proc_macro: don't pass a client-side function pointer through the server.

compiler/rustc_builtin_macros/src/proc_macro_harness.rs

@@ -294,50 +294,57 @@ fn mk_decls(cx: &mut ExtCtxt<'_>, macros: &[ProcMacro]) -> P<ast::Item> {
     // that we generate expressions. The position of each NodeId
     // in the 'proc_macros' Vec corresponds to its position
     // in the static array that will be generated
-    let decls = {
+    let decls = macros
-        let local_path = |cx: &ExtCtxt<'_>, sp: Span, name| {
+        .iter()
-            cx.expr_path(cx.path(sp.with_ctxt(span.ctxt()), vec![name]))
+        .map(|m| {
-        };
+            let harness_span = span;
-        let proc_macro_ty_method_path = |cx: &ExtCtxt<'_>, method| {
+            let span = match m {
-            cx.expr_path(cx.path(span, vec![proc_macro, bridge, client, proc_macro_ty, method]))
+                ProcMacro::Derive(m) => m.span,
-        };
+                ProcMacro::Attr(m) | ProcMacro::Bang(m) => m.span,
-        let attr_or_bang = |cx: &mut ExtCtxt<'_>, ca: &ProcMacroDef, ident| {
+            };
-            cx.resolver.declare_proc_macro(ca.id);
+            let local_path = |cx: &ExtCtxt<'_>, name| cx.expr_path(cx.path(span, vec![name]));
-            cx.expr_call(
+            let proc_macro_ty_method_path = |cx: &ExtCtxt<'_>, method| {
-                span,
+                cx.expr_path(cx.path(
-                proc_macro_ty_method_path(cx, ident),
+                    span.with_ctxt(harness_span.ctxt()),
-                vec![
+                    vec![proc_macro, bridge, client, proc_macro_ty, method],
-                    cx.expr_str(ca.span, ca.function_name.name),
+                ))
-                    local_path(cx, ca.span, ca.function_name),
+            };
-                ],
+            match m {
-            )
-        };
-        macros
-            .iter()
-            .map(|m| match m {
                 ProcMacro::Derive(cd) => {
                     cx.resolver.declare_proc_macro(cd.id);
                     cx.expr_call(
                         span,
                         proc_macro_ty_method_path(cx, custom_derive),
                         vec![
-                            cx.expr_str(cd.span, cd.trait_name),
+                            cx.expr_str(span, cd.trait_name),
                             cx.expr_vec_slice(
                                 span,
-                                cd.attrs
+                                cd.attrs.iter().map(|&s| cx.expr_str(span, s)).collect::<Vec<_>>(),
-                                    .iter()
-                                    .map(|&s| cx.expr_str(cd.span, s))
-                                    .collect::<Vec<_>>(),
                             ),
-                            local_path(cx, cd.span, cd.function_name),
+                            local_path(cx, cd.function_name),
                         ],
                     )
                 }
-                ProcMacro::Attr(ca) => attr_or_bang(cx, &ca, attr),
+                ProcMacro::Attr(ca) | ProcMacro::Bang(ca) => {
-                ProcMacro::Bang(ca) => attr_or_bang(cx, &ca, bang),
+                    cx.resolver.declare_proc_macro(ca.id);
-            })
+                    let ident = match m {
-            .collect()
+                        ProcMacro::Attr(_) => attr,
-    };
+                        ProcMacro::Bang(_) => bang,
+                        ProcMacro::Derive(_) => unreachable!(),
+                    };
+
+                    cx.expr_call(
+                        span,
+                        proc_macro_ty_method_path(cx, ident),
+                        vec![
+                            cx.expr_str(span, ca.function_name.name),
+                            local_path(cx, ca.function_name),
+                        ],
+                    )
+                }
+            }
+        })
+        .collect();
 
     let decls_static = cx
         .item_static(

compiler/rustc_expand/src/proc_macro.rs

@@ -14,7 +14,7 @@ use rustc_span::{Span, DUMMY_SP};
 const EXEC_STRATEGY: pm::bridge::server::SameThread = pm::bridge::server::SameThread;
 
