Control System. It contains
+//! all "base" types such as `Commit` and the `Backend` trait.
+
+#![warn(missing_docs)]
+#![forbid(unsafe_code)]
+#![deny(unused_must_use)]

From 3baace65c9b43fde074e8b471a687eac70b0f1cf Mon Sep 17 00:00:00 2001
From: Martin von Zweigbergk <martinvonz@ersc.io>
Date: Thu, 6 Aug 2026 13:07:56 -0700
Subject: [PATCH 2/5] core: move `dag_walk` and `dag_walk_async` from `jj-lib`

These modules are general-purpose algorithms with no dependencies on
the rest of `jj-lib`, so they can move to the new low-dependency
`jj-core` crate. `jj-lib` re-exports them, so its API is unchanged.

Part of #6284
---
 Cargo.lock                          | 10 ++++++++++
 Cargo.toml                          |  1 +
 core/Cargo.toml                     |  9 +++++++++
 {lib => core}/src/dag_walk.rs       |  0
 {lib => core}/src/dag_walk_async.rs |  0
 core/src/lib.rs                     |  3 +++
 lib/Cargo.toml                      |  1 +
 lib/src/lib.rs                      |  4 ++--
 8 files changed, 26 insertions(+), 2 deletions(-)
 rename {lib => core}/src/dag_walk.rs (100%)
 rename {lib => core}/src/dag_walk_async.rs (100%)

diff --git a/Cargo.lock b/Cargo.lock
index 13e38360b94..1c16e3931fc 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2638,6 +2638,15 @@ dependencies = [
 [[package]]
 name = "jj-core"
 version = "0.44.0"
+dependencies = [
+ "assert_matches",
+ "futures 0.3.33",
+ "indexmap",
+ "itertools 0.15.0",
+ "maplit",
+ "pollster",
+ "smallvec",
+]
 
 [[package]]
 name = "jj-lib"
@@ -2665,6 +2674,7 @@ dependencies = [
  "insta",
  "interim",
  "itertools 0.15.0",
+ "jj-core",
  "jj-lib-proc-macros",
  "maplit",
  "memchr",
diff --git a/Cargo.toml b/Cargo.toml
index bfeb2bc2124..1641b3e79dc 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -146,6 +146,7 @@ winreg = "0.56"
 
 # put all inter-workspace libraries, i.e. those that use 'path = ...' here in
 # their own (alphabetically sorted) block
+jj-core = { path = "core", version = "0.44.0" }
 jj-lib = { path = "lib", version = "0.44.0", default-features = false }
 jj-lib-proc-macros = { path = "lib/proc-macros", version = "0.44.0" }
 testutils = { path = "lib/testutils" }
diff --git a/core/Cargo.toml b/core/Cargo.toml
index ff3a82422fe..5eeb360c02a 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -12,6 +12,15 @@ categories.workspace = true
 keywords.workspace = true
 
 [dependencies]
+futures = { workspace = true }
+indexmap = { workspace = true }
+itertools = { workspace = true }
+smallvec = { workspace = true }
+
+[dev-dependencies]
+assert_matches = { workspace = true }
+maplit = { workspace = true }
+pollster = { workspace = true }
 
