diff --git a/cli/examples/custom-backend/main.rs b/cli/examples/custom-backend/main.rs
index 4bc1c797e..6cf0a0f25 100644
--- a/cli/examples/custom-backend/main.rs
+++ b/cli/examples/custom-backend/main.rs
@@ -166,7 +166,7 @@ impl Backend for JitBackend {
     fn write_commit(
         &self,
         contents: Commit,
-        sign_with: Option<SigningFn>,
+        sign_with: Option<&mut SigningFn>,
     ) -> BackendResult<(CommitId, Commit)> {
         self.inner.write_commit(contents, sign_with)
     }
diff --git a/lib/src/backend.rs b/lib/src/backend.rs
index 68ec07209..a81c183ea 100644
--- a/lib/src/backend.rs
+++ b/lib/src/backend.rs
@@ -148,7 +148,7 @@ content_hash! {
     }
 }
 
-pub type SigningFn = Box<dyn FnMut(&[u8]) -> SignResult<Vec<u8>>>;
+pub type SigningFn<'a> = dyn FnMut(&[u8]) -> SignResult<Vec<u8>> + 'a;
 
 /// Identifies a single legacy tree, which may have path-level conflicts, or a
 /// merge of multiple trees, where the individual trees do not have conflicts.
@@ -531,6 +531,6 @@ pub trait Backend: Send + Sync + Debug {
     fn write_commit(
         &self,
         contents: Commit,
-        sign_with: Option<SigningFn>,
+        sign_with: Option<&mut SigningFn>,
     ) -> BackendResult<(CommitId, Commit)>;
 }
diff --git a/lib/src/commit_builder.rs b/lib/src/commit_builder.rs
index 9f816e29e..67c50e48a 100644
--- a/lib/src/commit_builder.rs
+++ b/lib/src/commit_builder.rs
@@ -183,14 +183,13 @@ impl CommitBuilder<'_> {
             }
         }
 
-        let sign_settings = self.sign_settings;
+        let sign_settings = &self.sign_settings;
         let store = self.mut_repo.store();
 
-        let signing_fn = (store.signer().can_sign() && sign_settings.should_sign(&self.commit))
-            .then(|| {
+        let mut signing_fn = (store.signer().can_sign() && sign_settings.should_sign(&self.commit))
+            .then(|| -> Box<SigningFn> {
                 let store = store.clone();
                 Box::new(move |data: &_| store.signer().sign(data, sign_settings.key.as_deref()))
-                    as SigningFn
             });
 
         // Commit backend doesn't use secure_sig for writing and enforces it with an
@@ -198,7 +197,9 @@ impl CommitBuilder<'_> {
         // if we're rewriting a signed commit
         self.commit.secure_sig = None;
 
-        let commit = self.mut_repo.write_commit(self.commit, signing_fn)?;
+        let commit = self
+            .mut_repo
+            .write_commit(self.commit, signing_fn.as_deref_mut())?;
         if let Some(rewrite_source_id) = rewrite_source_id {
             self.mut_repo
                 .record_rewritten_commit(rewrite_source_id, commit.id().clone())
diff --git a/lib/src/git_backend.rs b/lib/src/git_backend.rs
index 866482f69..e0126a93a 100644
--- a/lib/src/git_backend.rs
+++ b/lib/src/git_backend.rs
@@ -904,7 +904,7 @@ impl Backend for GitBackend {
     fn write_commit(
         &self,
         mut contents: Commit,
-        mut sign_with: Option<SigningFn>,
+        mut sign_with: Option<&mut SigningFn>,
     ) -> BackendResult<(CommitId, Commit)> {
         assert!(contents.secure_sig.is_none(), "commit.secure_sig was set");
 
@@ -1668,12 +1668,14 @@ mod tests {
             secure_sig: None,
         };
 
-        let signer = Box::new(|data: &_| {
+        let mut signer = |data: &_| {
             let hash: String = blake2b_hash(data).encode_hex();
             Ok(format!("test sig\n\n\nhash={hash}").into_bytes())
-        });
+        };
 
-        let (id, commit) = backend.write_commit(commit, Some(signer)).unwrap();
+        let (id, commit) = backend
+            .write_commit(commit, Some(&mut signer as &mut SigningFn))
+            .unwrap();
 
         let git_repo = backend.git_repo();
         let obj = git_repo.find_object(id.as_bytes()).unwrap();
diff --git a/lib/src/local_backend.rs b/lib/src/local_backend.rs
index c2283e26e..09574b592 100644
--- a/lib/src/local_backend.rs
+++ b/lib/src/local_backend.rs
@@ -267,7 +267,7 @@ impl Backend for LocalBackend {
     fn write_commit(
         &self,
         mut commit: Commit,
-        sign_with: Option<SigningFn>,
+        sign_with: Option<&mut SigningFn>,
     ) -> BackendResult<(CommitId, Commit)> {
         assert!(commit.secure_sig.is_none(), "commit.secure_sig was set");
 
@@ -279,7 +279,7 @@ impl Backend for LocalBackend {
         let temp_file = NamedTempFile::new_in(&self.path).map_err(to_other_err)?;
 
         let mut proto = commit_to_proto(&commit);
-        if let Some(mut sign) = sign_with {
+        if let Some(sign) = sign_with {
             let data = proto.encode_to_vec();
             let sig = sign(&data).map_err(to_other_err)?;
             proto.secure_sig = Some(sig.clone());
diff --git a/lib/src/repo.rs b/lib/src/repo.rs
index 2765766ff..135927b04 100644
--- a/lib/src/repo.rs
+++ b/lib/src/repo.rs
@@ -801,7 +801,7 @@ impl MutableRepo {
     pub fn write_commit(
         &mut self,
         commit: backend::Commit,
-        sign_with: Option<SigningFn>,
+        sign_with: Option<&mut SigningFn>,
     ) -> BackendResult<Commit> {
         let commit = self.store().write_commit(commit, sign_with)?;
         self.add_head(&commit);
diff --git a/lib/src/store.rs b/lib/src/store.rs
index 0ab0149ce..f4810bd36 100644
--- a/lib/src/store.rs
+++ b/lib/src/store.rs
@@ -138,7 +138,7 @@ impl Store {
     pub fn write_commit(
         self: &Arc<Self>,
         commit: backend::Commit,
-        sign_with: Option<SigningFn>,
+        sign_with: Option<&mut SigningFn>,
     ) -> BackendResult<Commit> {
         assert!(!commit.parents.is_empty());
 
diff --git a/lib/testutils/src/test_backend.rs b/lib/testutils/src/test_backend.rs
index a3d63aeae..59ab5037d 100644
--- a/lib/testutils/src/test_backend.rs
+++ b/lib/testutils/src/test_backend.rs
@@ -276,7 +276,7 @@ impl Backend for TestBackend {
     fn write_commit(
         &self,
         mut contents: Commit,
-        mut sign_with: Option<SigningFn>,
+        mut sign_with: Option<&mut SigningFn>,
     ) -> BackendResult<(CommitId, Commit)> {
         assert!(contents.secure_sig.is_none(), "commit.secure_sig was set");