 pub struct BangProcMacro {
-    pub client: pm::bridge::client::Client<fn(pm::TokenStream) -> pm::TokenStream>,
+    pub client: pm::bridge::client::Client<pm::TokenStream, pm::TokenStream>,
 }
 
 impl base::BangProcMacro for BangProcMacro {
@@ -42,7 +42,7 @@ impl base::BangProcMacro for BangProcMacro {
 }
 
 pub struct AttrProcMacro {
-    pub client: pm::bridge::client::Client<fn(pm::TokenStream, pm::TokenStream) -> pm::TokenStream>,
+    pub client: pm::bridge::client::Client<(pm::TokenStream, pm::TokenStream), pm::TokenStream>,
 }
 
 impl base::AttrProcMacro for AttrProcMacro {
@@ -73,7 +73,7 @@ impl base::AttrProcMacro for AttrProcMacro {
 }
 
 pub struct DeriveProcMacro {
-    pub client: pm::bridge::client::Client<fn(pm::TokenStream) -> pm::TokenStream>,
+    pub client: pm::bridge::client::Client<pm::TokenStream, pm::TokenStream>,
 }
 
 impl MultiItemModifier for DeriveProcMacro {

library/proc_macro/src/bridge/client.rs

@@ -357,22 +357,33 @@ impl Bridge<'_> {
     }
 }
 
-/// A client-side "global object" (usually a function pointer),
+/// A client-side RPC entry-point, which may be using a different `proc_macro`
-/// which may be using a different `proc_macro` from the one
+/// from the one used by the server, but can be invoked compatibly.
-/// used by the server, but can be interacted with compatibly.
 ///
-/// N.B., `F` must have FFI-friendly memory layout (e.g., a pointer).
+/// Note that the (phantom) `I` ("input") and `O` ("output") type parameters
-/// The call ABI of function pointers used for `F` doesn't
+/// decorate the `Client<I, O>` with the RPC "interface" of the entry-point, but
-/// need to match between server and client, since it's only
+/// do not themselves participate in ABI, at all, only facilitate type-checking.
-/// passed between them and (eventually) called by the client.
+///
+/// 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)]
-#[derive(Copy, Clone)]
+pub struct Client<I, O> {
-pub struct Client<F> {
     // 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(Bridge<'_>, F) -> Buffer,
+
-    pub(super) f: F,
+    pub(super) run: extern "C" fn(Bridge<'_>) -> 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
+    }
 }
 
 /// Client-side helper for handling client panics, entering the bridge,
@@ -419,31 +430,31 @@ fn run_client<A: for<'a, 's> DecodeMut<'a, 's, ()>, R: Encode<()>>(
     buf
 }
 
-impl Client<fn(crate::TokenStream) -> crate::TokenStream> {
+impl Client<crate::TokenStream, crate::TokenStream> {
-    pub const fn expand1(f: fn(crate::TokenStream) -> crate::TokenStream) -> Self {
+    pub const fn expand1(f: impl Fn(crate::TokenStream) -> crate::TokenStream + Copy) -> Self {
-        extern "C" fn run(
+        Client {
-            bridge: Bridge<'_>,
+            get_handle_counters: HandleCounters::get,
-            f: impl FnOnce(crate::TokenStream) -> crate::TokenStream,
+            run: super::selfless_reify::reify_to_extern_c_fn_hrt_bridge(move |bridge| {
-        ) -> Buffer {
+                run_client(bridge, |input| f(crate::TokenStream(input)).0)
-            run_client(bridge, |input| f(crate::TokenStream(input)).0)
+            }),
+            _marker: PhantomData,
         }
-        Client { get_handle_counters: HandleCounters::get, run, f }
     }
 }
 
