Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

12 files changed, 2540 insertions, 0 deletions

diff --git a/library/proc_macro/src/bridge/arena.rs b/library/proc_macro/src/bridge/arena.rs
new file mode 100644
index 000000000..fa72d2816
--- /dev/null
+++ b/library/proc_macro/src/bridge/arena.rs
@@ -0,0 +1,113 @@
+//! A minimal arena allocator inspired by `rustc_arena::DroplessArena`.
+//!
+//! This is unfortunately a minimal re-implementation rather than a dependency
+//! as it is difficult to depend on crates from within `proc_macro`, due to it
+//! being built at the same time as `std`.
+
+use std::cell::{Cell, RefCell};
+use std::cmp;
+use std::mem::MaybeUninit;
+use std::ops::Range;
+use std::ptr;
+use std::slice;
+use std::str;
+
+// The arenas start with PAGE-sized chunks, and then each new chunk is twice as
+// big as its predecessor, up until we reach HUGE_PAGE-sized chunks, whereupon
+// we stop growing. This scales well, from arenas that are barely used up to
+// arenas that are used for 100s of MiBs. Note also that the chosen sizes match
+// the usual sizes of pages and huge pages on Linux.
+const PAGE: usize = 4096;
+const HUGE_PAGE: usize = 2 * 1024 * 1024;
+
+/// A minimal arena allocator inspired by `rustc_arena::DroplessArena`.
+///
+/// This is unfortunately a complete re-implementation rather than a dependency
+/// as it is difficult to depend on crates from within `proc_macro`, due to it
+/// being built at the same time as `std`.
+///
+/// This arena doesn't have support for allocating anything other than byte
+/// slices, as that is all that is necessary.
+pub(crate) struct Arena {
+    start: Cell<*mut MaybeUninit<u8>>,
+    end: Cell<*mut MaybeUninit<u8>>,
+    chunks: RefCell<Vec<Box<[MaybeUninit<u8>]>>>,
+}
+
+impl Arena {
+    pub(crate) fn new() -> Self {
+        Arena {
+            start: Cell::new(ptr::null_mut()),
+            end: Cell::new(ptr::null_mut()),
+            chunks: RefCell::new(Vec::new()),
+        }
+    }
+
+    /// Add a new chunk with at least `additional` free bytes.
+    #[inline(never)]
+    #[cold]
+    fn grow(&self, additional: usize) {
+        let mut chunks = self.chunks.borrow_mut();
+        let mut new_cap;
+        if let Some(last_chunk) = chunks.last_mut() {
+            // If the previous chunk's len is less than HUGE_PAGE
+            // bytes, then this chunk will be least double the previous
+            // chunk's size.
+            new_cap = last_chunk.len().min(HUGE_PAGE / 2);
+            new_cap *= 2;
+        } else {
+            new_cap = PAGE;
+        }
+        // Also ensure that this chunk can fit `additional`.
+        new_cap = cmp::max(additional, new_cap);
+
+        let mut chunk = Box::new_uninit_slice(new_cap);
+        let Range { start, end } = chunk.as_mut_ptr_range();
+        self.start.set(start);
+        self.end.set(end);
+        chunks.push(chunk);
+    }
+
+    /// Allocates a byte slice with specified size from the current memory
+    /// chunk. Returns `None` if there is no free space left to satisfy the
+    /// request.
+    fn alloc_raw_without_grow(&self, bytes: usize) -> Option<&mut [MaybeUninit<u8>]> {
+        let start = self.start.get().addr();
+        let old_end = self.end.get();
+        let end = old_end.addr();
+
+        let new_end = end.checked_sub(bytes)?;
+        if start <= new_end {
+            let new_end = old_end.with_addr(new_end);
+            self.end.set(new_end);
+            // SAFETY: `bytes` bytes starting at `new_end` were just reserved.
+            Some(unsafe { slice::from_raw_parts_mut(new_end, bytes) })
+        } else {
+            None
+        }
+    }
+
+    fn alloc_raw(&self, bytes: usize) -> &mut [MaybeUninit<u8>] {
+        if bytes == 0 {
+            return &mut [];
+        }
+
+        loop {
+            if let Some(a) = self.alloc_raw_without_grow(bytes) {
+                break a;
+            }
+            // No free space left. Allocate a new chunk to satisfy the request.
+            // On failure the grow will panic or abort.
+            self.grow(bytes);
+        }
+    }
+
+    pub(crate) fn alloc_str<'a>(&'a self, string: &str) -> &'a mut str {
+        let alloc = self.alloc_raw(string.len());
+        let bytes = MaybeUninit::write_slice(alloc, string.as_bytes());
+
+        // SAFETY: we convert from `&str` to `&[u8]`, clone it into the arena,
+        // and immediately convert the clone back to `&str`.
+        unsafe { str::from_utf8_unchecked_mut(bytes) }
+    }
+}
diff --git a/library/proc_macro/src/bridge/buffer.rs b/library/proc_macro/src/bridge/buffer.rs
new file mode 100644
index 000000000..48030f8d8
--- /dev/null
+++ b/library/proc_macro/src/bridge/buffer.rs
@@ -0,0 +1,156 @@
+//! Buffer management for same-process client<->server communication.
+
+use std::io::{self, Write};
+use std::mem;
+use std::ops::{Deref, DerefMut};
+use std::slice;
+
+#[repr(C)]
+pub struct Buffer {
+    data: *mut u8,
+    len: usize,
+    capacity: usize,
+    reserve: extern "C" fn(Buffer, usize) -> Buffer,
+    drop: extern "C" fn(Buffer),
+}
+
+unsafe impl Sync for Buffer {}
+unsafe impl Send for Buffer {}
+
+impl Default for Buffer {
+    #[inline]
+    fn default() -> Self {
+        Self::from(vec![])
+    }
+}
+
+impl Deref for Buffer {
+    type Target = [u8];
+    #[inline]
+    fn deref(&self) -> &[u8] {
+        unsafe { slice::from_raw_parts(self.data as *const u8, self.len) }
+    }
+}
+
+impl DerefMut for Buffer {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut [u8] {
+        unsafe { slice::from_raw_parts_mut(self.data, self.len) }
+    }
+}
+
+impl Buffer {
+    #[inline]
+    pub(super) fn new() -> Self {
+        Self::default()
+    }
+
+    #[inline]
+    pub(super) fn clear(&mut self) {
+        self.len = 0;
+    }
+
+    #[inline]
+    pub(super) fn take(&mut self) -> Self {
+        mem::take(self)
+    }
+
+    // We have the array method separate from extending from a slice. This is
+    // 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.
+    #[inline]
+    pub(super) fn extend_from_array<const N: usize>(&mut self, xs: &[u8; N]) {
+        if xs.len() > (self.capacity - self.len) {
+            let b = self.take();
+            *self = (b.reserve)(b, xs.len());
+        }
+        unsafe {
+            xs.as_ptr().copy_to_nonoverlapping(self.data.add(self.len), xs.len());
+            self.len += xs.len();
+        }
+    }
+
+    #[inline]
+    pub(super) fn extend_from_slice(&mut self, xs: &[u8]) {
+        if xs.len() > (self.capacity - self.len) {
+            let b = self.take();
+            *self = (b.reserve)(b, xs.len());
+        }
+        unsafe {
+            xs.as_ptr().copy_to_nonoverlapping(self.data.add(self.len), xs.len());
+            self.len += xs.len();
+        }
+    }
+
+    #[inline]
+    pub(super) fn push(&mut self, v: u8) {
+        // 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.
+        if self.len == self.capacity {
+            let b = self.take();
+            *self = (b.reserve)(b, 1);
+        }
+        unsafe {
+            *self.data.add(self.len) = v;
+            self.len += 1;
+        }
+    }
+}
+
+impl Write for Buffer {
+    #[inline]
+    fn write(&mut self, xs: &[u8]) -> io::Result<usize> {
+        self.extend_from_slice(xs);
+        Ok(xs.len())
+    }
+
+    #[inline]
+    fn write_all(&mut self, xs: &[u8]) -> io::Result<()> {
+        self.extend_from_slice(xs);
+        Ok(())
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+impl Drop for Buffer {
+    #[inline]
+    fn drop(&mut self) {
+        let b = self.take();
+        (b.drop)(b);
+    }
+}
+
+impl From<Vec<u8>> for Buffer {
+    fn from(mut v: Vec<u8>) -> 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> {
+            unsafe {
+                let Buffer { data, len, capacity, .. } = b;
+                mem::forget(b);
+                Vec::from_raw_parts(data, len, capacity)
+            }
+        }
+
+        extern "C" fn reserve(b: Buffer, additional: usize) -> Buffer {
+            let mut v = to_vec(b);
+            v.reserve(additional);
+            Buffer::from(v)
+        }
+
+        extern "C" fn drop(b: Buffer) {
+            mem::drop(to_vec(b));
+        }
+
+        Buffer { data, len, capacity, reserve, drop }
+    }
+}
diff --git a/library/proc_macro/src/bridge/client.rs b/library/proc_macro/src/bridge/client.rs
new file mode 100644
index 000000000..1516f084a
--- /dev/null
+++ b/library/proc_macro/src/bridge/client.rs
@@ -0,0 +1,510 @@
+//! Client-side types.
+
+use super::*;
+
+use std::marker::PhantomData;
+
+macro_rules! define_handles {
+    (
+        'owned: $($oty:ident,)*
+        'interned: $($ity:ident,)*
+    ) => {
+        #[repr(C)]
+        #[allow(non_snake_case)]
+        pub struct HandleCounters {
+            $($oty: AtomicUsize,)*
+            $($ity: AtomicUsize,)*
+        }
+
+        impl HandleCounters {
+            // FIXME(eddyb) use a reference to the `static COUNTERS`, instead of
+            // a wrapper `fn` pointer, once `const fn` can reference `static`s.
+            extern "C" fn get() -> &'static Self {
+                static COUNTERS: HandleCounters = HandleCounters {
+                    $($oty: AtomicUsize::new(1),)*
+                    $($ity: AtomicUsize::new(1),)*
+                };
+                &COUNTERS
+            }
+        }
+
+        // FIXME(eddyb) generate the definition of `HandleStore` in `server.rs`.
+        #[allow(non_snake_case)]
+        pub(super) struct HandleStore<S: server::Types> {
+            $($oty: handle::OwnedStore<S::$oty>,)*
+            $($ity: handle::InternedStore<S::$ity>,)*
+        }
+
+        impl<S: server::Types> HandleStore<S> {
+            pub(super) fn new(handle_counters: &'static HandleCounters) -> Self {
+                HandleStore {
+                    $($oty: handle::OwnedStore::new(&handle_counters.$oty),)*
+                    $($ity: handle::InternedStore::new(&handle_counters.$ity),)*
+                }
+            }
+        }
+
+        $(
+            pub(crate) struct $oty {
+                handle: handle::Handle,
+                // Prevent Send and Sync impls. `!Send`/`!Sync` is the usual
+                // way of doing this, but that requires unstable features.
+                // rust-analyzer uses this code and avoids unstable features.
+                _marker: PhantomData<*mut ()>,
+            }
+
+            // Forward `Drop::drop` to the inherent `drop` method.