 [lints]
 workspace = true
diff --git a/lib/src/dag_walk.rs b/core/src/dag_walk.rs
similarity index 100%
rename from lib/src/dag_walk.rs
rename to core/src/dag_walk.rs
diff --git a/lib/src/dag_walk_async.rs b/core/src/dag_walk_async.rs
similarity index 100%
rename from lib/src/dag_walk_async.rs
rename to core/src/dag_walk_async.rs
diff --git a/core/src/lib.rs b/core/src/lib.rs
index 02eb60d1a21..44125be0ecd 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -18,3 +18,6 @@
 #![warn(missing_docs)]
 #![forbid(unsafe_code)]
 #![deny(unused_must_use)]
+
+pub mod dag_walk;
+pub mod dag_walk_async;
diff --git a/lib/Cargo.toml b/lib/Cargo.toml
index 152fc1700c5..457a4bb2c4c 100644
--- a/lib/Cargo.toml
+++ b/lib/Cargo.toml
@@ -47,6 +47,7 @@ hashbrown = { workspace = true }
 indexmap = { workspace = true }
 interim = { workspace = true }
 itertools = { workspace = true }
+jj-core = { workspace = true }
 jj-lib-proc-macros = { workspace = true }
 maplit = { workspace = true }
 memchr = { workspace = true }
diff --git a/lib/src/lib.rs b/lib/src/lib.rs
index 5ed1381c8f0..0f2f5dbc860 100644
--- a/lib/src/lib.rs
+++ b/lib/src/lib.rs
@@ -40,8 +40,8 @@ pub mod conflict_labels;
 pub mod conflicts;
 pub mod converge;
 pub mod copies;
-pub mod dag_walk;
-pub mod dag_walk_async;
+pub use jj_core::dag_walk;
+pub use jj_core::dag_walk_async;
 pub mod default_backend_factories;
 pub mod default_index;
 pub mod default_submodule_store;

From a16db415aac48ff540ac261ef3de171bfe75e06a Mon Sep 17 00:00:00 2001
From: Martin von Zweigbergk <martinvonz@ersc.io>
Date: Thu, 6 Aug 2026 13:17:37 -0700
Subject: [PATCH 3/5] core: move `diff` module from `jj-lib`

`jj-lib` re-exports it, so its API is unchanged. The `hashbrown`
dependency moves along with it since `diff` was its only user in
`jj-lib`.

Part of #6284
---
 Cargo.lock                          | 5 +++--
 core/Cargo.toml                     | 7 +++++++
 {lib => core}/benches/diff_bench.rs | 2 +-
 {lib => core}/src/diff.rs           | 0
 core/src/lib.rs                     | 1 +
 lib/Cargo.toml                      | 7 -------
 lib/src/lib.rs                      | 2 +-
 7 files changed, 13 insertions(+), 11 deletions(-)
 rename {lib => core}/benches/diff_bench.rs (99%)
 rename {lib => core}/src/diff.rs (100%)

diff --git a/Cargo.lock b/Cargo.lock
index 1c16e3931fc..e43d289f6fa 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2640,7 +2640,10 @@ name = "jj-core"
 version = "0.44.0"
 dependencies = [
  "assert_matches",
+ "bstr",
+ "criterion",
  "futures 0.3.33",
+ "hashbrown 0.17.1",
  "indexmap",
  "itertools 0.15.0",
  "maplit",
@@ -2658,7 +2661,6 @@ dependencies = [
  "bstr",
  "chrono",
  "clru",
- "criterion",
  "digest 0.10.7",
  "dunce",
  "either",
@@ -2668,7 +2670,6 @@ dependencies = [
  "gix",
  "gix-ignore",
  "globset",
- "hashbrown 0.17.1",
  "indexmap",
  "indoc",
  "insta",
diff --git a/core/Cargo.toml b/core/Cargo.toml
index 5eeb360c02a..29fa2ace826 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -11,14 +11,21 @@ documentation.workspace = true
 categories.workspace = true
 keywords.workspace = true
 
+[[bench]]
+name = "diff_bench"
+harness = false
+
 [dependencies]
+bstr = { workspace = true }
 futures = { workspace = true }
+hashbrown = { workspace = true }
 indexmap = { workspace = true }
 itertools = { workspace = true }
 smallvec = { workspace = true }
 