-impl Client<fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream> {
+impl Client<(crate::TokenStream, crate::TokenStream), crate::TokenStream> {
     pub const fn expand2(
-        f: fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream,
+        f: impl Fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream + Copy,
     ) -> Self {
-        extern "C" fn run(
+        Client {
-            bridge: Bridge<'_>,
+            get_handle_counters: HandleCounters::get,
-            f: impl FnOnce(crate::TokenStream, crate::TokenStream) -> crate::TokenStream,
+            run: super::selfless_reify::reify_to_extern_c_fn_hrt_bridge(move |bridge| {
-        ) -> Buffer {
+                run_client(bridge, |(input, input2)| {
-            run_client(bridge, |(input, input2)| {
+                    f(crate::TokenStream(input), crate::TokenStream(input2)).0
-                f(crate::TokenStream(input), crate::TokenStream(input2)).0
+                })
-            })
+            }),
+            _marker: PhantomData,
         }
-        Client { get_handle_counters: HandleCounters::get, run, f }
     }
 }
 
@@ -453,17 +464,17 @@ pub enum ProcMacro {
     CustomDerive {
         trait_name: &'static str,
         attributes: &'static [&'static str],
-        client: Client<fn(crate::TokenStream) -> crate::TokenStream>,
+        client: Client<crate::TokenStream, crate::TokenStream>,
     },
 
     Attr {
         name: &'static str,
-        client: Client<fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream>,
+        client: Client<(crate::TokenStream, crate::TokenStream), crate::TokenStream>,
     },
 
     Bang {
         name: &'static str,
-        client: Client<fn(crate::TokenStream) -> crate::TokenStream>,
+        client: Client<crate::TokenStream, crate::TokenStream>,
     },
 }
 
@@ -479,21 +490,21 @@ impl ProcMacro {
     pub const fn custom_derive(
         trait_name: &'static str,
         attributes: &'static [&'static str],
-        expand: fn(crate::TokenStream) -> crate::TokenStream,
+        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: fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream,
+        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: fn(crate::TokenStream) -> crate::TokenStream,
+        expand: impl Fn(crate::TokenStream) -> crate::TokenStream + Copy,
     ) -> Self {
         ProcMacro::Bang { name, client: Client::expand1(expand) }
     }

library/proc_macro/src/bridge/mod.rs

@@ -208,6 +208,8 @@ mod handle;
 mod rpc;
 #[allow(unsafe_code)]
 mod scoped_cell;
+#[allow(unsafe_code)]
+mod selfless_reify;
 #[forbid(unsafe_code)]
 pub mod server;
 

library/proc_macro/src/bridge/selfless_reify.rs

@@ -0,0 +1,83 @@
+//! 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(Bridge<'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 `Bridge`, that'd help.
+    fn reify_to_extern_c_fn_hrt_bridge<R> for extern "C" fn(bridge: super::Bridge<'_>) -> R;
+}

library/proc_macro/src/bridge/server.rs

@@ -118,12 +118,11 @@ macro_rules! define_dispatcher_impl {
 with_api!(Self, self_, define_dispatcher_impl);
 
 pub trait ExecutionStrategy {
-    fn run_bridge_and_client<D: Copy + Send + 'static>(
+    fn run_bridge_and_client(
         &self,
         dispatcher: &mut impl DispatcherTrait,
         input: Buffer,
-        run_client: extern "C" fn(Bridge<'_>, D) -> Buffer,
+        run_client: extern "C" fn(Bridge<'_>) -> Buffer,
-        client_data: D,
         force_show_panics: bool,
     ) -> Buffer;
 }
@@ -131,25 +130,21 @@ pub trait ExecutionStrategy {
 pub struct SameThread;
 
 impl ExecutionStrategy for SameThread {
-    fn run_bridge_and_client<D: Copy + Send + 'static>(
+    fn run_bridge_and_client(
         &self,
         dispatcher: &mut impl DispatcherTrait,
         input: Buffer,
-        run_client: extern "C" fn(Bridge<'_>, D) -> Buffer,
+        run_client: extern "C" fn(Bridge<'_>) -> Buffer,
-        client_data: D,
         force_show_panics: bool,
     ) -> Buffer {
         let mut dispatch = |buf| dispatcher.dispatch(buf);
 