+            impl Drop for $oty {
+                fn drop(&mut self) {
+                    $oty {
+                        handle: self.handle,
+                        _marker: PhantomData,
+                    }.drop();
+                }
+            }
+
+            impl<S> Encode<S> for $oty {
+                fn encode(self, w: &mut Writer, s: &mut S) {
+                    let handle = self.handle;
+                    mem::forget(self);
+                    handle.encode(w, s);
+                }
+            }
+
+            impl<S: server::Types> DecodeMut<'_, '_, HandleStore<server::MarkedTypes<S>>>
+                for Marked<S::$oty, $oty>
+            {
+                fn decode(r: &mut Reader<'_>, s: &mut HandleStore<server::MarkedTypes<S>>) -> Self {
+                    s.$oty.take(handle::Handle::decode(r, &mut ()))
+                }
+            }
+
+            impl<S> Encode<S> for &$oty {
+                fn encode(self, w: &mut Writer, s: &mut S) {
+                    self.handle.encode(w, s);
+                }
+            }
+
+            impl<'s, S: server::Types> Decode<'_, 's, HandleStore<server::MarkedTypes<S>>>
+                for &'s Marked<S::$oty, $oty>
+            {
+                fn decode(r: &mut Reader<'_>, s: &'s HandleStore<server::MarkedTypes<S>>) -> Self {
+                    &s.$oty[handle::Handle::decode(r, &mut ())]
+                }
+            }
+
+            impl<S> Encode<S> for &mut $oty {
+                fn encode(self, w: &mut Writer, s: &mut S) {
+                    self.handle.encode(w, s);
+                }
+            }
+
+            impl<'s, S: server::Types> DecodeMut<'_, 's, HandleStore<server::MarkedTypes<S>>>
+                for &'s mut Marked<S::$oty, $oty>
+            {
+                fn decode(
+                    r: &mut Reader<'_>,
+                    s: &'s mut HandleStore<server::MarkedTypes<S>>
+                ) -> Self {
+                    &mut s.$oty[handle::Handle::decode(r, &mut ())]
+                }
+            }
+
+            impl<S: server::Types> Encode<HandleStore<server::MarkedTypes<S>>>
+                for Marked<S::$oty, $oty>
+            {
+                fn encode(self, w: &mut Writer, s: &mut HandleStore<server::MarkedTypes<S>>) {
+                    s.$oty.alloc(self).encode(w, s);
+                }
+            }
+
+            impl<S> DecodeMut<'_, '_, S> for $oty {
+                fn decode(r: &mut Reader<'_>, s: &mut S) -> Self {
+                    $oty {
+                        handle: handle::Handle::decode(r, s),
+                        _marker: PhantomData,
+                    }
+                }
+            }
+        )*
+
+        $(
+            #[derive(Copy, Clone, PartialEq, Eq, Hash)]
+            pub(crate) struct $ity {
+                handle: handle::Handle,
+                // Prevent Send and Sync impls. `!Send`/`!Sync` is the usual
+                // way of doing this, but that requires unstable features.
+                // rust-analyzer uses this code and avoids unstable features.
+                _marker: PhantomData<*mut ()>,
+            }
+
+            impl<S> Encode<S> for $ity {
+                fn encode(self, w: &mut Writer, s: &mut S) {
+                    self.handle.encode(w, s);
+                }
+            }
+
+            impl<S: server::Types> DecodeMut<'_, '_, HandleStore<server::MarkedTypes<S>>>
+                for Marked<S::$ity, $ity>
+            {
+                fn decode(r: &mut Reader<'_>, s: &mut HandleStore<server::MarkedTypes<S>>) -> Self {
+                    s.$ity.copy(handle::Handle::decode(r, &mut ()))
+                }
+            }
+
+            impl<S: server::Types> Encode<HandleStore<server::MarkedTypes<S>>>
+                for Marked<S::$ity, $ity>
+            {
+                fn encode(self, w: &mut Writer, s: &mut HandleStore<server::MarkedTypes<S>>) {
+                    s.$ity.alloc(self).encode(w, s);
+                }
+            }
+
+            impl<S> DecodeMut<'_, '_, S> for $ity {
+                fn decode(r: &mut Reader<'_>, s: &mut S) -> Self {
+                    $ity {
+                        handle: handle::Handle::decode(r, s),
+                        _marker: PhantomData,
+                    }
+                }
+            }
+        )*
+    }
+}
+define_handles! {
+    'owned:
+    FreeFunctions,
+    TokenStream,
+    SourceFile,
+    MultiSpan,
+    Diagnostic,
+
+    'interned:
+    Span,
+}
+
+// FIXME(eddyb) generate these impls by pattern-matching on the
+// names of methods - also could use the presence of `fn drop`
+// to distinguish between 'owned and 'interned, above.
+// Alternatively, special "modes" could be listed of types in with_api
+// instead of pattern matching on methods, here and in server decl.
+
+impl Clone for TokenStream {
+    fn clone(&self) -> Self {
+        self.clone()
+    }
+}
+
+impl Clone for SourceFile {
+    fn clone(&self) -> Self {
+        self.clone()
+    }
+}
+
+impl Span {
+    pub(crate) fn def_site() -> Span {
+        Bridge::with(|bridge| bridge.globals.def_site)
+    }
+
+    pub(crate) fn call_site() -> Span {
+        Bridge::with(|bridge| bridge.globals.call_site)
+    }
+
+    pub(crate) fn mixed_site() -> Span {
+        Bridge::with(|bridge| bridge.globals.mixed_site)
+    }
+}
+
+impl fmt::Debug for Span {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&self.debug())
+    }
+}
+
+pub(crate) use super::symbol::Symbol;
+
+macro_rules! define_client_side {
+    ($($name:ident {
+        $(fn $method:ident($($arg:ident: $arg_ty:ty),* $(,)?) $(-> $ret_ty:ty)*;)*
+    }),* $(,)?) => {
+        $(impl $name {
+            $(pub(crate) fn $method($($arg: $arg_ty),*) $(-> $ret_ty)* {
+                Bridge::with(|bridge| {
+                    let mut buf = bridge.cached_buffer.take();
+
+                    buf.clear();
+                    api_tags::Method::$name(api_tags::$name::$method).encode(&mut buf, &mut ());
+                    reverse_encode!(buf; $($arg),*);
+
+                    buf = bridge.dispatch.call(buf);
+
+                    let r = Result::<_, PanicMessage>::decode(&mut &buf[..], &mut ());
+
+                    bridge.cached_buffer = buf;
+
+                    r.unwrap_or_else(|e| panic::resume_unwind(e.into()))
+                })
+            })*
+        })*
+    }
+}
+with_api!(self, self, define_client_side);
+
+struct Bridge<'a> {
+    /// Reusable buffer (only `clear`-ed, never shrunk), primarily
+    /// used for making requests.
+    cached_buffer: Buffer,
+
+    /// Server-side function that the client uses to make requests.
+    dispatch: closure::Closure<'a, Buffer, Buffer>,
+
+    /// Provided globals for this macro expansion.
+    globals: ExpnGlobals<Span>,
+}
+
+impl<'a> !Send for Bridge<'a> {}
+impl<'a> !Sync for Bridge<'a> {}
+
+enum BridgeState<'a> {
+    /// No server is currently connected to this client.
+    NotConnected,
+
+    /// A server is connected and available for requests.
+    Connected(Bridge<'a>),
+
+    /// Access to the bridge is being exclusively acquired
+    /// (e.g., during `BridgeState::with`).
+    InUse,
+}
+
+enum BridgeStateL {}
+
+impl<'a> scoped_cell::ApplyL<'a> for BridgeStateL {
+    type Out = BridgeState<'a>;
+}
+
+thread_local! {
+    static BRIDGE_STATE: scoped_cell::ScopedCell<BridgeStateL> =
+        scoped_cell::ScopedCell::new(BridgeState::NotConnected);
+}
+
+impl BridgeState<'_> {
+    /// Take exclusive control of the thread-local
+    /// `BridgeState`, and pass it to `f`, mutably.
+    /// The state will be restored after `f` exits, even
+    /// by panic, including modifications made to it by `f`.
+    ///
+    /// N.B., while `f` is running, the thread-local state
+    /// is `BridgeState::InUse`.
+    fn with<R>(f: impl FnOnce(&mut BridgeState<'_>) -> R) -> R {
+        BRIDGE_STATE.with(|state| {
+            state.replace(BridgeState::InUse, |mut state| {
+                // FIXME(#52812) pass `f` directly to `replace` when `RefMutL` is gone
+                f(&mut *state)
+            })
+        })
+    }
+}
+
+impl Bridge<'_> {
+    fn with<R>(f: impl FnOnce(&mut Bridge<'_>) -> R) -> R {
+        BridgeState::with(|state| match state {
+            BridgeState::NotConnected => {
+                panic!("procedural macro API is used outside of a procedural macro");
+            }
+            BridgeState::InUse => {
+                panic!("procedural macro API is used while it's already in use");
+            }
+            BridgeState::Connected(bridge) => f(bridge),
+        })
+    }
+}
+
+pub(crate) fn is_available() -> bool {
+    BridgeState::with(|state| match state {
+        BridgeState::Connected(_) | BridgeState::InUse => true,
+        BridgeState::NotConnected => false,
+    })
+}
+
+/// A client-side RPC entry-point, which may be using a different `proc_macro`
+/// from the one used by the server, but can be invoked compatibly.
+///
+/// Note that the (phantom) `I` ("input") and `O` ("output") type parameters
+/// decorate the `Client<I, O>` with the RPC "interface" of the entry-point, but
+/// do not themselves participate in ABI, at all, only facilitate type-checking.
+///
+/// E.g. `Client<TokenStream, TokenStream>` is the common proc macro interface,
+/// used for `#[proc_macro] fn foo(input: TokenStream) -> TokenStream`,
+/// indicating that the RPC input and output will be serialized token streams,
+/// and forcing the use of APIs that take/return `S::TokenStream`, server-side.
+#[repr(C)]
+pub struct Client<I, O> {
+    // FIXME(eddyb) use a reference to the `static COUNTERS`, instead of
+    // 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(BridgeConfig<'_>) -> Buffer,
+
+    pub(super) _marker: PhantomData<fn(I) -> O>,
+}
+
+impl<I, O> Copy for Client<I, O> {}
+impl<I, O> Clone for Client<I, O> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+fn maybe_install_panic_hook(force_show_panics: bool) {
+    // Hide the default panic output within `proc_macro` expansions.
+    // NB. the server can't do this because it may use a different libstd.
+    static HIDE_PANICS_DURING_EXPANSION: Once = Once::new();
+    HIDE_PANICS_DURING_EXPANSION.call_once(|| {
+        let prev = panic::take_hook();
+        panic::set_hook(Box::new(move |info| {
+            let show = BridgeState::with(|state| match state {
+                BridgeState::NotConnected => true,
+                BridgeState::Connected(_) | BridgeState::InUse => force_show_panics,
+            });
+            if show {
+                prev(info)
+            }
+        }));
+    });
+}
+
+/// Client-side helper for handling client panics, entering the bridge,
+/// deserializing input and serializing output.
+// FIXME(eddyb) maybe replace `Bridge::enter` with this?
+fn run_client<A: for<'a, 's> DecodeMut<'a, 's, ()>, R: Encode<()>>(
+    config: BridgeConfig<'_>,
+    f: impl FnOnce(A) -> R,
+) -> Buffer {
+    let BridgeConfig { input: mut buf, dispatch, force_show_panics, .. } = config;
+
+    panic::catch_unwind(panic::AssertUnwindSafe(|| {
+        maybe_install_panic_hook(force_show_panics);
+
+        // Make sure the symbol store is empty before decoding inputs.
+        Symbol::invalidate_all();
+
+        let reader = &mut &buf[..];
+        let (globals, input) = <(ExpnGlobals<Span>, A)>::decode(reader, &mut ());
+
+        // Put the buffer we used for input back in the `Bridge` for requests.
+        let new_state =
+            BridgeState::Connected(Bridge { cached_buffer: buf.take(), dispatch, globals });
+
+        BRIDGE_STATE.with(|state| {
+            state.set(new_state, || {
+                let output = f(input);
+
+                // Take the `cached_buffer` back out, for the output value.
