add Never type to allow foreign trait implementations

This is a newtype to workaround `sanakirja::Storable` not being
able to be implemented on `std::convert::Infallible` due to
"foreign trait on foreign type" rules.

database/src/key.rs

@@ -71,7 +71,6 @@ pub struct DupKey<P, S> {
     pub secondary: S,
 }
 
-//---------------------------------------------------------------------------------------------------- Impl
 /// Implement `Key` on most primitive types.
 ///
 /// `Key::DUPLICATE` is always `false`.
@@ -85,12 +84,7 @@ macro_rules! impl_key {
             impl Key for $t {
                 const DUPLICATE: bool = false;
                 type Primary = $t;
-                // This 0 variant enum is unconstructable,
+                type Secondary = crate::never::Never;
-                // and "has the same role as the ! “never” type":
-                // <https://doc.rust-lang.org/std/convert/enum.Infallible.html#future-compatibility>.
-                //
-                // FIXME: Use the `!` type when stable.
-                type Secondary = std::convert::Infallible;
 
                 #[inline(always)]
                 fn primary(self) -> Self::Primary {

database/src/lib.rs

@@ -254,6 +254,9 @@ pub use key::{DupKey, Key};
 
 mod macros;
 
+mod never;
+pub(crate) use never::Never;
+
 pub mod ops;
 
 mod pod;

database/src/never.rs

@@ -0,0 +1,34 @@
+//! The never `!` type, in stable Rust.
+
+//---------------------------------------------------------------------------------------------------- Table
+/// Unconstructable never `!` type.
+///
+/// This is just a newtype around [`Infallible`], it is needed
+/// because `cuprate_database` needs to implement serialization
+/// traits (even if not used) to satisfy traits bounds, but
+/// we cannot implement a foreign trait (e.g [`sanakirja::Storable`])
+/// on a foreign type ([`Infallible`]).
+///
+/// This needs to be a local type to `cuprate_database`,
+/// so unfortunately it cannot be in `cuprate_helper`.
+///
+/// `Infallible` is a 0 variant enum that is unconstructable,
+/// it "has the same role as the ! “never” type":
+/// <https://doc.rust-lang.org/std/convert/enum.Infallible.html#future-compatibility>.
+///
+/// FIXME: Use the `!` type when stable.
+#[derive(Debug)]
+pub(crate) struct Never(std::convert::Infallible);
+
+//---------------------------------------------------------------------------------------------------- Impl
+cfg_if::cfg_if! {
+    if #[cfg(all(feature = "sanakirja", not(feature = "heed")))] {
+        impl sanakirja::Storable for Never {
+            type PageReferences = core::iter::Empty<u64>;
+
+            fn page_references(&self) -> Self::PageReferences {
+                unreachable!()
+            }
+        }
+    }
+}

database/src/pod.rs

@@ -111,7 +111,7 @@ mod private {
     impl<const N: usize> Sealed for [u8; N] {}
 
     impl_sealed! {
-        std::convert::Infallible,
+        crate::never::Never,
         Vec<u8>,
         Box<[u8]>,
         std::sync::Arc<[u8]>,
@@ -133,10 +133,10 @@ mod private {
 }
 
 //---------------------------------------------------------------------------------------------------- Pod Impl (bytes)
-// Implement for `Infallible`.
+// Implement for `Never`.
-// This type is `!` and should never be constructable,
+// This type is never be constructable,
 // so all these functions will just panic.
-impl Pod for std::convert::Infallible {
+impl Pod for crate::never::Never {
     #[cold]
     #[inline(never)]
     fn as_bytes(&self) -> impl AsRef<[u8]> {