-        run_client(
+        run_client(Bridge {
-            Bridge {
+            cached_buffer: input,
-                cached_buffer: input,
+            dispatch: (&mut dispatch).into(),
-                dispatch: (&mut dispatch).into(),
+            force_show_panics,
-                force_show_panics,
+            _marker: marker::PhantomData,
-                _marker: marker::PhantomData,
+        })
-            },
-            client_data,
-        )
     }
 }
 
@@ -159,12 +154,11 @@ impl ExecutionStrategy for SameThread {
 pub struct CrossThread1;
 
 impl ExecutionStrategy for CrossThread1 {
-    fn run_bridge_and_client<D: Copy + Send + 'static>(
+    fn run_bridge_and_client(
         &self,
         dispatcher: &mut impl DispatcherTrait,
         input: Buffer,
-        run_client: extern "C" fn(Bridge<'_>, D) -> Buffer,
+        run_client: extern "C" fn(Bridge<'_>) -> Buffer,
-        client_data: D,
         force_show_panics: bool,
     ) -> Buffer {
         use std::sync::mpsc::channel;
@@ -178,15 +172,12 @@ impl ExecutionStrategy for CrossThread1 {
                 res_rx.recv().unwrap()
             };
 
-            run_client(
+            run_client(Bridge {
-                Bridge {
+                cached_buffer: input,
-                    cached_buffer: input,
+                dispatch: (&mut dispatch).into(),
-                    dispatch: (&mut dispatch).into(),
+                force_show_panics,
-                    force_show_panics,
+                _marker: marker::PhantomData,
-                    _marker: marker::PhantomData,
+            })
-                },
-                client_data,
-            )
         });
 
         for b in req_rx {
@@ -200,12 +191,11 @@ impl ExecutionStrategy for CrossThread1 {
 pub struct CrossThread2;
 
 impl ExecutionStrategy for CrossThread2 {
-    fn run_bridge_and_client<D: Copy + Send + 'static>(
+    fn run_bridge_and_client(
         &self,
         dispatcher: &mut impl DispatcherTrait,
         input: Buffer,
-        run_client: extern "C" fn(Bridge<'_>, D) -> Buffer,
+        run_client: extern "C" fn(Bridge<'_>) -> Buffer,
-        client_data: D,
         force_show_panics: bool,
     ) -> Buffer {
         use std::sync::{Arc, Mutex};
@@ -231,15 +221,12 @@ impl ExecutionStrategy for CrossThread2 {
                 }
             };
 
-            let r = run_client(
+            let r = run_client(Bridge {
-                Bridge {
+                cached_buffer: input,
-                    cached_buffer: input,
+                dispatch: (&mut dispatch).into(),
-                    dispatch: (&mut dispatch).into(),
+                force_show_panics,
-                    force_show_panics,
+                _marker: marker::PhantomData,
-                    _marker: marker::PhantomData,
+            });
-                },
-                client_data,
-            );
 
             // Wake up the server so it can exit the dispatch loop.
             drop(state2);
@@ -268,14 +255,12 @@ fn run_server<
     S: Server,
     I: Encode<HandleStore<MarkedTypes<S>>>,
     O: for<'a, 's> DecodeMut<'a, 's, HandleStore<MarkedTypes<S>>>,
-    D: Copy + Send + 'static,
 >(
     strategy: &impl ExecutionStrategy,
     handle_counters: &'static client::HandleCounters,
     server: S,
     input: I,
-    run_client: extern "C" fn(Bridge<'_>, D) -> Buffer,
+    run_client: extern "C" fn(Bridge<'_>) -> Buffer,
-    client_data: D,
     force_show_panics: bool,
 ) -> Result<O, PanicMessage> {
     let mut dispatcher =
@@ -284,18 +269,12 @@ fn run_server<
     let mut buf = Buffer::new();
     input.encode(&mut buf, &mut dispatcher.handle_store);
 
-    buf = strategy.run_bridge_and_client(
+    buf = strategy.run_bridge_and_client(&mut dispatcher, buf, run_client, force_show_panics);
-        &mut dispatcher,
-        buf,
-        run_client,
-        client_data,
-        force_show_panics,
-    );
 