+                buf = Bridge::with(|bridge| bridge.cached_buffer.take());
+
+                // HACK(eddyb) Separate encoding a success value (`Ok(output)`)
+                // from encoding a panic (`Err(e: PanicMessage)`) to avoid
+                // having handles outside the `bridge.enter(|| ...)` scope, and
+                // to catch panics that could happen while encoding the success.
+                //
+                // Note that panics should be impossible beyond this point, but
+                // this is defensively trying to avoid any accidental panicking
+                // reaching the `extern "C"` (which should `abort` but might not
+                // at the moment, so this is also potentially preventing UB).
+                buf.clear();
+                Ok::<_, ()>(output).encode(&mut buf, &mut ());
+            })
+        })
+    }))
+    .map_err(PanicMessage::from)
+    .unwrap_or_else(|e| {
+        buf.clear();
+        Err::<(), _>(e).encode(&mut buf, &mut ());
+    });
+
+    // Now that a response has been serialized, invalidate all symbols
+    // registered with the interner.
+    Symbol::invalidate_all();
+    buf
+}
+
+impl Client<crate::TokenStream, crate::TokenStream> {
+    pub const fn expand1(f: impl Fn(crate::TokenStream) -> crate::TokenStream + Copy) -> Self {
+        Client {
+            get_handle_counters: HandleCounters::get,
+            run: super::selfless_reify::reify_to_extern_c_fn_hrt_bridge(move |bridge| {
+                run_client(bridge, |input| f(crate::TokenStream(Some(input))).0)
+            }),
+            _marker: PhantomData,
+        }
+    }
+}
+
+impl Client<(crate::TokenStream, crate::TokenStream), crate::TokenStream> {
+    pub const fn expand2(
+        f: impl Fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream + Copy,
+    ) -> Self {
+        Client {
+            get_handle_counters: HandleCounters::get,
+            run: super::selfless_reify::reify_to_extern_c_fn_hrt_bridge(move |bridge| {
+                run_client(bridge, |(input, input2)| {
+                    f(crate::TokenStream(Some(input)), crate::TokenStream(Some(input2))).0
+                })
+            }),
+            _marker: PhantomData,
+        }
+    }
+}
+
+#[repr(C)]
+#[derive(Copy, Clone)]
+pub enum ProcMacro {
+    CustomDerive {
+        trait_name: &'static str,
+        attributes: &'static [&'static str],
+        client: Client<crate::TokenStream, crate::TokenStream>,
+    },
+
+    Attr {
+        name: &'static str,
+        client: Client<(crate::TokenStream, crate::TokenStream), crate::TokenStream>,
+    },
+
+    Bang {
+        name: &'static str,
+        client: Client<crate::TokenStream, crate::TokenStream>,
+    },
+}
+
+impl ProcMacro {
+    pub fn name(&self) -> &'static str {
+        match self {
+            ProcMacro::CustomDerive { trait_name, .. } => trait_name,
+            ProcMacro::Attr { name, .. } => name,
+            ProcMacro::Bang { name, .. } => name,
+        }
+    }
+
+    pub const fn custom_derive(
+        trait_name: &'static str,
+        attributes: &'static [&'static str],
+        expand: impl Fn(crate::TokenStream) -> crate::TokenStream + Copy,
+    ) -> Self {
+        ProcMacro::CustomDerive { trait_name, attributes, client: Client::expand1(expand) }
+    }
+
+    pub const fn attr(
+        name: &'static str,
+        expand: impl Fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream + Copy,
+    ) -> Self {
+        ProcMacro::Attr { name, client: Client::expand2(expand) }
+    }
+
+    pub const fn bang(
+        name: &'static str,
+        expand: impl Fn(crate::TokenStream) -> crate::TokenStream + Copy,
+    ) -> Self {
+        ProcMacro::Bang { name, client: Client::expand1(expand) }
+    }
+}
diff --git a/library/proc_macro/src/bridge/closure.rs b/library/proc_macro/src/bridge/closure.rs
new file mode 100644
index 000000000..d371ae3ce
--- /dev/null
+++ b/library/proc_macro/src/bridge/closure.rs
@@ -0,0 +1,32 @@
+//! Closure type (equivalent to `&mut dyn FnMut(A) -> R`) that's `repr(C)`.
+
+use std::marker::PhantomData;
+
+#[repr(C)]
+pub struct Closure<'a, A, R> {
+    call: unsafe extern "C" fn(*mut Env, A) -> R,
+    env: *mut Env,
+    // Prevent Send and Sync impls. `!Send`/`!Sync` is the usual way of doing
+    // this, but that requires unstable features. rust-analyzer uses this code
+    // and avoids unstable features.
+    //
+    // The `'a` lifetime parameter represents the lifetime of `Env`.
+    _marker: PhantomData<*mut &'a mut ()>,
+}
+
+struct Env;
+
+impl<'a, A, R, F: FnMut(A) -> R> From<&'a mut F> for Closure<'a, A, R> {
+    fn from(f: &'a mut F) -> Self {
+        unsafe extern "C" fn call<A, R, F: FnMut(A) -> R>(env: *mut Env, arg: A) -> R {
+            (*(env as *mut _ as *mut F))(arg)
+        }
+        Closure { call: call::<A, R, F>, env: f as *mut _ as *mut Env, _marker: PhantomData }
+    }
+}
+
+impl<'a, A, R> Closure<'a, A, R> {
+    pub fn call(&mut self, arg: A) -> R {
+        unsafe { (self.call)(self.env, arg) }
+    }
+}
diff --git a/library/proc_macro/src/bridge/fxhash.rs b/library/proc_macro/src/bridge/fxhash.rs
new file mode 100644
index 000000000..4b1e412e2
--- /dev/null
+++ b/library/proc_macro/src/bridge/fxhash.rs
@@ -0,0 +1,117 @@
+//! This is a copy of the `rustc_hash` crate, adapted to work as a module.
+//!
+//! If in the future it becomes more reasonable to add dependencies to
+//! `proc_macro`, this module should be removed and replaced with a dependency
+//! on the `rustc_hash` crate.
+
+use std::collections::HashMap;
+use std::convert::TryInto;
+use std::default::Default;
+use std::hash::BuildHasherDefault;
+use std::hash::Hasher;
+use std::mem::size_of;
+use std::ops::BitXor;
+
+/// Type alias for a hashmap using the `fx` hash algorithm.
+pub type FxHashMap<K, V> = HashMap<K, V, BuildHasherDefault<FxHasher>>;
+
+/// A speedy hash algorithm for use within rustc. The hashmap in liballoc
+/// by default uses SipHash which isn't quite as speedy as we want. In the
+/// compiler we're not really worried about DOS attempts, so we use a fast
+/// non-cryptographic hash.
+///
+/// This is the same as the algorithm used by Firefox -- which is a homespun
+/// one not based on any widely-known algorithm -- though modified to produce
+/// 64-bit hash values instead of 32-bit hash values. It consistently
+/// out-performs an FNV-based hash within rustc itself -- the collision rate is
+/// similar or slightly worse than FNV, but the speed of the hash function
+/// itself is much higher because it works on up to 8 bytes at a time.
+pub struct FxHasher {
+    hash: usize,
+}
+
+#[cfg(target_pointer_width = "32")]
+const K: usize = 0x9e3779b9;
+#[cfg(target_pointer_width = "64")]
+const K: usize = 0x517cc1b727220a95;
+
+impl Default for FxHasher {
+    #[inline]
+    fn default() -> FxHasher {
+        FxHasher { hash: 0 }
+    }
+}
+
+impl FxHasher {
+    #[inline]
+    fn add_to_hash(&mut self, i: usize) {
+        self.hash = self.hash.rotate_left(5).bitxor(i).wrapping_mul(K);
+    }
+}
+
+impl Hasher for FxHasher {
+    #[inline]
+    fn write(&mut self, mut bytes: &[u8]) {
+        #[cfg(target_pointer_width = "32")]
+        let read_usize = |bytes: &[u8]| u32::from_ne_bytes(bytes[..4].try_into().unwrap());
+        #[cfg(target_pointer_width = "64")]
+        let read_usize = |bytes: &[u8]| u64::from_ne_bytes(bytes[..8].try_into().unwrap());
+
+        let mut hash = FxHasher { hash: self.hash };
+        assert!(size_of::<usize>() <= 8);
+        while bytes.len() >= size_of::<usize>() {
+            hash.add_to_hash(read_usize(bytes) as usize);
+            bytes = &bytes[size_of::<usize>()..];
+        }
+        if (size_of::<usize>() > 4) && (bytes.len() >= 4) {
+            hash.add_to_hash(u32::from_ne_bytes(bytes[..4].try_into().unwrap()) as usize);
+            bytes = &bytes[4..];
+        }
+        if (size_of::<usize>() > 2) && bytes.len() >= 2 {
+            hash.add_to_hash(u16::from_ne_bytes(bytes[..2].try_into().unwrap()) as usize);
+            bytes = &bytes[2..];
+        }
+        if (size_of::<usize>() > 1) && bytes.len() >= 1 {
+            hash.add_to_hash(bytes[0] as usize);
+        }
+        self.hash = hash.hash;
+    }
+
+    #[inline]
+    fn write_u8(&mut self, i: u8) {
+        self.add_to_hash(i as usize);
+    }
+
+    #[inline]
+    fn write_u16(&mut self, i: u16) {
+        self.add_to_hash(i as usize);
+    }
+
+    #[inline]
+    fn write_u32(&mut self, i: u32) {
+        self.add_to_hash(i as usize);
+    }
+
+    #[cfg(target_pointer_width = "32")]
+    #[inline]
+    fn write_u64(&mut self, i: u64) {
+        self.add_to_hash(i as usize);
+        self.add_to_hash((i >> 32) as usize);
+    }
+
+    #[cfg(target_pointer_width = "64")]
+    #[inline]
+    fn write_u64(&mut self, i: u64) {
+        self.add_to_hash(i as usize);
+    }
+
+    #[inline]
+    fn write_usize(&mut self, i: usize) {
+        self.add_to_hash(i);
+    }
+
+    #[inline]
+    fn finish(&self) -> u64 {
+        self.hash as u64
+    }
+}
diff --git a/library/proc_macro/src/bridge/handle.rs b/library/proc_macro/src/bridge/handle.rs
new file mode 100644
index 000000000..00954107b
--- /dev/null
+++ b/library/proc_macro/src/bridge/handle.rs
@@ -0,0 +1,75 @@
+//! Server-side handles and storage for per-handle data.
+
+use std::collections::BTreeMap;
+use std::hash::Hash;
+use std::num::NonZeroU32;
+use std::ops::{Index, IndexMut};
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+use super::fxhash::FxHashMap;
+
+pub(super) type Handle = NonZeroU32;
+
+/// A store that associates values of type `T` with numeric handles. A value can
+/// be looked up using its handle.
+pub(super) struct OwnedStore<T: 'static> {
+    counter: &'static AtomicUsize,
+    data: BTreeMap<Handle, T>,
+}
+
+impl<T> OwnedStore<T> {
+    pub(super) fn new(counter: &'static AtomicUsize) -> Self {
+        // Ensure the handle counter isn't 0, which would panic later,
+        // when `NonZeroU32::new` (aka `Handle::new`) is called in `alloc`.