 [dev-dependencies]
 assert_matches = { workspace = true }
+criterion = { workspace = true }
 maplit = { workspace = true }
 pollster = { workspace = true }
 
diff --git a/lib/benches/diff_bench.rs b/core/benches/diff_bench.rs
similarity index 99%
rename from lib/benches/diff_bench.rs
rename to core/benches/diff_bench.rs
index c1c8bf4b4ed..0a104331cc1 100644
--- a/lib/benches/diff_bench.rs
+++ b/core/benches/diff_bench.rs
@@ -2,7 +2,7 @@ use criterion::BenchmarkId;
 use criterion::Criterion;
 use criterion::criterion_group;
 use criterion::criterion_main;
-use jj_lib::diff;
+use jj_core::diff;
 
 fn unchanged_lines(count: usize) -> (String, String) {
     let mut lines = vec![];
diff --git a/lib/src/diff.rs b/core/src/diff.rs
similarity index 100%
rename from lib/src/diff.rs
rename to core/src/diff.rs
diff --git a/core/src/lib.rs b/core/src/lib.rs
index 44125be0ecd..42ad60497ed 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -21,3 +21,4 @@
 
 pub mod dag_walk;
 pub mod dag_walk_async;
+pub mod diff;
diff --git a/lib/Cargo.toml b/lib/Cargo.toml
index 457a4bb2c4c..ed2b49765f1 100644
--- a/lib/Cargo.toml
+++ b/lib/Cargo.toml
@@ -14,7 +14,6 @@ readme = { workspace = true }
 
 include = [
     "/LICENSE",
-    "/benches/",
     "/src/",
     "/tests/",
     "!*.pending-snap",
@@ -25,10 +24,6 @@ include = [
 [[test]]
 name = "runner"
 
-[[bench]]
-name = "diff_bench"
-harness = false
-
 [dependencies]
 async-trait = { workspace = true }
 blake2 = { workspace = true }
@@ -43,7 +38,6 @@ futures = { workspace = true }
 gix = { workspace = true, optional = true }
 gix-ignore = { workspace = true }
 globset = { workspace = true }
-hashbrown = { workspace = true }
 indexmap = { workspace = true }
 interim = { workspace = true }
 itertools = { workspace = true }
@@ -80,7 +74,6 @@ winreg = { workspace = true }
 
 [dev-dependencies]
 assert_matches = { workspace = true }
-criterion = { workspace = true }
 eyre = { workspace = true }
 indoc = { workspace = true }
 insta = { workspace = true }
diff --git a/lib/src/lib.rs b/lib/src/lib.rs
index 0f2f5dbc860..30e611ef95b 100644
--- a/lib/src/lib.rs
+++ b/lib/src/lib.rs
@@ -45,7 +45,7 @@ pub use jj_core::dag_walk_async;
 pub mod default_backend_factories;
 pub mod default_index;
 pub mod default_submodule_store;
-pub mod diff;
+pub use jj_core::diff;
 pub mod diff_presentation;
 pub mod dsl_util;
 pub(crate) mod eol;

From aaf5cc8c31b844280d171a3be5453c2d8336e08a Mon Sep 17 00:00:00 2001
From: Martin von Zweigbergk <martinvonz@ersc.io>
Date: Thu, 6 Aug 2026 13:18:04 -0700
Subject: [PATCH 4/5] core: move `hex_util` module from `jj-lib`

`jj-lib` re-exports it, so its API is unchanged.

Part of #6284
---
 {lib => core}/src/hex_util.rs | 0
 core/src/lib.rs               | 1 +
 lib/src/lib.rs                | 2 +-
 3 files changed, 2 insertions(+), 1 deletion(-)
 rename {lib => core}/src/hex_util.rs (100%)

diff --git a/lib/src/hex_util.rs b/core/src/hex_util.rs
similarity index 100%
rename from lib/src/hex_util.rs
rename to core/src/hex_util.rs
diff --git a/core/src/lib.rs b/core/src/lib.rs
index 42ad60497ed..b840b4fc8f1 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -22,3 +22,4 @@
 pub mod dag_walk;
 pub mod dag_walk_async;
 pub mod diff;
+pub mod hex_util;
diff --git a/lib/src/lib.rs b/lib/src/lib.rs
index 30e611ef95b..7c8297095ce 100644
--- a/lib/src/lib.rs
+++ b/lib/src/lib.rs
@@ -68,7 +68,7 @@ pub mod gitignore;
 pub mod gpg_signing;
 pub mod graph;
 pub mod graph_dominators;
-pub mod hex_util;
+pub use jj_core::hex_util;
 pub mod id_prefix;
 pub mod index;
 pub mod iter_util;

