diff --git a/library/proc_macro/src/bridge/client.rs b/library/proc_macro/src/bridge/client.rs
index 8f4a79b389f6..ddc9e0d4dee0 100644
--- a/library/proc_macro/src/bridge/client.rs
+++ b/library/proc_macro/src/bridge/client.rs
@@ -129,7 +129,7 @@ struct Bridge<'a> {
     cached_buffer: Buffer,
 
     /// Server-side function that the client uses to make requests.
-    dispatch: closure::Closure<'a, Buffer, Buffer>,
+    dispatch: closure::Closure<'a>,
 
     /// Provided globals for this macro expansion.
     globals: ExpnGlobals<Span>,
diff --git a/library/proc_macro/src/bridge/closure.rs b/library/proc_macro/src/bridge/closure.rs
index e5133907854b..88c4dd6630b1 100644
--- a/library/proc_macro/src/bridge/closure.rs
+++ b/library/proc_macro/src/bridge/closure.rs
@@ -1,10 +1,12 @@
-//! Closure type (equivalent to `&mut dyn FnMut(A) -> R`) that's `repr(C)`.
+//! Closure type (equivalent to `&mut dyn FnMut(Buffer) -> Buffer`) that's `repr(C)`.
 
 use std::marker::PhantomData;
 
+use super::Buffer;
+
 #[repr(C)]
-pub(super) struct Closure<'a, A, R> {
-    call: unsafe extern "C" fn(*mut Env, A) -> R,
+pub(super) struct Closure<'a> {
+    call: extern "C" fn(*mut Env, Buffer) -> Buffer,
     env: *mut Env,
     // Prevent Send and Sync impls.
     //
@@ -14,17 +16,17 @@ pub(super) struct Closure<'a, A, R> {
 
 struct Env;
 
-impl<'a, A, R, F: FnMut(A) -> R> From<&'a mut F> for Closure<'a, A, R> {
+impl<'a, F: FnMut(Buffer) -> Buffer> From<&'a mut F> for Closure<'a> {
     fn from(f: &'a mut F) -> Self {
-        unsafe extern "C" fn call<A, R, F: FnMut(A) -> R>(env: *mut Env, arg: A) -> R {
+        extern "C" fn call<F: FnMut(Buffer) -> Buffer>(env: *mut Env, arg: Buffer) -> Buffer {
             unsafe { (*(env as *mut _ as *mut F))(arg) }
         }
-        Closure { call: call::<A, R, F>, env: f as *mut _ as *mut Env, _marker: PhantomData }
+        Closure { call: call::<F>, env: f as *mut _ as *mut Env, _marker: PhantomData }
     }
 }
 
-impl<'a, A, R> Closure<'a, A, R> {
-    pub(super) fn call(&mut self, arg: A) -> R {
-        unsafe { (self.call)(self.env, arg) }
+impl<'a> Closure<'a> {
+    pub(super) fn call(&mut self, arg: Buffer) -> Buffer {
+        (self.call)(self.env, arg)
     }
 }
diff --git a/library/proc_macro/src/bridge/mod.rs b/library/proc_macro/src/bridge/mod.rs
index 244ab7d81b02..d9529b63e8e4 100644
--- a/library/proc_macro/src/bridge/mod.rs
+++ b/library/proc_macro/src/bridge/mod.rs
@@ -126,7 +126,7 @@ pub struct BridgeConfig<'a> {
     input: Buffer,
 
     /// Server-side function that the client uses to make requests.
-    dispatch: closure::Closure<'a, Buffer, Buffer>,
+    dispatch: closure::Closure<'a>,
 
     /// If 'true', always invoke the default panic hook
     force_show_panics: bool,