+        assert_ne!(counter.load(Ordering::SeqCst), 0);
+
+        OwnedStore { counter, data: BTreeMap::new() }
+    }
+}
+
+impl<T> OwnedStore<T> {
+    pub(super) fn alloc(&mut self, x: T) -> Handle {
+        let counter = self.counter.fetch_add(1, Ordering::SeqCst);
+        let handle = Handle::new(counter as u32).expect("`proc_macro` handle counter overflowed");
+        assert!(self.data.insert(handle, x).is_none());
+        handle
+    }
+
+    pub(super) fn take(&mut self, h: Handle) -> T {
+        self.data.remove(&h).expect("use-after-free in `proc_macro` handle")
+    }
+}
+
+impl<T> Index<Handle> for OwnedStore<T> {
+    type Output = T;
+    fn index(&self, h: Handle) -> &T {
+        self.data.get(&h).expect("use-after-free in `proc_macro` handle")
+    }
+}
+
+impl<T> IndexMut<Handle> for OwnedStore<T> {
+    fn index_mut(&mut self, h: Handle) -> &mut T {
+        self.data.get_mut(&h).expect("use-after-free in `proc_macro` handle")
+    }
+}
+
+/// Like `OwnedStore`, but avoids storing any value more than once.
+pub(super) struct InternedStore<T: 'static> {
+    owned: OwnedStore<T>,
+    interner: FxHashMap<T, Handle>,
+}
+
+impl<T: Copy + Eq + Hash> InternedStore<T> {
+    pub(super) fn new(counter: &'static AtomicUsize) -> Self {
+        InternedStore { owned: OwnedStore::new(counter), interner: FxHashMap::default() }
+    }
+
+    pub(super) fn alloc(&mut self, x: T) -> Handle {
+        let owned = &mut self.owned;
+        *self.interner.entry(x).or_insert_with(|| owned.alloc(x))
+    }
+
+    pub(super) fn copy(&mut self, h: Handle) -> T {
+        self.owned[h]
+    }
+}
diff --git a/library/proc_macro/src/bridge/mod.rs b/library/proc_macro/src/bridge/mod.rs
new file mode 100644
index 000000000..5cde966bf
--- /dev/null
+++ b/library/proc_macro/src/bridge/mod.rs
@@ -0,0 +1,524 @@
+//! Internal interface for communicating between a `proc_macro` client
+//! (a proc macro crate) and a `proc_macro` server (a compiler front-end).
+//!
+//! Serialization (with C ABI buffers) and unique integer handles are employed
+//! to allow safely interfacing between two copies of `proc_macro` built
+//! (from the same source) by different compilers with potentially mismatching
+//! Rust ABIs (e.g., stage0/bin/rustc vs stage1/bin/rustc during bootstrap).
+
+#![deny(unsafe_code)]
+
+use crate::{Delimiter, Level, LineColumn, Spacing};
+use std::fmt;
+use std::hash::Hash;
+use std::marker;
+use std::mem;
+use std::ops::Bound;
+use std::panic;
+use std::sync::atomic::AtomicUsize;
+use std::sync::Once;
+use std::thread;
+
+/// Higher-order macro describing the server RPC API, allowing automatic
+/// generation of type-safe Rust APIs, both client-side and server-side.
+///
+/// `with_api!(MySelf, my_self, my_macro)` expands to:
+/// ```rust,ignore (pseudo-code)
+/// my_macro! {
+///     // ...
+///     Literal {
+///         // ...
+///         fn character(ch: char) -> MySelf::Literal;
+///         // ...
+///         fn span(my_self: &MySelf::Literal) -> MySelf::Span;
+///         fn set_span(my_self: &mut MySelf::Literal, span: MySelf::Span);
+///     },
+///     // ...
+/// }
+/// ```
+///
+/// The first two arguments serve to customize the arguments names
+/// and argument/return types, to enable several different usecases:
+///
+/// If `my_self` is just `self`, then each `fn` signature can be used
+/// as-is for a method. If it's anything else (`self_` in practice),
+/// then the signatures don't have a special `self` argument, and
+/// can, therefore, have a different one introduced.
+///
+/// If `MySelf` is just `Self`, then the types are only valid inside
+/// a trait or a trait impl, where the trait has associated types
+/// for each of the API types. If non-associated types are desired,
+/// a module name (`self` in practice) can be used instead of `Self`.
+macro_rules! with_api {
+    ($S:ident, $self:ident, $m:ident) => {
+        $m! {
+            FreeFunctions {
+                fn drop($self: $S::FreeFunctions);
+                fn track_env_var(var: &str, value: Option<&str>);
+                fn track_path(path: &str);
+                fn literal_from_str(s: &str) -> Result<Literal<$S::Span, $S::Symbol>, ()>;
+            },
+            TokenStream {
+                fn drop($self: $S::TokenStream);
+                fn clone($self: &$S::TokenStream) -> $S::TokenStream;
+                fn is_empty($self: &$S::TokenStream) -> bool;
+                fn expand_expr($self: &$S::TokenStream) -> Result<$S::TokenStream, ()>;
+                fn from_str(src: &str) -> $S::TokenStream;
+                fn to_string($self: &$S::TokenStream) -> String;
+                fn from_token_tree(
+                    tree: TokenTree<$S::TokenStream, $S::Span, $S::Symbol>,
+                ) -> $S::TokenStream;
+                fn concat_trees(
+                    base: Option<$S::TokenStream>,
+                    trees: Vec<TokenTree<$S::TokenStream, $S::Span, $S::Symbol>>,
+                ) -> $S::TokenStream;
+                fn concat_streams(
+                    base: Option<$S::TokenStream>,
+                    streams: Vec<$S::TokenStream>,
+                ) -> $S::TokenStream;
+                fn into_trees(
+                    $self: $S::TokenStream
+                ) -> Vec<TokenTree<$S::TokenStream, $S::Span, $S::Symbol>>;
+            },
+            SourceFile {
+                fn drop($self: $S::SourceFile);
+                fn clone($self: &$S::SourceFile) -> $S::SourceFile;
+                fn eq($self: &$S::SourceFile, other: &$S::SourceFile) -> bool;
+                fn path($self: &$S::SourceFile) -> String;
+                fn is_real($self: &$S::SourceFile) -> bool;
+            },
+            MultiSpan {
+                fn drop($self: $S::MultiSpan);
+                fn new() -> $S::MultiSpan;
+                fn push($self: &mut $S::MultiSpan, span: $S::Span);
+            },
+            Diagnostic {
+                fn drop($self: $S::Diagnostic);
+                fn new(level: Level, msg: &str, span: $S::MultiSpan) -> $S::Diagnostic;
+                fn sub(
+                    $self: &mut $S::Diagnostic,
+                    level: Level,
+                    msg: &str,
+                    span: $S::MultiSpan,
+                );
+                fn emit($self: $S::Diagnostic);
+            },
+            Span {
+                fn debug($self: $S::Span) -> String;
+                fn source_file($self: $S::Span) -> $S::SourceFile;
+                fn parent($self: $S::Span) -> Option<$S::Span>;
+                fn source($self: $S::Span) -> $S::Span;
+                fn start($self: $S::Span) -> LineColumn;
+                fn end($self: $S::Span) -> LineColumn;
+                fn before($self: $S::Span) -> $S::Span;
+                fn after($self: $S::Span) -> $S::Span;
+                fn join($self: $S::Span, other: $S::Span) -> Option<$S::Span>;
+                fn subspan($self: $S::Span, start: Bound<usize>, end: Bound<usize>) -> Option<$S::Span>;
+                fn resolved_at($self: $S::Span, at: $S::Span) -> $S::Span;
+                fn source_text($self: $S::Span) -> Option<String>;
+                fn save_span($self: $S::Span) -> usize;
+                fn recover_proc_macro_span(id: usize) -> $S::Span;
+            },
+            Symbol {
+                fn normalize_and_validate_ident(string: &str) -> Result<$S::Symbol, ()>;
+            },
+        }
+    };
+}
+
+// FIXME(eddyb) this calls `encode` for each argument, but in reverse,
+// to match the ordering in `reverse_decode`.
+macro_rules! reverse_encode {
+    ($writer:ident;) => {};
+    ($writer:ident; $first:ident $(, $rest:ident)*) => {
+        reverse_encode!($writer; $($rest),*);
+        $first.encode(&mut $writer, &mut ());
+    }
+}
+
+// FIXME(eddyb) this calls `decode` for each argument, but in reverse,
+// to avoid borrow conflicts from borrows started by `&mut` arguments.
+macro_rules! reverse_decode {
+    ($reader:ident, $s:ident;) => {};
+    ($reader:ident, $s:ident; $first:ident: $first_ty:ty $(, $rest:ident: $rest_ty:ty)*) => {
+        reverse_decode!($reader, $s; $($rest: $rest_ty),*);
+        let $first = <$first_ty>::decode(&mut $reader, $s);
+    }
+}
+
+#[allow(unsafe_code)]
+mod arena;
+#[allow(unsafe_code)]
+mod buffer;
+#[forbid(unsafe_code)]
+pub mod client;
+#[allow(unsafe_code)]
+mod closure;
+#[forbid(unsafe_code)]
+mod fxhash;
+#[forbid(unsafe_code)]
+mod handle;
+#[macro_use]
+#[forbid(unsafe_code)]
+mod rpc;
+#[allow(unsafe_code)]
+mod scoped_cell;
+#[allow(unsafe_code)]
+mod selfless_reify;
+#[forbid(unsafe_code)]
+pub mod server;
+#[allow(unsafe_code)]
+mod symbol;
+
+use buffer::Buffer;
+pub use rpc::PanicMessage;
+use rpc::{Decode, DecodeMut, Encode, Reader, Writer};
+
+/// Configuration for establishing an active connection between a server and a
+/// client.  The server creates the bridge config (`run_server` in `server.rs`),
+/// then passes it to the client through the function pointer in the `run` field
+/// of `client::Client`. The client constructs a local `Bridge` from the config
+/// in TLS during its execution (`Bridge::{enter, with}` in `client.rs`).
+#[repr(C)]
+pub struct BridgeConfig<'a> {
+    /// Buffer used to pass initial input to the client.
+    input: Buffer,
+
+    /// Server-side function that the client uses to make requests.
+    dispatch: closure::Closure<'a, Buffer, Buffer>,
+
+    /// If 'true', always invoke the default panic hook
+    force_show_panics: bool,
+
+    // Prevent Send and Sync impls. `!Send`/`!Sync` is the usual way of doing
+    // this, but that requires unstable features. rust-analyzer uses this code
+    // and avoids unstable features.
+    _marker: marker::PhantomData<*mut ()>,
+}
+
+#[forbid(unsafe_code)]
+#[allow(non_camel_case_types)]
+mod api_tags {
+    use super::rpc::{DecodeMut, Encode, Reader, Writer};
+
+    macro_rules! declare_tags {
+        ($($name:ident {
+            $(fn $method:ident($($arg:ident: $arg_ty:ty),* $(,)?) $(-> $ret_ty:ty)*;)*
+        }),* $(,)?) => {
+            $(
+                pub(super) enum $name {
+                    $($method),*
+                }
+                rpc_encode_decode!(enum $name { $($method),* });
+            )*
+
+            pub(super) enum Method {
+                $($name($name)),*
+            }
+            rpc_encode_decode!(enum Method { $($name(m)),* });
+        }
+    }
+    with_api!(self, self, declare_tags);
+}
+
+/// Helper to wrap associated types to allow trait impl dispatch.
+/// That is, normally a pair of impls for `T::Foo` and `T::Bar`
+/// can overlap, but if the impls are, instead, on types like
+/// `Marked<T::Foo, Foo>` and `Marked<T::Bar, Bar>`, they can't.
+trait Mark {
+    type Unmarked;
+    fn mark(unmarked: Self::Unmarked) -> Self;
+}
+
+/// Unwrap types wrapped by `Mark::mark` (see `Mark` for details).
