diff --git a/library/proc_macro/src/bridge/buffer.rs b/library/proc_macro/src/bridge/buffer.rs
index fb05a02dc4571..5c2bb8a9ca0ac 100644
--- a/library/proc_macro/src/bridge/buffer.rs
+++ b/library/proc_macro/src/bridge/buffer.rs
@@ -5,38 +5,40 @@ use std::mem;
 use std::ops::{Deref, DerefMut};
 use std::slice;
 
+// `T` is always `u8` in practice. Attempts to remove `T` caused mild
+// performance regressions, surprisingly enough.
 #[repr(C)]
-pub struct Buffer {
-    data: *mut u8,
+pub struct Buffer<T: Copy> {
+    data: *mut T,
     len: usize,
     capacity: usize,
-    reserve: extern "C" fn(Buffer, usize) -> Buffer,
-    drop: extern "C" fn(Buffer),
+    reserve: extern "C" fn(Buffer<T>, usize) -> Buffer<T>,
+    drop: extern "C" fn(Buffer<T>),
 }
 
-unsafe impl Sync for Buffer {}
-unsafe impl Send for Buffer {}
+unsafe impl<T: Copy + Sync> Sync for Buffer<T> {}
+unsafe impl<T: Copy + Send> Send for Buffer<T> {}
 
-impl Default for Buffer {
+impl<T: Copy> Default for Buffer<T> {
     fn default() -> Self {
         Self::from(vec![])
     }
 }
 
-impl Deref for Buffer {
-    type Target = [u8];
-    fn deref(&self) -> &[u8] {
-        unsafe { slice::from_raw_parts(self.data as *const u8, self.len) }
+impl<T: Copy> Deref for Buffer<T> {
+    type Target = [T];
+    fn deref(&self) -> &[T] {
+        unsafe { slice::from_raw_parts(self.data as *const T, self.len) }
     }
 }
 
-impl DerefMut for Buffer {
-    fn deref_mut(&mut self) -> &mut [u8] {
+impl<T: Copy> DerefMut for Buffer<T> {
+    fn deref_mut(&mut self) -> &mut [T] {
         unsafe { slice::from_raw_parts_mut(self.data, self.len) }
     }
 }
 
-impl Buffer {
+impl<T: Copy> Buffer<T> {
     pub(super) fn new() -> Self {
         Self::default()
     }
@@ -53,7 +55,7 @@ impl Buffer {
     // because in the case of small arrays, codegen can be more efficient
     // (avoiding a memmove call). With extend_from_slice, LLVM at least
     // currently is not able to make that optimization.
-    pub(super) fn extend_from_array<const N: usize>(&mut self, xs: &[u8; N]) {
+    pub(super) fn extend_from_array<const N: usize>(&mut self, xs: &[T; N]) {
         if xs.len() > (self.capacity - self.len) {
             let b = self.take();
             *self = (b.reserve)(b, xs.len());
@@ -64,7 +66,7 @@ impl Buffer {
         }
     }
 
-    pub(super) fn extend_from_slice(&mut self, xs: &[u8]) {
+    pub(super) fn extend_from_slice(&mut self, xs: &[T]) {
         if xs.len() > (self.capacity - self.len) {
             let b = self.take();
             *self = (b.reserve)(b, xs.len());
@@ -75,7 +77,7 @@ impl Buffer {
         }
     }
 
-    pub(super) fn push(&mut self, v: u8) {
+    pub(super) fn push(&mut self, v: T) {
         // The code here is taken from Vec::push, and we know that reserve()
         // will panic if we're exceeding isize::MAX bytes and so there's no need
         // to check for overflow.
@@ -90,7 +92,7 @@ impl Buffer {
     }
 }
 
-impl Write for Buffer {
+impl Write for Buffer<u8> {
     fn write(&mut self, xs: &[u8]) -> io::Result<usize> {
         self.extend_from_slice(xs);
         Ok(xs.len())
@@ -106,21 +108,21 @@ impl Write for Buffer {
     }
 }
 
-impl Drop for Buffer {
+impl<T: Copy> Drop for Buffer<T> {
     fn drop(&mut self) {
         let b = self.take();
         (b.drop)(b);
     }
 }
 
