diff --git a/tokio/src/runtime/builder.rs b/tokio/src/runtime/builder.rs
index ae9f6d68e33..2622a61da24 100644
--- a/tokio/src/runtime/builder.rs
+++ b/tokio/src/runtime/builder.rs
@@ -2,6 +2,8 @@ use crate::runtime::handle::Handle;
 use crate::runtime::shell::Shell;
 use crate::runtime::{blocking, io, time, Callback, Runtime, Spawner};
 
+#[cfg(loom)]
+use crate::loom::sync::Arc;
 use std::fmt;
 #[cfg(not(loom))]
 use std::sync::Arc;
@@ -65,7 +67,7 @@ pub struct Builder {
     pub(super) before_stop: Option<Callback>,
 }
 
-pub(crate) type ThreadNameFn = Arc<dyn Fn() -> String + Send + Sync + 'static>;
+pub(crate) type ThreadNameFn = Arc<Box<dyn Fn() -> String + Send + Sync + 'static>>;
 
 #[derive(Debug, Clone, Copy)]
 enum Kind {
@@ -96,7 +98,7 @@ impl Builder {
             num_threads: crate::loom::sys::num_cpus(),
 
             // Default thread name
-            thread_name: Arc::new(|| "tokio-runtime-worker".into()),
+            thread_name: Arc::new(Box::new(|| "tokio-runtime-worker".into())),
 
             // Do not set a stack size by default
             thread_stack_size: None,
@@ -171,7 +173,7 @@ impl Builder {
     /// ```
     pub fn thread_name(&mut self, val: impl Into<String>) -> &mut Self {
         let val = val.into();
-        self.thread_name = Arc::new(move || val.clone());
+        self.thread_name = Arc::new(Box::new(move || val.clone()));
         self
     }
 
@@ -199,7 +201,7 @@ impl Builder {
     where
         F: Fn() -> String + Send + Sync + 'static,
     {
-        self.thread_name = Arc::new(f);
+        self.thread_name = Arc::new(Box::new(f));
         self
     }