+trait Unmark {
+    type Unmarked;
+    fn unmark(self) -> Self::Unmarked;
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+struct Marked<T, M> {
+    value: T,
+    _marker: marker::PhantomData<M>,
+}
+
+impl<T, M> Mark for Marked<T, M> {
+    type Unmarked = T;
+    fn mark(unmarked: Self::Unmarked) -> Self {
+        Marked { value: unmarked, _marker: marker::PhantomData }
+    }
+}
+impl<T, M> Unmark for Marked<T, M> {
+    type Unmarked = T;
+    fn unmark(self) -> Self::Unmarked {
+        self.value
+    }
+}
+impl<'a, T, M> Unmark for &'a Marked<T, M> {
+    type Unmarked = &'a T;
+    fn unmark(self) -> Self::Unmarked {
+        &self.value
+    }
+}
+impl<'a, T, M> Unmark for &'a mut Marked<T, M> {
+    type Unmarked = &'a mut T;
+    fn unmark(self) -> Self::Unmarked {
+        &mut self.value
+    }
+}
+
+impl<T: Mark> Mark for Vec<T> {
+    type Unmarked = Vec<T::Unmarked>;
+    fn mark(unmarked: Self::Unmarked) -> Self {
+        // Should be a no-op due to std's in-place collect optimizations.
+        unmarked.into_iter().map(T::mark).collect()
+    }
+}
+impl<T: Unmark> Unmark for Vec<T> {
+    type Unmarked = Vec<T::Unmarked>;
+    fn unmark(self) -> Self::Unmarked {
+        // Should be a no-op due to std's in-place collect optimizations.
+        self.into_iter().map(T::unmark).collect()
+    }
+}
+
+macro_rules! mark_noop {
+    ($($ty:ty),* $(,)?) => {
+        $(
+            impl Mark for $ty {
+                type Unmarked = Self;
+                fn mark(unmarked: Self::Unmarked) -> Self {
+                    unmarked
+                }
+            }
+            impl Unmark for $ty {
+                type Unmarked = Self;
+                fn unmark(self) -> Self::Unmarked {
+                    self
+                }
+            }
+        )*
+    }
+}
+mark_noop! {
+    (),
+    bool,
+    char,
+    &'_ [u8],
+    &'_ str,
+    String,
+    u8,
+    usize,
+    Delimiter,
+    LitKind,
+    Level,
+    LineColumn,
+    Spacing,
+}
+
+rpc_encode_decode!(
+    enum Delimiter {
+        Parenthesis,
+        Brace,
+        Bracket,
+        None,
+    }
+);
+rpc_encode_decode!(
+    enum Level {
+        Error,
+        Warning,
+        Note,
+        Help,
+    }
+);
+rpc_encode_decode!(struct LineColumn { line, column });
+rpc_encode_decode!(
+    enum Spacing {
+        Alone,
+        Joint,
+    }
+);
+
+#[derive(Copy, Clone, Eq, PartialEq, Debug)]
+pub enum LitKind {
+    Byte,
+    Char,
+    Integer,
+    Float,
+    Str,
+    StrRaw(u8),
+    ByteStr,
+    ByteStrRaw(u8),
+    Err,
+}
+
+rpc_encode_decode!(
+    enum LitKind {
+        Byte,
+        Char,
+        Integer,
+        Float,
+        Str,
+        StrRaw(n),
+        ByteStr,
+        ByteStrRaw(n),
+        Err,
+    }
+);
+
+macro_rules! mark_compound {
+    (struct $name:ident <$($T:ident),+> { $($field:ident),* $(,)? }) => {
+        impl<$($T: Mark),+> Mark for $name <$($T),+> {
+            type Unmarked = $name <$($T::Unmarked),+>;
+            fn mark(unmarked: Self::Unmarked) -> Self {
+                $name {
+                    $($field: Mark::mark(unmarked.$field)),*
+                }
+            }
+        }
+
+        impl<$($T: Unmark),+> Unmark for $name <$($T),+> {
+            type Unmarked = $name <$($T::Unmarked),+>;
+            fn unmark(self) -> Self::Unmarked {
+                $name {
+                    $($field: Unmark::unmark(self.$field)),*
+                }
+            }
+        }
+    };
+    (enum $name:ident <$($T:ident),+> { $($variant:ident $(($field:ident))?),* $(,)? }) => {
+        impl<$($T: Mark),+> Mark for $name <$($T),+> {
+            type Unmarked = $name <$($T::Unmarked),+>;
+            fn mark(unmarked: Self::Unmarked) -> Self {
+                match unmarked {
+                    $($name::$variant $(($field))? => {
+                        $name::$variant $((Mark::mark($field)))?
+                    })*
+                }
+            }
+        }
+
+        impl<$($T: Unmark),+> Unmark for $name <$($T),+> {
+            type Unmarked = $name <$($T::Unmarked),+>;
+            fn unmark(self) -> Self::Unmarked {
+                match self {
+                    $($name::$variant $(($field))? => {
+                        $name::$variant $((Unmark::unmark($field)))?
+                    })*
+                }
+            }
+        }
+    }
+}
+
+macro_rules! compound_traits {
+    ($($t:tt)*) => {
+        rpc_encode_decode!($($t)*);
+        mark_compound!($($t)*);
+    };
+}
+
+compound_traits!(
+    enum Bound<T> {
+        Included(x),
+        Excluded(x),
+        Unbounded,
+    }
+);
+
+compound_traits!(
+    enum Option<T> {
+        Some(t),
+        None,
+    }
+);
+
+compound_traits!(
+    enum Result<T, E> {
+        Ok(t),
+        Err(e),
+    }
+);
+
+#[derive(Copy, Clone)]
+pub struct DelimSpan<Span> {
+    pub open: Span,
+    pub close: Span,
+    pub entire: Span,
+}
+
+impl<Span: Copy> DelimSpan<Span> {
+    pub fn from_single(span: Span) -> Self {
+        DelimSpan { open: span, close: span, entire: span }
+    }
+}
+
+compound_traits!(struct DelimSpan<Span> { open, close, entire });
+
+#[derive(Clone)]
+pub struct Group<TokenStream, Span> {
+    pub delimiter: Delimiter,
+    pub stream: Option<TokenStream>,
+    pub span: DelimSpan<Span>,
+}
+
+compound_traits!(struct Group<TokenStream, Span> { delimiter, stream, span });
+
+#[derive(Clone)]
+pub struct Punct<Span> {
+    pub ch: u8,
+    pub joint: bool,
+    pub span: Span,
+}
+
+compound_traits!(struct Punct<Span> { ch, joint, span });
+
+#[derive(Copy, Clone, Eq, PartialEq)]
+pub struct Ident<Span, Symbol> {
+    pub sym: Symbol,
+    pub is_raw: bool,
+    pub span: Span,
+}
+
+compound_traits!(struct Ident<Span, Symbol> { sym, is_raw, span });
+
+#[derive(Clone, Eq, PartialEq)]
+pub struct Literal<Span, Symbol> {
+    pub kind: LitKind,
+    pub symbol: Symbol,
+    pub suffix: Option<Symbol>,
+    pub span: Span,
+}
+
+compound_traits!(struct Literal<Sp, Sy> { kind, symbol, suffix, span });
+
+#[derive(Clone)]
+pub enum TokenTree<TokenStream, Span, Symbol> {
+    Group(Group<TokenStream, Span>),
+    Punct(Punct<Span>),
+    Ident(Ident<Span, Symbol>),
+    Literal(Literal<Span, Symbol>),
+}
+
+compound_traits!(
+    enum TokenTree<TokenStream, Span, Symbol> {
+        Group(tt),
+        Punct(tt),
+        Ident(tt),
+        Literal(tt),
+    }
+);
+
+/// Globals provided alongside the initial inputs for a macro expansion.
+/// Provides values such as spans which are used frequently to avoid RPC.
+#[derive(Clone)]
+pub struct ExpnGlobals<Span> {
+    pub def_site: Span,
+    pub call_site: Span,
+    pub mixed_site: Span,
+}
+
+compound_traits!(
+    struct ExpnGlobals<Span> { def_site, call_site, mixed_site }
+);
diff --git a/library/proc_macro/src/bridge/rpc.rs b/library/proc_macro/src/bridge/rpc.rs
new file mode 100644
index 000000000..e9d7a46c0
--- /dev/null
+++ b/library/proc_macro/src/bridge/rpc.rs
@@ -0,0 +1,304 @@
+//! Serialization for client-server communication.
+
+use std::any::Any;
+use std::char;
+use std::io::Write;
+use std::num::NonZeroU32;
+use std::str;
+
+pub(super) type Writer = super::buffer::Buffer;
+
+pub(super) trait Encode<S>: Sized {
+    fn encode(self, w: &mut Writer, s: &mut S);
+}
+
+pub(super) type Reader<'a> = &'a [u8];
+
+pub(super) trait Decode<'a, 's, S>: Sized {
+    fn decode(r: &mut Reader<'a>, s: &'s S) -> Self;
+}
+
+pub(super) trait DecodeMut<'a, 's, S>: Sized {
+    fn decode(r: &mut Reader<'a>, s: &'s mut S) -> Self;
+}
+
+macro_rules! rpc_encode_decode {
+    (le $ty:ty) => {
+        impl<S> Encode<S> for $ty {
+            fn encode(self, w: &mut Writer, _: &mut S) {
+                w.extend_from_array(&self.to_le_bytes());
+            }
+        }
+
+        impl<S> DecodeMut<'_, '_, S> for $ty {
+            fn decode(r: &mut Reader<'_>, _: &mut S) -> Self {
+                const N: usize = ::std::mem::size_of::<$ty>();
+
+                let mut bytes = [0; N];
+                bytes.copy_from_slice(&r[..N]);
+                *r = &r[N..];
+
+                Self::from_le_bytes(bytes)
+            }
+        }
+    };
+    (struct $name:ident $(<$($T:ident),+>)? { $($field:ident),* $(,)? }) => {
+        impl<S, $($($T: Encode<S>),+)?> Encode<S> for $name $(<$($T),+>)? {
+            fn encode(self, w: &mut Writer, s: &mut S) {
+                $(self.$field.encode(w, s);)*
+            }
+        }
+
+        impl<'a, S, $($($T: for<'s> DecodeMut<'a, 's, S>),+)?> DecodeMut<'a, '_, S>
+            for $name $(<$($T),+>)?
+        {
+            fn decode(r: &mut Reader<'a>, s: &mut S) -> Self {
+                $name {
+                    $($field: DecodeMut::decode(r, s)),*
+                }
+            }
+        }
+    };
+    (enum $name:ident $(<$($T:ident),+>)? { $($variant:ident $(($field:ident))*),* $(,)? }) => {
+        impl<S, $($($T: Encode<S>),+)?> Encode<S> for $name $(<$($T),+>)? {
+            fn encode(self, w: &mut Writer, s: &mut S) {
+                // HACK(eddyb): `Tag` enum duplicated between the
+                // two impls as there's no other place to stash it.
+                #[allow(non_upper_case_globals)]
+                mod tag {
+                    #[repr(u8)] enum Tag { $($variant),* }
+
+                    $(pub const $variant: u8 = Tag::$variant as u8;)*
+                }
+
+                match self {
+                    $($name::$variant $(($field))* => {
+                        tag::$variant.encode(w, s);
+                        $($field.encode(w, s);)*
+                    })*
+                }
+            }
+        }
+
+        impl<'a, S, $($($T: for<'s> DecodeMut<'a, 's, S>),+)?> DecodeMut<'a, '_, S>
+            for $name $(<$($T),+>)?
+        {
+            fn decode(r: &mut Reader<'a>, s: &mut S) -> Self {
+                // HACK(eddyb): `Tag` enum duplicated between the
+                // two impls as there's no other place to stash it.
