runtime, macros: unify NaN-boxed value API under ValueVariant trait

fix-macros/src/lib.rs

@@ -0,0 +1,39 @@
+extern crate proc_macro;
+
+// Adapted from `__unelide_lifetimes` in `gc-arena-derive`.
+// Licensed under the MIT license.
+// See: https://github.com/kyren/gc-arena
+#[proc_macro]
+pub fn unelide_lifetimes(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    use quote::ToTokens;
+    use syn::parse::{Parse, ParseStream};
+    use syn::visit_mut::VisitMut;
+
+    struct Input {
+        lt: syn::Lifetime,
+        ty: syn::Type,
+    }
+
+    impl Parse for Input {
+        fn parse(input: ParseStream) -> syn::Result<Self> {
+            let lt: syn::Lifetime = input.parse()?;
+            let _: syn::Token!(;) = input.parse()?;
+            let ty: syn::Type = input.parse()?;
+            Ok(Self { lt, ty })
+        }
+    }
+
+    struct UnelideLifetimes(syn::Lifetime);
+
+    impl VisitMut for UnelideLifetimes {
+        fn visit_lifetime_mut(&mut self, i: &mut syn::Lifetime) {
+            if i.ident == "_" {
+                *i = self.0.clone();
+            }
+        }
+    }
+
+    let mut input = syn::parse_macro_input!(input as Input);
+    UnelideLifetimes(input.lt).visit_type_mut(&mut input.ty);
+    input.ty.to_token_stream().into()
+}