From e627b08786cb24c966e8f4f4da92b34035dde9fe Mon Sep 17 00:00:00 2001
From: Erik Johnston <erikj@element.io>
Date: Tue, 17 Feb 2026 14:48:59 +0100
Subject: [PATCH] Add cargo.lock to Rust build hash (#19470)

This is so that when we update dependencies etc we correctly ensure that
the Rust library has been rebuilt.
---
 changelog.d/19470.misc |  1 +
 rust/build.rs          | 14 ++++++++++++++
 synapse/util/rust.py   | 19 +++++++++++++------
 3 files changed, 28 insertions(+), 6 deletions(-)
 create mode 100644 changelog.d/19470.misc

diff --git a/changelog.d/19470.misc b/changelog.d/19470.misc
new file mode 100644
index 0000000000..e253d89b17
--- /dev/null
+++ b/changelog.d/19470.misc
@@ -0,0 +1 @@
+Correctly refuse to start if the Rust workspace config has changed and the Rust library has not been rebuilt.
diff --git a/rust/build.rs b/rust/build.rs
index ef370e6b41..8755f3bfa3 100644
--- a/rust/build.rs
+++ b/rust/build.rs
@@ -29,6 +29,13 @@ fn main() -> Result<(), std::io::Error> {
         }
     }
 
+    // Manually add Cargo.toml's, Cargo.lock and build.rs to the hash, since changes to
+    // these files should also invalidate the built module.
+    paths.push("Cargo.toml".to_string());
+    paths.push("../Cargo.lock".to_string());
+    paths.push("../Cargo.toml".to_string());
+    paths.push("build.rs".to_string());
+
     paths.sort();
 
     let mut hasher = Blake2b512::new();
@@ -41,5 +48,12 @@ fn main() -> Result<(), std::io::Error> {
     let hex_digest = hex::encode(hasher.finalize());
     println!("cargo:rustc-env=SYNAPSE_RUST_DIGEST={hex_digest}");
 
+    // The default rules don't pick up trivial changes to the workspace config
+    // files, but we need to rebuild if those change to pick up the changed
+    // hashes.
+    println!("cargo::rerun-if-changed=.");
+    println!("cargo::rerun-if-changed=../Cargo.lock");
+    println!("cargo::rerun-if-changed=../Cargo.toml");
+
     Ok(())
 }
diff --git a/synapse/util/rust.py b/synapse/util/rust.py
index d1e1a259e4..a7c9e976ea 100644
--- a/synapse/util/rust.py
+++ b/synapse/util/rust.py
@@ -40,24 +40,24 @@ def check_rust_lib_up_to_date() -> None:
         return None
 
     # Get the hash of all Rust source files
-    rust_path = os.path.join(synapse_root, "rust", "src")
+    rust_path = os.path.join(synapse_root, "rust")
     if not os.path.exists(rust_path):
         return None
 
-    hash = _hash_rust_files_in_directory(rust_path)
+    hash = _hash_rust_files_in_directory(synapse_root)
 
     if hash != get_rust_file_digest():
         raise Exception("Rust module outdated. Please rebuild using `poetry install`")
 
 
-def _hash_rust_files_in_directory(directory: str) -> str:
+def _hash_rust_files_in_directory(synapse_root: str) -> str:
     """Get the hash of all files in a directory (recursively)"""
 
-    directory = os.path.abspath(directory)
+    src_directory = os.path.abspath(os.path.join(synapse_root, "rust", "src"))
 
     paths = []
 
-    dirs = [directory]
+    dirs = [src_directory]
     while dirs:
         dir = dirs.pop()
         with os.scandir(dir) as d:
@@ -67,13 +67,20 @@ def _hash_rust_files_in_directory(directory: str) -> str:
                 else:
                     paths.append(entry.path)
 
+    # Manually add Cargo.toml's, Cargo.lock and build.rs to the hash, since
+    # changes to these files should also invalidate the built module.
+    paths.append(os.path.join(synapse_root, "rust", "Cargo.toml"))
+    paths.append(os.path.join(synapse_root, "rust", "build.rs"))
+    paths.append(os.path.join(synapse_root, "Cargo.lock"))
+    paths.append(os.path.join(synapse_root, "Cargo.toml"))
+
     # We sort to make sure that we get a consistent and well-defined ordering.
     paths.sort()
 
     hasher = blake2b()
 
     for path in paths:
-        with open(os.path.join(directory, path), "rb") as f:
+        with open(path, "rb") as f:
             hasher.update(f.read())
 
     return hasher.hexdigest()