+                #[allow(non_upper_case_globals)]
+                mod tag {
+                    #[repr(u8)] enum Tag { $($variant),* }
+
+                    $(pub const $variant: u8 = Tag::$variant as u8;)*
+                }
+
+                match u8::decode(r, s) {
+                    $(tag::$variant => {
+                        $(let $field = DecodeMut::decode(r, s);)*
+                        $name::$variant $(($field))*
+                    })*
+                    _ => unreachable!(),
+                }
+            }
+        }
+    }
+}
+
+impl<S> Encode<S> for () {
+    fn encode(self, _: &mut Writer, _: &mut S) {}
+}
+
+impl<S> DecodeMut<'_, '_, S> for () {
+    fn decode(_: &mut Reader<'_>, _: &mut S) -> Self {}
+}
+
+impl<S> Encode<S> for u8 {
+    fn encode(self, w: &mut Writer, _: &mut S) {
+        w.push(self);
+    }
+}
+
+impl<S> DecodeMut<'_, '_, S> for u8 {
+    fn decode(r: &mut Reader<'_>, _: &mut S) -> Self {
+        let x = r[0];
+        *r = &r[1..];
+        x
+    }
+}
+
+rpc_encode_decode!(le u32);
+rpc_encode_decode!(le usize);
+
+impl<S> Encode<S> for bool {
+    fn encode(self, w: &mut Writer, s: &mut S) {
+        (self as u8).encode(w, s);
+    }
+}
+
+impl<S> DecodeMut<'_, '_, S> for bool {
+    fn decode(r: &mut Reader<'_>, s: &mut S) -> Self {
+        match u8::decode(r, s) {
+            0 => false,
+            1 => true,
+            _ => unreachable!(),
+        }
+    }
+}
+
+impl<S> Encode<S> for char {
+    fn encode(self, w: &mut Writer, s: &mut S) {
+        (self as u32).encode(w, s);
+    }
+}
+
+impl<S> DecodeMut<'_, '_, S> for char {
+    fn decode(r: &mut Reader<'_>, s: &mut S) -> Self {
+        char::from_u32(u32::decode(r, s)).unwrap()
+    }
+}
+
+impl<S> Encode<S> for NonZeroU32 {
+    fn encode(self, w: &mut Writer, s: &mut S) {
+        self.get().encode(w, s);
+    }
+}
+
+impl<S> DecodeMut<'_, '_, S> for NonZeroU32 {
+    fn decode(r: &mut Reader<'_>, s: &mut S) -> Self {
+        Self::new(u32::decode(r, s)).unwrap()
+    }
+}
+
+impl<S, A: Encode<S>, B: Encode<S>> Encode<S> for (A, B) {
+    fn encode(self, w: &mut Writer, s: &mut S) {
+        self.0.encode(w, s);
+        self.1.encode(w, s);
+    }
+}
+
+impl<'a, S, A: for<'s> DecodeMut<'a, 's, S>, B: for<'s> DecodeMut<'a, 's, S>> DecodeMut<'a, '_, S>
+    for (A, B)
+{
+    fn decode(r: &mut Reader<'a>, s: &mut S) -> Self {
+        (DecodeMut::decode(r, s), DecodeMut::decode(r, s))
+    }
+}
+
+impl<S> Encode<S> for &[u8] {
+    fn encode(self, w: &mut Writer, s: &mut S) {
+        self.len().encode(w, s);
+        w.write_all(self).unwrap();
+    }
+}
+
+impl<'a, S> DecodeMut<'a, '_, S> for &'a [u8] {
+    fn decode(r: &mut Reader<'a>, s: &mut S) -> Self {
+        let len = usize::decode(r, s);
+        let xs = &r[..len];
+        *r = &r[len..];
+        xs
+    }
+}
+
+impl<S> Encode<S> for &str {
+    fn encode(self, w: &mut Writer, s: &mut S) {
+        self.as_bytes().encode(w, s);
+    }
+}
+
+impl<'a, S> DecodeMut<'a, '_, S> for &'a str {
+    fn decode(r: &mut Reader<'a>, s: &mut S) -> Self {
+        str::from_utf8(<&[u8]>::decode(r, s)).unwrap()
+    }
+}
+
+impl<S> Encode<S> for String {
+    fn encode(self, w: &mut Writer, s: &mut S) {
+        self[..].encode(w, s);
+    }
+}
+
+impl<S> DecodeMut<'_, '_, S> for String {
+    fn decode(r: &mut Reader<'_>, s: &mut S) -> Self {
+        <&str>::decode(r, s).to_string()
+    }
+}
+
+impl<S, T: Encode<S>> Encode<S> for Vec<T> {
+    fn encode(self, w: &mut Writer, s: &mut S) {
+        self.len().encode(w, s);
+        for x in self {
+            x.encode(w, s);
+        }
+    }
+}
+
+impl<'a, S, T: for<'s> DecodeMut<'a, 's, S>> DecodeMut<'a, '_, S> for Vec<T> {
+    fn decode(r: &mut Reader<'a>, s: &mut S) -> Self {
+        let len = usize::decode(r, s);
+        let mut vec = Vec::with_capacity(len);
+        for _ in 0..len {
+            vec.push(T::decode(r, s));
+        }
+        vec
+    }
+}
+
+/// Simplified version of panic payloads, ignoring
+/// types other than `&'static str` and `String`.
+pub enum PanicMessage {
+    StaticStr(&'static str),
+    String(String),
+    Unknown,
+}
+
+impl From<Box<dyn Any + Send>> for PanicMessage {
+    fn from(payload: Box<dyn Any + Send + 'static>) -> Self {
+        if let Some(s) = payload.downcast_ref::<&'static str>() {
+            return PanicMessage::StaticStr(s);
+        }
+        if let Ok(s) = payload.downcast::<String>() {
+            return PanicMessage::String(*s);
+        }
+        PanicMessage::Unknown
+    }
+}
+
+impl Into<Box<dyn Any + Send>> for PanicMessage {
+    fn into(self) -> Box<dyn Any + Send> {
+        match self {
+            PanicMessage::StaticStr(s) => Box::new(s),
+            PanicMessage::String(s) => Box::new(s),
+            PanicMessage::Unknown => {
+                struct UnknownPanicMessage;
+                Box::new(UnknownPanicMessage)
+            }
+        }
+    }
+}
+
+impl PanicMessage {
+    pub fn as_str(&self) -> Option<&str> {
+        match self {
+            PanicMessage::StaticStr(s) => Some(s),
+            PanicMessage::String(s) => Some(s),
+            PanicMessage::Unknown => None,
+        }
+    }
+}
+
+impl<S> Encode<S> for PanicMessage {
+    fn encode(self, w: &mut Writer, s: &mut S) {
+        self.as_str().encode(w, s);
+    }
+}
+
+impl<S> DecodeMut<'_, '_, S> for PanicMessage {
+    fn decode(r: &mut Reader<'_>, s: &mut S) -> Self {
+        match Option::<String>::decode(r, s) {
+            Some(s) => PanicMessage::String(s),
+            None => PanicMessage::Unknown,
+        }
+    }
+}
diff --git a/library/proc_macro/src/bridge/scoped_cell.rs b/library/proc_macro/src/bridge/scoped_cell.rs
new file mode 100644
index 000000000..2cde1f65a
--- /dev/null
+++ b/library/proc_macro/src/bridge/scoped_cell.rs
@@ -0,0 +1,81 @@
+//! `Cell` variant for (scoped) existential lifetimes.
+
+use std::cell::Cell;
+use std::mem;
+use std::ops::{Deref, DerefMut};
+
+/// Type lambda application, with a lifetime.
+#[allow(unused_lifetimes)]
+pub trait ApplyL<'a> {
+    type Out;
+}
+
+/// Type lambda taking a lifetime, i.e., `Lifetime -> Type`.
+pub trait LambdaL: for<'a> ApplyL<'a> {}
+
+impl<T: for<'a> ApplyL<'a>> LambdaL for T {}
+
+// HACK(eddyb) work around projection limitations with a newtype
+// FIXME(#52812) replace with `&'a mut <T as ApplyL<'b>>::Out`
+pub struct RefMutL<'a, 'b, T: LambdaL>(&'a mut <T as ApplyL<'b>>::Out);
+
+impl<'a, 'b, T: LambdaL> Deref for RefMutL<'a, 'b, T> {
+    type Target = <T as ApplyL<'b>>::Out;
+    fn deref(&self) -> &Self::Target {
+        self.0
+    }
+}
+
+impl<'a, 'b, T: LambdaL> DerefMut for RefMutL<'a, 'b, T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.0
+    }
+}
+
+pub struct ScopedCell<T: LambdaL>(Cell<<T as ApplyL<'static>>::Out>);
+
+impl<T: LambdaL> ScopedCell<T> {
+    pub const fn new(value: <T as ApplyL<'static>>::Out) -> Self {
+        ScopedCell(Cell::new(value))
+    }
+
+    /// Sets the value in `self` to `replacement` while
+    /// running `f`, which gets the old value, mutably.
+    /// The old value will be restored after `f` exits, even
+    /// by panic, including modifications made to it by `f`.
+    pub fn replace<'a, R>(
+        &self,
+        replacement: <T as ApplyL<'a>>::Out,
+        f: impl for<'b, 'c> FnOnce(RefMutL<'b, 'c, T>) -> R,
+    ) -> R {
+        /// Wrapper that ensures that the cell always gets filled
+        /// (with the original state, optionally changed by `f`),
+        /// even if `f` had panicked.
+        struct PutBackOnDrop<'a, T: LambdaL> {
+            cell: &'a ScopedCell<T>,
+            value: Option<<T as ApplyL<'static>>::Out>,
+        }
+
+        impl<'a, T: LambdaL> Drop for PutBackOnDrop<'a, T> {
+            fn drop(&mut self) {
+                self.cell.0.set(self.value.take().unwrap());
+            }
+        }
+
+        let mut put_back_on_drop = PutBackOnDrop {
+            cell: self,
+            value: Some(self.0.replace(unsafe {
+                let erased = mem::transmute_copy(&replacement);
+                mem::forget(replacement);
+                erased
+            })),
+        };
+
+        f(RefMutL(put_back_on_drop.value.as_mut().unwrap()))
+    }
+
+    /// Sets the value in `self` to `value` while running `f`.
+    pub fn set<R>(&self, value: <T as ApplyL<'_>>::Out, f: impl FnOnce() -> R) -> R {
+        self.replace(value, |_| f())
+    }
+}
diff --git a/library/proc_macro/src/bridge/selfless_reify.rs b/library/proc_macro/src/bridge/selfless_reify.rs
new file mode 100644
index 000000000..907ad256e
--- /dev/null
+++ b/library/proc_macro/src/bridge/selfless_reify.rs
@@ -0,0 +1,84 @@
+//! Abstraction for creating `fn` pointers from any callable that *effectively*
+//! has the equivalent of implementing `Default`, even if the compiler neither
+//! provides `Default` nor allows reifying closures (i.e. creating `fn` pointers)
+//! other than those with absolutely no captures.
+//!