-impl From<Vec<u8>> for Buffer {
-    fn from(mut v: Vec<u8>) -> Self {
+impl<T: Copy> From<Vec<T>> for Buffer<T> {
+    fn from(mut v: Vec<T>) -> Self {
         let (data, len, capacity) = (v.as_mut_ptr(), v.len(), v.capacity());
         mem::forget(v);
 
         // This utility function is nested in here because it can *only*
         // be safely called on `Buffer`s created by *this* `proc_macro`.
-        fn to_vec(b: Buffer) -> Vec<u8> {
+        fn to_vec<T: Copy>(b: Buffer<T>) -> Vec<T> {
             unsafe {
                 let Buffer { data, len, capacity, .. } = b;
                 mem::forget(b);
@@ -128,13 +130,13 @@ impl From<Vec<u8>> for Buffer {
             }
         }
 
-        extern "C" fn reserve(b: Buffer, additional: usize) -> Buffer {
+        extern "C" fn reserve<T: Copy>(b: Buffer<T>, additional: usize) -> Buffer<T> {
             let mut v = to_vec(b);
             v.reserve(additional);
             Buffer::from(v)
         }
 
-        extern "C" fn drop(b: Buffer) {
+        extern "C" fn drop<T: Copy>(b: Buffer<T>) {
             mem::drop(to_vec(b));
         }
 
diff --git a/library/proc_macro/src/bridge/client.rs b/library/proc_macro/src/bridge/client.rs
index c38457ac6712d..9721a09fa7781 100644
--- a/library/proc_macro/src/bridge/client.rs
+++ b/library/proc_macro/src/bridge/client.rs
@@ -374,7 +374,7 @@ pub struct Client<I, O> {
     // a wrapper `fn` pointer, once `const fn` can reference `static`s.
     pub(super) get_handle_counters: extern "C" fn() -> &'static HandleCounters,
 
-    pub(super) run: extern "C" fn(Bridge<'_>) -> Buffer,
+    pub(super) run: extern "C" fn(Bridge<'_>) -> Buffer<u8>,
 
     pub(super) _marker: PhantomData<fn(I) -> O>,
 }
@@ -392,7 +392,7 @@ impl<I, O> Clone for Client<I, O> {
 fn run_client<A: for<'a, 's> DecodeMut<'a, 's, ()>, R: Encode<()>>(
     mut bridge: Bridge<'_>,
     f: impl FnOnce(A) -> R,
-) -> Buffer {
+) -> Buffer<u8> {
     // The initial `cached_buffer` contains the input.
     let mut buf = bridge.cached_buffer.take();
 
diff --git a/library/proc_macro/src/bridge/mod.rs b/library/proc_macro/src/bridge/mod.rs
index 4d3e89ba09356..3d42b834e50a0 100644
--- a/library/proc_macro/src/bridge/mod.rs
+++ b/library/proc_macro/src/bridge/mod.rs
@@ -226,10 +226,10 @@ use rpc::{Decode, DecodeMut, Encode, Reader, Writer};
 pub struct Bridge<'a> {
     /// Reusable buffer (only `clear`-ed, never shrunk), primarily
     /// used for making requests, but also for passing input to client.
-    cached_buffer: Buffer,
+    cached_buffer: Buffer<u8>,
 
     /// Server-side function that the client uses to make requests.
-    dispatch: closure::Closure<'a, Buffer, Buffer>,
+    dispatch: closure::Closure<'a, Buffer<u8>, Buffer<u8>>,
 
     /// If 'true', always invoke the default panic hook
     force_show_panics: bool,
diff --git a/library/proc_macro/src/bridge/rpc.rs b/library/proc_macro/src/bridge/rpc.rs
index f79e016400fb1..d50564d01a5d2 100644
--- a/library/proc_macro/src/bridge/rpc.rs
+++ b/library/proc_macro/src/bridge/rpc.rs
@@ -7,7 +7,7 @@ use std::num::NonZeroU32;
 use std::ops::Bound;
 use std::str;
 
