diff --git a/crates/proc-macro-srv-cli/src/main_loop.rs b/crates/proc-macro-srv-cli/src/main_loop.rs
index ba1fcd8e336a..569070766f1c 100644
--- a/crates/proc-macro-srv-cli/src/main_loop.rs
+++ b/crates/proc-macro-srv-cli/src/main_loop.rs
@@ -21,18 +21,16 @@ pub(crate) fn run() -> io::Result<()> {
         }
     }
 
-    let read_request =
-        |buf: &mut String| msg::Request::read(read_json, &mut io::stdin().lock(), buf);
-
+    let mut buf = String::new();
+    let mut read_request = || msg::Request::read(read_json, &mut io::stdin().lock(), &mut buf);
     let write_response = |msg: msg::Response| msg.write(write_json, &mut io::stdout().lock());
 
     let env = EnvSnapshot::default();
-    let mut srv = proc_macro_srv::ProcMacroSrv::new(&env);
-    let mut buf = String::new();
+    let srv = proc_macro_srv::ProcMacroSrv::new(&env);
 
     let mut span_mode = SpanMode::Id;
 
-    while let Some(req) = read_request(&mut buf)? {
+    while let Some(req) = read_request()? {
         let res = match req {
             msg::Request::ListMacros { dylib_path } => {
                 msg::Response::ListMacros(srv.list_macros(&dylib_path).map(|macros| {
diff --git a/crates/proc-macro-srv/src/lib.rs b/crates/proc-macro-srv/src/lib.rs
index 7ae75713ebfe..f28821b4afc5 100644
--- a/crates/proc-macro-srv/src/lib.rs
+++ b/crates/proc-macro-srv/src/lib.rs
@@ -35,6 +35,7 @@ use std::{
     ffi::OsString,
     fs,
     path::{Path, PathBuf},
+    sync::{Arc, Mutex, PoisonError},
     thread,
 };
 
@@ -53,7 +54,7 @@ pub enum ProcMacroKind {
 pub const RUSTC_VERSION_STRING: &str = env!("RUSTC_VERSION");
 
 pub struct ProcMacroSrv<'env> {
-    expanders: HashMap<Utf8PathBuf, dylib::Expander>,
+    expanders: Mutex<HashMap<Utf8PathBuf, Arc<dylib::Expander>>>,
     env: &'env EnvSnapshot,
 }
 
@@ -67,7 +68,7 @@ const EXPANDER_STACK_SIZE: usize = 8 * 1024 * 1024;
 
 impl ProcMacroSrv<'_> {
     pub fn expand<S: ProcMacroSrvSpan>(
-        &mut self,
+        &self,
         lib: impl AsRef<Utf8Path>,
         env: Vec<(String, String)>,
         current_dir: Option<impl AsRef<Path>>,
@@ -118,29 +119,37 @@ impl ProcMacroSrv<'_> {
     }
 
     pub fn list_macros(
-        &mut self,
+        &self,
         dylib_path: &Utf8Path,
     ) -> Result<Vec<(String, ProcMacroKind)>, String> {
         let expander = self.expander(dylib_path)?;
         Ok(expander.list_macros())
     }
 
-    fn expander(&mut self, path: &Utf8Path) -> Result<&dylib::Expander, String> {
+    fn expander(&self, path: &Utf8Path) -> Result<Arc<dylib::Expander>, String> {
         let expander = || {
-            dylib::Expander::new(path)
-                .map_err(|err| format!("Cannot create expander for {path}: {err}",))
+            let expander = dylib::Expander::new(path)
+                .map_err(|err| format!("Cannot create expander for {path}: {err}",));
+            expander.map(Arc::new)
         };
 
-        Ok(match self.expanders.entry(path.to_path_buf()) {
-            Entry::Vacant(v) => v.insert(expander()?),
-            Entry::Occupied(mut e) => {
-                let time = fs::metadata(path).and_then(|it| it.modified()).ok();
-                if Some(e.get().modified_time()) != time {
-                    e.insert(expander()?);
+        Ok(
+            match self
+                .expanders
+                .lock()
+                .unwrap_or_else(PoisonError::into_inner)
+                .entry(path.to_path_buf())
+            {
+                Entry::Vacant(v) => v.insert(expander()?).clone(),
+                Entry::Occupied(mut e) => {
+                    let time = fs::metadata(path).and_then(|it| it.modified()).ok();
+                    if Some(e.get().modified_time()) != time {
+                        e.insert(expander()?);
+                    }
+                    e.get().clone()
                 }
-                e.into_mut()
-            }
-        })
+            },
+        )
     }
 }
 
diff --git a/crates/proc-macro-srv/src/tests/utils.rs b/crates/proc-macro-srv/src/tests/utils.rs
index 4ce4544243ab..1b085520d565 100644
--- a/crates/proc-macro-srv/src/tests/utils.rs
+++ b/crates/proc-macro-srv/src/tests/utils.rs
@@ -107,7 +107,7 @@ fn assert_expand_impl(
 pub(crate) fn list() -> Vec<String> {
     let dylib_path = proc_macro_test_dylib_path();
     let env = EnvSnapshot::default();
-    let mut srv = ProcMacroSrv::new(&env);
+    let srv = ProcMacroSrv::new(&env);
     let res = srv.list_macros(&dylib_path).unwrap();
     res.into_iter().map(|(name, kind)| format!("{name} [{kind:?}]")).collect()
 }