+//! More specifically, for a closure-like type to be "effectively `Default`":
+//! * it must be a ZST (zero-sized type): no information contained within, so
+//!   that `Default`'s return value (if it were implemented) is unambiguous
+//! * it must be `Copy`: no captured "unique ZST tokens" or any other similar
+//!   types that would make duplicating values at will unsound
+//!   * combined with the ZST requirement, this confers a kind of "telecopy"
+//!     ability: similar to `Copy`, but without keeping the value around, and
+//!     instead "reconstructing" it (a noop given it's a ZST) when needed
+//! * it must be *provably* inhabited: no captured uninhabited types or any
+//!   other types that cannot be constructed by the user of this abstraction
+//!   * the proof is a value of the closure-like type itself, in a sense the
+//!     "seed" for the "telecopy" process made possible by ZST + `Copy`
+//!   * this requirement is the only reason an abstraction limited to a specific
+//!     usecase is required: ZST + `Copy` can be checked with *at worst* a panic
+//!     at the "attempted `::default()` call" time, but that doesn't guarantee
+//!     that the value can be soundly created, and attempting to use the typical
+//!     "proof ZST token" approach leads yet again to having a ZST + `Copy` type
+//!     that is not proof of anything without a value (i.e. isomorphic to a
+//!     newtype of the type it's trying to prove the inhabitation of)
+//!
+//! A more flexible (and safer) solution to the general problem could exist once
+//! `const`-generic parameters can have type parameters in their types:
+//!
+//! ```rust,ignore (needs future const-generics)
+//! extern "C" fn ffi_wrapper<
+//!     A, R,
+//!     F: Fn(A) -> R,
+//!     const f: F, // <-- this `const`-generic is not yet allowed
+//! >(arg: A) -> R {
+//!     f(arg)
+//! }
+//! ```
+
+use std::mem;
+
+// FIXME(eddyb) this could be `trait` impls except for the `const fn` requirement.
+macro_rules! define_reify_functions {
+    ($(
+        fn $name:ident $(<$($param:ident),*>)?
+            for $(extern $abi:tt)? fn($($arg:ident: $arg_ty:ty),*) -> $ret_ty:ty;
+    )+) => {
+        $(pub const fn $name<
+            $($($param,)*)?
+            F: Fn($($arg_ty),*) -> $ret_ty + Copy
+        >(f: F) -> $(extern $abi)? fn($($arg_ty),*) -> $ret_ty {
+            // FIXME(eddyb) describe the `F` type (e.g. via `type_name::<F>`) once panic
+            // formatting becomes possible in `const fn`.
+            assert!(mem::size_of::<F>() == 0, "selfless_reify: closure must be zero-sized");
+
+            $(extern $abi)? fn wrapper<
+                $($($param,)*)?
+                F: Fn($($arg_ty),*) -> $ret_ty + Copy
+            >($($arg: $arg_ty),*) -> $ret_ty {
+                let f = unsafe {
+                    // SAFETY: `F` satisfies all criteria for "out of thin air"
+                    // reconstructability (see module-level doc comment).
+                    mem::MaybeUninit::<F>::uninit().assume_init()
+                };
+                f($($arg),*)
+            }
+            let _f_proof = f;
+            wrapper::<
+                $($($param,)*)?
+                F
+            >
+        })+
+    }
+}
+
+define_reify_functions! {
+    fn _reify_to_extern_c_fn_unary<A, R> for extern "C" fn(arg: A) -> R;
+
+    // HACK(eddyb) this abstraction is used with `for<'a> fn(BridgeConfig<'a>)
+    // -> T` but that doesn't work with just `reify_to_extern_c_fn_unary`
+    // because of the `fn` pointer type being "higher-ranked" (i.e. the
+    // `for<'a>` binder).
+    // FIXME(eddyb) try to remove the lifetime from `BridgeConfig`, that'd help.
+    fn reify_to_extern_c_fn_hrt_bridge<R> for extern "C" fn(bridge: super::BridgeConfig<'_>) -> R;
+}
diff --git a/library/proc_macro/src/bridge/server.rs b/library/proc_macro/src/bridge/server.rs
new file mode 100644
index 000000000..e068ec60b
--- /dev/null
+++ b/library/proc_macro/src/bridge/server.rs
@@ -0,0 +1,339 @@
+//! Server-side traits.
+
+use super::*;
+
+use std::marker::PhantomData;
+
+// FIXME(eddyb) generate the definition of `HandleStore` in `server.rs`.
+use super::client::HandleStore;
+
+pub trait Types {
+    type FreeFunctions: 'static;
+    type TokenStream: 'static + Clone;
+    type SourceFile: 'static + Clone;
+    type MultiSpan: 'static;
+    type Diagnostic: 'static;
+    type Span: 'static + Copy + Eq + Hash;
+    type Symbol: 'static;
+}
+
+/// Declare an associated fn of one of the traits below, adding necessary
+/// default bodies.
+macro_rules! associated_fn {
+    (fn drop(&mut self, $arg:ident: $arg_ty:ty)) =>
+        (fn drop(&mut self, $arg: $arg_ty) { mem::drop($arg) });
+
+    (fn clone(&mut self, $arg:ident: $arg_ty:ty) -> $ret_ty:ty) =>
+        (fn clone(&mut self, $arg: $arg_ty) -> $ret_ty { $arg.clone() });
+
+    ($($item:tt)*) => ($($item)*;)
+}
+
+macro_rules! declare_server_traits {
+    ($($name:ident {
+        $(fn $method:ident($($arg:ident: $arg_ty:ty),* $(,)?) $(-> $ret_ty:ty)?;)*
+    }),* $(,)?) => {
+        $(pub trait $name: Types {
+            $(associated_fn!(fn $method(&mut self, $($arg: $arg_ty),*) $(-> $ret_ty)?);)*
+        })*
+
+        pub trait Server: Types $(+ $name)* {
+            fn globals(&mut self) -> ExpnGlobals<Self::Span>;
+
+            /// Intern a symbol received from RPC
+            fn intern_symbol(ident: &str) -> Self::Symbol;
+
+            /// Recover the string value of a symbol, and invoke a callback with it.
+            fn with_symbol_string(symbol: &Self::Symbol, f: impl FnOnce(&str));
+        }
+    }
+}
+with_api!(Self, self_, declare_server_traits);
+
+pub(super) struct MarkedTypes<S: Types>(S);
+
+impl<S: Server> Server for MarkedTypes<S> {
+    fn globals(&mut self) -> ExpnGlobals<Self::Span> {
+        <_>::mark(Server::globals(&mut self.0))
+    }
+    fn intern_symbol(ident: &str) -> Self::Symbol {
+        <_>::mark(S::intern_symbol(ident))
+    }
+    fn with_symbol_string(symbol: &Self::Symbol, f: impl FnOnce(&str)) {
+        S::with_symbol_string(symbol.unmark(), f)
+    }
+}
+
+macro_rules! define_mark_types_impls {
+    ($($name:ident {
+        $(fn $method:ident($($arg:ident: $arg_ty:ty),* $(,)?) $(-> $ret_ty:ty)?;)*
+    }),* $(,)?) => {
+        impl<S: Types> Types for MarkedTypes<S> {
+            $(type $name = Marked<S::$name, client::$name>;)*
+        }
+
+        $(impl<S: $name> $name for MarkedTypes<S> {
+            $(fn $method(&mut self, $($arg: $arg_ty),*) $(-> $ret_ty)? {
+                <_>::mark($name::$method(&mut self.0, $($arg.unmark()),*))
+            })*
+        })*
+    }
+}
+with_api!(Self, self_, define_mark_types_impls);
+
+struct Dispatcher<S: Types> {
+    handle_store: HandleStore<S>,
+    server: S,
+}
+
+macro_rules! define_dispatcher_impl {
+    ($($name:ident {
+        $(fn $method:ident($($arg:ident: $arg_ty:ty),* $(,)?) $(-> $ret_ty:ty)?;)*
+    }),* $(,)?) => {
+        // FIXME(eddyb) `pub` only for `ExecutionStrategy` below.
+        pub trait DispatcherTrait {
+            // HACK(eddyb) these are here to allow `Self::$name` to work below.
+            $(type $name;)*
+
+            fn dispatch(&mut self, buf: Buffer) -> Buffer;
+        }
+
+        impl<S: Server> DispatcherTrait for Dispatcher<MarkedTypes<S>> {
+            $(type $name = <MarkedTypes<S> as Types>::$name;)*
+
+            fn dispatch(&mut self, mut buf: Buffer) -> Buffer {
+                let Dispatcher { handle_store, server } = self;
+
+                let mut reader = &buf[..];
+                match api_tags::Method::decode(&mut reader, &mut ()) {
+                    $(api_tags::Method::$name(m) => match m {
+                        $(api_tags::$name::$method => {
+                            let mut call_method = || {
+                                reverse_decode!(reader, handle_store; $($arg: $arg_ty),*);
+                                $name::$method(server, $($arg),*)
+                            };
+                            // HACK(eddyb) don't use `panic::catch_unwind` in a panic.
+                            // If client and server happen to use the same `libstd`,
+                            // `catch_unwind` asserts that the panic counter was 0,
+                            // even when the closure passed to it didn't panic.
+                            let r = if thread::panicking() {
+                                Ok(call_method())
+                            } else {
+                                panic::catch_unwind(panic::AssertUnwindSafe(call_method))
+                                    .map_err(PanicMessage::from)
+                            };
+
+                            buf.clear();
+                            r.encode(&mut buf, handle_store);
+                        })*
+                    }),*
+                }
+                buf
+            }
+        }
+    }
+}
+with_api!(Self, self_, define_dispatcher_impl);
+
+pub trait ExecutionStrategy {
+    fn run_bridge_and_client(
+        &self,
+        dispatcher: &mut impl DispatcherTrait,
+        input: Buffer,
+        run_client: extern "C" fn(BridgeConfig<'_>) -> Buffer,
+        force_show_panics: bool,
+    ) -> Buffer;
+}
+
+pub struct MaybeCrossThread<P> {
+    cross_thread: bool,
+    marker: PhantomData<P>,
+}
+
+impl<P> MaybeCrossThread<P> {
+    pub const fn new(cross_thread: bool) -> Self {
+        MaybeCrossThread { cross_thread, marker: PhantomData }
+    }
+}
+
+impl<P> ExecutionStrategy for MaybeCrossThread<P>
+where
+    P: MessagePipe<Buffer> + Send + 'static,
+{
+    fn run_bridge_and_client(
+        &self,
+        dispatcher: &mut impl DispatcherTrait,
+        input: Buffer,
+        run_client: extern "C" fn(BridgeConfig<'_>) -> Buffer,
+        force_show_panics: bool,
+    ) -> Buffer {
+        if self.cross_thread {
+            <CrossThread<P>>::new().run_bridge_and_client(
+                dispatcher,
+                input,
+                run_client,
+                force_show_panics,
+            )
+        } else {
+            SameThread.run_bridge_and_client(dispatcher, input, run_client, force_show_panics)
+        }
+    }
+}
+
+pub struct SameThread;
+
+impl ExecutionStrategy for SameThread {
+    fn run_bridge_and_client(
+        &self,
+        dispatcher: &mut impl DispatcherTrait,
+        input: Buffer,
+        run_client: extern "C" fn(BridgeConfig<'_>) -> Buffer,
+        force_show_panics: bool,
+    ) -> Buffer {
+        let mut dispatch = |buf| dispatcher.dispatch(buf);
+
+        run_client(BridgeConfig {
+            input,
+            dispatch: (&mut dispatch).into(),
+            force_show_panics,
+            _marker: marker::PhantomData,
+        })
+    }
+}
+
+pub struct CrossThread<P>(PhantomData<P>);
+
+impl<P> CrossThread<P> {
+    pub const fn new() -> Self {
+        CrossThread(PhantomData)
+    }
+}
+
+impl<P> ExecutionStrategy for CrossThread<P>
+where
+    P: MessagePipe<Buffer> + Send + 'static,
+{
+    fn run_bridge_and_client(
+        &self,
+        dispatcher: &mut impl DispatcherTrait,
+        input: Buffer,
+        run_client: extern "C" fn(BridgeConfig<'_>) -> Buffer,
+        force_show_panics: bool,
+    ) -> Buffer {
+        let (mut server, mut client) = P::new();
+
+        let join_handle = thread::spawn(move || {
+            let mut dispatch = |b: Buffer| -> Buffer {
+                client.send(b);
+                client.recv().expect("server died while client waiting for reply")
+            };
+
+            run_client(BridgeConfig {
+                input,
+                dispatch: (&mut dispatch).into(),
+                force_show_panics,
+                _marker: marker::PhantomData,
+            })
+        });
+
+        while let Some(b) = server.recv() {
+            server.send(dispatcher.dispatch(b));
+        }
+
+        join_handle.join().unwrap()
+    }
+}
+
+/// A message pipe used for communicating between server and client threads.