-pub(super) type Writer = super::buffer::Buffer;
+pub(super) type Writer = super::buffer::Buffer<u8>;
 
 pub(super) trait Encode<S>: Sized {
     fn encode(self, w: &mut Writer, s: &mut S);
diff --git a/library/proc_macro/src/bridge/server.rs b/library/proc_macro/src/bridge/server.rs
index cbddf39da44d2..059bf27d96eb8 100644
--- a/library/proc_macro/src/bridge/server.rs
+++ b/library/proc_macro/src/bridge/server.rs
@@ -78,12 +78,12 @@ macro_rules! define_dispatcher_impl {
         pub trait DispatcherTrait {
             // HACK(eddyb) these are here to allow `Self::$name` to work below.
             $(type $name;)*
-            fn dispatch(&mut self, buf: Buffer) -> Buffer;
+            fn dispatch(&mut self, buf: Buffer<u8>) -> Buffer<u8>;
         }
 
         impl<S: Server> DispatcherTrait for Dispatcher<MarkedTypes<S>> {
             $(type $name = <MarkedTypes<S> as Types>::$name;)*
-            fn dispatch(&mut self, mut buf: Buffer) -> Buffer {
+            fn dispatch(&mut self, mut buf: Buffer<u8>) -> Buffer<u8> {
                 let Dispatcher { handle_store, server } = self;
 
                 let mut reader = &buf[..];
@@ -121,10 +121,10 @@ pub trait ExecutionStrategy {
     fn run_bridge_and_client(
         &self,
         dispatcher: &mut impl DispatcherTrait,
-        input: Buffer,
-        run_client: extern "C" fn(Bridge<'_>) -> Buffer,
+        input: Buffer<u8>,
+        run_client: extern "C" fn(Bridge<'_>) -> Buffer<u8>,
         force_show_panics: bool,
-    ) -> Buffer;
+    ) -> Buffer<u8>;
 }
 
 pub struct SameThread;
@@ -133,10 +133,10 @@ impl ExecutionStrategy for SameThread {
     fn run_bridge_and_client(
         &self,
         dispatcher: &mut impl DispatcherTrait,
-        input: Buffer,
-        run_client: extern "C" fn(Bridge<'_>) -> Buffer,
+        input: Buffer<u8>,
+        run_client: extern "C" fn(Bridge<'_>) -> Buffer<u8>,
         force_show_panics: bool,
-    ) -> Buffer {
+    ) -> Buffer<u8> {
         let mut dispatch = |buf| dispatcher.dispatch(buf);
 
         run_client(Bridge {
@@ -157,10 +157,10 @@ impl ExecutionStrategy for CrossThread1 {
     fn run_bridge_and_client(
         &self,
         dispatcher: &mut impl DispatcherTrait,
-        input: Buffer,
-        run_client: extern "C" fn(Bridge<'_>) -> Buffer,
+        input: Buffer<u8>,
+        run_client: extern "C" fn(Bridge<'_>) -> Buffer<u8>,
         force_show_panics: bool,
-    ) -> Buffer {
+    ) -> Buffer<u8> {
         use std::sync::mpsc::channel;
 
         let (req_tx, req_rx) = channel();
@@ -194,10 +194,10 @@ impl ExecutionStrategy for CrossThread2 {
     fn run_bridge_and_client(
         &self,
         dispatcher: &mut impl DispatcherTrait,
-        input: Buffer,
-        run_client: extern "C" fn(Bridge<'_>) -> Buffer,
+        input: Buffer<u8>,
+        run_client: extern "C" fn(Bridge<'_>) -> Buffer<u8>,
         force_show_panics: bool,
-    ) -> Buffer {
+    ) -> Buffer<u8> {
         use std::sync::{Arc, Mutex};
 
         enum State<T> {
@@ -260,7 +260,7 @@ fn run_server<
     handle_counters: &'static client::HandleCounters,
     server: S,
     input: I,
-    run_client: extern "C" fn(Bridge<'_>) -> Buffer,
+    run_client: extern "C" fn(Bridge<'_>) -> Buffer<u8>,
     force_show_panics: bool,
 ) -> Result<O, PanicMessage> {
     let mut dispatcher =