From ceab14c61f6658a5b03903c8956a460e8a347df7 Mon Sep 17 00:00:00 2001
From: Martin von Zweigbergk <martinvonz@ersc.io>
Date: Thu, 6 Aug 2026 13:22:30 -0700
Subject: [PATCH 5/5] core: move `str_util` module from `jj-lib`

`jj-lib` re-exports it, so its API is unchanged. The tests need the
`TestResult` alias that `jj-lib` has in its crate-level `tests`
module, so `jj-core` gets a copy of that module.

Part of #6284
---
 Cargo.lock                    | 6 ++++++
 core/Cargo.toml               | 6 ++++++
 core/src/lib.rs               | 7 +++++++
 {lib => core}/src/str_util.rs | 0
 lib/src/lib.rs                | 2 +-
 5 files changed, 20 insertions(+), 1 deletion(-)
 rename {lib => core}/src/str_util.rs (100%)

diff --git a/Cargo.lock b/Cargo.lock
index e43d289f6fa..95ca7dc8c42 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2642,13 +2642,19 @@ dependencies = [
  "assert_matches",
  "bstr",
  "criterion",
+ "either",
+ "eyre",
  "futures 0.3.33",
+ "globset",
  "hashbrown 0.17.1",
  "indexmap",
+ "insta",
  "itertools 0.15.0",
  "maplit",
  "pollster",
+ "regex",
  "smallvec",
+ "thiserror 2.0.19",
 ]
 
 [[package]]
diff --git a/core/Cargo.toml b/core/Cargo.toml
index 29fa2ace826..c70da7b8c97 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -17,15 +17,21 @@ harness = false
 
 [dependencies]
 bstr = { workspace = true }
+either = { workspace = true }
 futures = { workspace = true }
+globset = { workspace = true }
 hashbrown = { workspace = true }
 indexmap = { workspace = true }
 itertools = { workspace = true }
+regex = { workspace = true }
 smallvec = { workspace = true }
+thiserror = { workspace = true }
 
 [dev-dependencies]
 assert_matches = { workspace = true }
 criterion = { workspace = true }
+eyre = { workspace = true }
+insta = { workspace = true }
 maplit = { workspace = true }
 pollster = { workspace = true }
 
diff --git a/core/src/lib.rs b/core/src/lib.rs
index b840b4fc8f1..ade500f8555 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -23,3 +23,10 @@ pub mod dag_walk;
 pub mod dag_walk_async;
 pub mod diff;
 pub mod hex_util;
+pub mod str_util;
+
+#[cfg(test)]
+mod tests {
+    // Copied from `testutils::TestResult` to remove dependency cycle.
+    pub type TestResult<T = ()> = eyre::Result<T>;
+}
diff --git a/lib/src/str_util.rs b/core/src/str_util.rs
similarity index 100%
rename from lib/src/str_util.rs
rename to core/src/str_util.rs
diff --git a/lib/src/lib.rs b/lib/src/lib.rs
index 7c8297095ce..34fd1479501 100644
--- a/lib/src/lib.rs
+++ b/lib/src/lib.rs
@@ -107,7 +107,7 @@ pub mod simple_op_store;
 pub mod ssh_signing;
 pub mod stacked_table;
 pub mod store;
-pub mod str_util;
+pub use jj_core::str_util;
 pub mod submodule_store;
 #[cfg(feature = "testing")]
 pub mod test_signing_backend;