+pub trait MessagePipe<T>: Sized {
+    /// Create a new pair of endpoints for the message pipe.
+    fn new() -> (Self, Self);
+
+    /// Send a message to the other endpoint of this pipe.
+    fn send(&mut self, value: T);
+
+    /// Receive a message from the other endpoint of this pipe.
+    ///
+    /// Returns `None` if the other end of the pipe has been destroyed, and no
+    /// message was received.
+    fn recv(&mut self) -> Option<T>;
+}
+
+fn run_server<
+    S: Server,
+    I: Encode<HandleStore<MarkedTypes<S>>>,
+    O: for<'a, 's> DecodeMut<'a, 's, HandleStore<MarkedTypes<S>>>,
+>(
+    strategy: &impl ExecutionStrategy,
+    handle_counters: &'static client::HandleCounters,
+    server: S,
+    input: I,
+    run_client: extern "C" fn(BridgeConfig<'_>) -> Buffer,
+    force_show_panics: bool,
+) -> Result<O, PanicMessage> {
+    let mut dispatcher =
+        Dispatcher { handle_store: HandleStore::new(handle_counters), server: MarkedTypes(server) };
+
+    let globals = dispatcher.server.globals();
+
+    let mut buf = Buffer::new();
+    (globals, input).encode(&mut buf, &mut dispatcher.handle_store);
+
+    buf = strategy.run_bridge_and_client(&mut dispatcher, buf, run_client, force_show_panics);
+
+    Result::decode(&mut &buf[..], &mut dispatcher.handle_store)
+}
+
+impl client::Client<crate::TokenStream, crate::TokenStream> {
+    pub fn run<S>(
+        &self,
+        strategy: &impl ExecutionStrategy,
+        server: S,
+        input: S::TokenStream,
+        force_show_panics: bool,
+    ) -> Result<S::TokenStream, PanicMessage>
+    where
+        S: Server,
+        S::TokenStream: Default,
+    {
+        let client::Client { get_handle_counters, run, _marker } = *self;
+        run_server(
+            strategy,
+            get_handle_counters(),
+            server,
+            <MarkedTypes<S> as Types>::TokenStream::mark(input),
+            run,
+            force_show_panics,
+        )
+        .map(|s| <Option<<MarkedTypes<S> as Types>::TokenStream>>::unmark(s).unwrap_or_default())
+    }
+}
+
+impl client::Client<(crate::TokenStream, crate::TokenStream), crate::TokenStream> {
+    pub fn run<S>(
+        &self,
+        strategy: &impl ExecutionStrategy,
+        server: S,
+        input: S::TokenStream,
+        input2: S::TokenStream,
+        force_show_panics: bool,
+    ) -> Result<S::TokenStream, PanicMessage>
+    where
+        S: Server,
+        S::TokenStream: Default,
+    {
+        let client::Client { get_handle_counters, run, _marker } = *self;
+        run_server(
+            strategy,
+            get_handle_counters(),
+            server,
+            (
+                <MarkedTypes<S> as Types>::TokenStream::mark(input),
+                <MarkedTypes<S> as Types>::TokenStream::mark(input2),
+            ),
+            run,
+            force_show_panics,
+        )
+        .map(|s| <Option<<MarkedTypes<S> as Types>::TokenStream>>::unmark(s).unwrap_or_default())
+    }
+}
diff --git a/library/proc_macro/src/bridge/symbol.rs b/library/proc_macro/src/bridge/symbol.rs
new file mode 100644
index 000000000..930c11145
--- /dev/null
+++ b/library/proc_macro/src/bridge/symbol.rs
@@ -0,0 +1,205 @@
+//! Client-side interner used for symbols.
+//!
+//! This is roughly based on the symbol interner from `rustc_span` and the
+//! DroplessArena from `rustc_arena`. It is unfortunately a complete
+//! copy/re-implementation rather than a dependency as it is difficult to depend
+//! on crates from within `proc_macro`, due to it being built at the same time
+//! as `std`.
+//!
+//! If at some point in the future it becomes easier to add dependencies to
+//! proc_macro, this module should probably be removed or simplified.
+
+use std::cell::RefCell;
+use std::num::NonZeroU32;
+use std::str;
+
+use super::*;
+
+/// Handle for a symbol string stored within the Interner.
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+pub struct Symbol(NonZeroU32);
+
+impl !Send for Symbol {}
+impl !Sync for Symbol {}
+
+impl Symbol {
+    /// Intern a new `Symbol`
+    pub(crate) fn new(string: &str) -> Self {
+        INTERNER.with_borrow_mut(|i| i.intern(string))
+    }
+
+    /// Create a new `Symbol` for an identifier.
+    ///
+    /// Validates and normalizes before converting it to a symbol.
+    pub(crate) fn new_ident(string: &str, is_raw: bool) -> Self {
+        // Fast-path: check if this is a valid ASCII identifier
+        if Self::is_valid_ascii_ident(string.as_bytes()) {
+            if is_raw && !Self::can_be_raw(string) {
+                panic!("`{}` cannot be a raw identifier", string);
+            }
+            return Self::new(string);
+        }
+
+        // Slow-path: If the string is already ASCII we're done, otherwise ask
+        // our server to do this for us over RPC.
+        // We don't need to check for identifiers which can't be raw here,
+        // because all of them are ASCII.
+        if string.is_ascii() {
+            Err(())
+        } else {
+            client::Symbol::normalize_and_validate_ident(string)
+        }
+        .unwrap_or_else(|_| panic!("`{:?}` is not a valid identifier", string))
+    }
+
+    /// Run a callback with the symbol's string value.
+    pub(crate) fn with<R>(self, f: impl FnOnce(&str) -> R) -> R {
+        INTERNER.with_borrow(|i| f(i.get(self)))
+    }
+
+    /// Clear out the thread-local symbol interner, making all previously
+    /// created symbols invalid such that `with` will panic when called on them.
+    pub(crate) fn invalidate_all() {
+        INTERNER.with_borrow_mut(|i| i.clear());
+    }
+
+    /// Check if the ident is a valid ASCII identifier.
+    ///
+    /// This is a short-circuit which is cheap to implement within the
+    /// proc-macro client to avoid RPC when creating simple idents, but may
+    /// return `false` for a valid identifier if it contains non-ASCII
+    /// characters.
+    fn is_valid_ascii_ident(bytes: &[u8]) -> bool {
+        matches!(bytes.first(), Some(b'_' | b'a'..=b'z' | b'A'..=b'Z'))
+            && bytes[1..]
+                .iter()
+                .all(|b| matches!(b, b'_' | b'a'..=b'z' | b'A'..=b'Z' | b'0'..=b'9'))
+    }
+
+    // Mimics the behaviour of `Symbol::can_be_raw` from `rustc_span`
+    fn can_be_raw(string: &str) -> bool {
+        match string {
+            "_" | "super" | "self" | "Self" | "crate" => false,
+            _ => true,
+        }
+    }
+}
+
+impl fmt::Debug for Symbol {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.with(|s| fmt::Debug::fmt(s, f))
+    }
+}
+
+impl ToString for Symbol {
+    fn to_string(&self) -> String {
+        self.with(|s| s.to_owned())
+    }
+}
+
+impl fmt::Display for Symbol {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.with(|s| fmt::Display::fmt(s, f))
+    }
+}
+
+impl<S> Encode<S> for Symbol {
+    fn encode(self, w: &mut Writer, s: &mut S) {
+        self.with(|sym| sym.encode(w, s))
+    }
+}
+
+impl<S: server::Server> DecodeMut<'_, '_, client::HandleStore<server::MarkedTypes<S>>>
+    for Marked<S::Symbol, Symbol>
+{
+    fn decode(r: &mut Reader<'_>, s: &mut client::HandleStore<server::MarkedTypes<S>>) -> Self {
+        Mark::mark(S::intern_symbol(<&str>::decode(r, s)))
+    }
+}
+
+impl<S: server::Server> Encode<client::HandleStore<server::MarkedTypes<S>>>
+    for Marked<S::Symbol, Symbol>
+{
+    fn encode(self, w: &mut Writer, s: &mut client::HandleStore<server::MarkedTypes<S>>) {
+        S::with_symbol_string(&self.unmark(), |sym| sym.encode(w, s))
+    }
+}
+
+impl<S> DecodeMut<'_, '_, S> for Symbol {
+    fn decode(r: &mut Reader<'_>, s: &mut S) -> Self {
+        Symbol::new(<&str>::decode(r, s))
+    }
+}
+
+thread_local! {
+    static INTERNER: RefCell<Interner> = RefCell::new(Interner {
+        arena: arena::Arena::new(),
+        names: fxhash::FxHashMap::default(),
+        strings: Vec::new(),
+        // Start with a base of 1 to make sure that `NonZeroU32` works.
+        sym_base: NonZeroU32::new(1).unwrap(),
+    });
+}
+
+/// Basic interner for a `Symbol`, inspired by the one in `rustc_span`.
+struct Interner {
+    arena: arena::Arena,
+    // SAFETY: These `'static` lifetimes are actually references to data owned
+    // by the Arena. This is safe, as we never return them as static references
+    // from `Interner`.
+    names: fxhash::FxHashMap<&'static str, Symbol>,
+    strings: Vec<&'static str>,
+    // The offset to apply to symbol names stored in the interner. This is used
+    // to ensure that symbol names are not re-used after the interner is
+    // cleared.
+    sym_base: NonZeroU32,
+}
+
+impl Interner {
+    fn intern(&mut self, string: &str) -> Symbol {
+        if let Some(&name) = self.names.get(string) {
+            return name;
+        }
+
+        let name = Symbol(
+            self.sym_base
+                .checked_add(self.strings.len() as u32)
+                .expect("`proc_macro` symbol name overflow"),
+        );
+
+        let string: &str = self.arena.alloc_str(string);
+
+        // SAFETY: we can extend the arena allocation to `'static` because we
+        // only access these while the arena is still alive.
+        let string: &'static str = unsafe { &*(string as *const str) };
+        self.strings.push(string);
+        self.names.insert(string, name);
+        name
+    }
+
+    /// Read a symbol's value from the store while it is held.
+    fn get(&self, symbol: Symbol) -> &str {
+        // NOTE: Subtract out the offset which was added to make the symbol
+        // nonzero and prevent symbol name re-use.
+        let name = symbol
+            .0
+            .get()
+            .checked_sub(self.sym_base.get())
+            .expect("use-after-free of `proc_macro` symbol");
+        self.strings[name as usize]
+    }
+
+    /// Clear all symbols from the store, invalidating them such that `get` will
+    /// panic if they are accessed in the future.
+    fn clear(&mut self) {
+        // NOTE: Be careful not to panic here, as we may be called on the client
+        // when a `catch_unwind` isn't installed.
+        self.sym_base = self.sym_base.saturating_add(self.strings.len() as u32);
+        self.names.clear();
+        self.strings.clear();
+
+        // SAFETY: This is cleared after the names and strings tables are
+        // cleared out, so no references into the arena should remain.
+        self.arena = arena::Arena::new();
+    }
+}