Add Index::DANGLING for two-phase initialization

src/arena.rs

@@ -28,6 +28,15 @@ pub struct Index {
 }
 
 impl Index {
+    /// Represents an `Index` that is unlikely to be in use. This is useful for
+    /// programs that want to do two-phase initialization in safe Rust. Avoid
+    /// using this value to represent the absence of an `Index`: prefer
+    /// `Option<Index>`.
+    pub const DANGLING: Self = Self {
+        slot: u32::MAX,
+        generation: Generation::DANGLING,
+    };
+
     /// Convert this `Index` to an equivalent `u64` representation. Mostly
     /// useful for passing to code outside of Rust.
     #[allow(clippy::integer_arithmetic)]

src/generation.rs

@@ -12,6 +12,11 @@ use core::num::NonZeroU32;
 pub(crate) struct Generation(NonZeroU32);
 
 impl Generation {
+    /// Represents a generation that is unlikely to be used. This is useful for
+    /// programs that want to do two-phase initialization in safe Rust.
+    // This is safe because the maximum value of a u32 is not zero.
+    pub(crate) const DANGLING: Self = unsafe { Generation(NonZeroU32::new_unchecked(u32::MAX)) };
+
     #[must_use]
     pub(crate) fn first() -> Self {
         // This is safe because 1 is not zero.