Update library functions with stability attributes

This may not be strictly minimal, but all unstable functions also need a
`rustc_const_unstable` attribute.

library/proc_macro/src/bridge/client.rs

@@ -401,6 +401,7 @@ fn run_client<A: for<'a, 's> DecodeMut<'a, 's, ()>, R: Encode<()>>(
 }
 
 impl Client<fn(crate::TokenStream) -> crate::TokenStream> {
+    #[allow_internal_unstable(const_fn)]
     pub const fn expand1(f: fn(crate::TokenStream) -> crate::TokenStream) -> Self {
         extern "C" fn run(
             bridge: Bridge<'_>,
@@ -413,6 +414,7 @@ impl Client<fn(crate::TokenStream) -> crate::TokenStream> {
 }
 
 impl Client<fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream> {
+    #[allow_internal_unstable(const_fn)]
     pub const fn expand2(
         f: fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream,
     ) -> Self {
@@ -457,6 +459,7 @@ impl ProcMacro {
         }
     }
 
+    #[allow_internal_unstable(const_fn)]
     pub const fn custom_derive(
         trait_name: &'static str,
         attributes: &'static [&'static str],
@@ -465,6 +468,7 @@ impl ProcMacro {
         ProcMacro::CustomDerive { trait_name, attributes, client: Client::expand1(expand) }
     }
 
+    #[allow_internal_unstable(const_fn)]
     pub const fn attr(
         name: &'static str,
         expand: fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream,
@@ -472,6 +476,7 @@ impl ProcMacro {
         ProcMacro::Attr { name, client: Client::expand2(expand) }
     }
 
+    #[allow_internal_unstable(const_fn)]
     pub const fn bang(
         name: &'static str,
         expand: fn(crate::TokenStream) -> crate::TokenStream,

library/proc_macro/src/bridge/scoped_cell.rs

@@ -35,6 +35,7 @@ impl<'a, 'b, T: LambdaL> DerefMut for RefMutL<'a, 'b, T> {
 pub struct ScopedCell<T: LambdaL>(Cell<<T as ApplyL<'static>>::Out>);
 
 impl<T: LambdaL> ScopedCell<T> {
+    #[allow_internal_unstable(const_fn)]
     pub const fn new(value: <T as ApplyL<'static>>::Out) -> Self {
         ScopedCell(Cell::new(value))
     }

library/proc_macro/src/lib.rs

@@ -20,6 +20,7 @@
 )]
 #![feature(nll)]
 #![feature(staged_api)]
+#![feature(const_fn)]
 #![feature(allow_internal_unstable)]
 #![feature(decl_macro)]
 #![feature(extern_types)]