     Result::decode(&mut &buf[..], &mut dispatcher.handle_store)
 }
 
-impl client::Client<fn(crate::TokenStream) -> crate::TokenStream> {
+impl client::Client<crate::TokenStream, crate::TokenStream> {
     pub fn run<S: Server>(
         &self,
         strategy: &impl ExecutionStrategy,
@@ -303,21 +282,20 @@ impl client::Client<fn(crate::TokenStream) -> crate::TokenStream> {
         input: S::TokenStream,
         force_show_panics: bool,
     ) -> Result<S::TokenStream, PanicMessage> {
-        let client::Client { get_handle_counters, run, f } = *self;
+        let client::Client { get_handle_counters, run, _marker } = *self;
         run_server(
             strategy,
             get_handle_counters(),
             server,
             <MarkedTypes<S> as Types>::TokenStream::mark(input),
             run,
-            f,
             force_show_panics,
         )
         .map(<MarkedTypes<S> as Types>::TokenStream::unmark)
     }
 }
 
-impl client::Client<fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream> {
+impl client::Client<(crate::TokenStream, crate::TokenStream), crate::TokenStream> {
     pub fn run<S: Server>(
         &self,
         strategy: &impl ExecutionStrategy,
@@ -326,7 +304,7 @@ impl client::Client<fn(crate::TokenStream, crate::TokenStream) -> crate::TokenSt
         input2: S::TokenStream,
         force_show_panics: bool,
     ) -> Result<S::TokenStream, PanicMessage> {
-        let client::Client { get_handle_counters, run, f } = *self;
+        let client::Client { get_handle_counters, run, _marker } = *self;
         run_server(
             strategy,
             get_handle_counters(),
@@ -336,7 +314,6 @@ impl client::Client<fn(crate::TokenStream, crate::TokenStream) -> crate::TokenSt
                 <MarkedTypes<S> as Types>::TokenStream::mark(input2),
             ),
             run,
-            f,
             force_show_panics,
         )
         .map(<MarkedTypes<S> as Types>::TokenStream::unmark)

src/test/ui/proc-macro/signature.rs

@@ -8,6 +8,6 @@ extern crate proc_macro;
 
 #[proc_macro_derive(A)]
 pub unsafe extern "C" fn foo(a: i32, b: u32) -> u32 {
-    //~^ ERROR: mismatched types
+    //~^ ERROR: expected a `Fn<(proc_macro::TokenStream,)>` closure, found `unsafe extern "C" fn
     loop {}
 }

src/test/ui/proc-macro/signature.stderr

@@ -1,20 +1,23 @@
-error[E0308]: mismatched types
+error[E0277]: expected a `Fn<(proc_macro::TokenStream,)>` closure, found `unsafe extern "C" fn(i32, u32) -> u32 {foo}`
   --> $DIR/signature.rs:10:1
    |
 LL | / pub unsafe extern "C" fn foo(a: i32, b: u32) -> u32 {
 LL | |
 LL | |     loop {}
 LL | | }
-   | |_^ expected normal fn, found unsafe fn
+   | | ^
+   | | |
+   | |_call the function in a closure: `|| unsafe { /* code */ }`
+   |   required by a bound introduced by this call
    |
-   = note: expected fn pointer `fn(proc_macro::TokenStream) -> proc_macro::TokenStream`
+   = help: the trait `Fn<(proc_macro::TokenStream,)>` is not implemented for `unsafe extern "C" fn(i32, u32) -> u32 {foo}`
-                 found fn item `unsafe extern "C" fn(i32, u32) -> u32 {foo}`
+   = note: unsafe function cannot be called generically without an unsafe block
-note: associated function defined here
+note: required by a bound in `ProcMacro::custom_derive`
   --> $SRC_DIR/proc_macro/src/bridge/client.rs:LL:COL
    |
-LL |     pub const fn custom_derive(
+LL |         expand: impl Fn(crate::TokenStream) -> crate::TokenStream + Copy,
-   |                  ^^^^^^^^^^^^^
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `ProcMacro::custom_derive`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0277`.