Try and fail to make nodes object-safe in node graph

node-graph/gcore/src/value.rs

@@ -0,0 +1,65 @@
+use std::{borrow::Borrow, marker::PhantomData};
+
+use const_default::ConstDefault;
+
+use crate::{Exec, Node};
+
+pub struct IntNode<const N: u32>;
+impl<const N: u32> Node for IntNode<N> {
+    type Input<'o> = ();
+    type Output<'i> = u32;
+    fn eval<'a, I: Borrow<Self::Input<'a>>>(&self, _input: I) -> u32 {
+        N
+    }
+}
+
+#[derive(Default)]
+pub struct ValueNode<T>(T);
+impl<T> Node for ValueNode<T> {
+    type Input<'i> = () where T: 'i;
+    type Output<'o> = &'o T where T: 'o;
+    fn eval<'a, I: Borrow<Self::Input<'a>>>(&'a self, _input: I) -> &T {
+        &self.0
+    }
+}
+impl<T> ValueNode<T> {
+    pub const fn new(value: T) -> ValueNode<T> {
+        ValueNode(value)
+    }
+}
+
+#[derive(Default)]
+pub struct DefaultNode<T>(PhantomData<T>);
+impl<T: Default> Node for DefaultNode<T> {
+    type Input<'i> = () where T: 'i;
+    type Output<'o> = T where T: 'o;
+    fn eval<'a, I: Borrow<Self::Input<'a>>>(&'a self, _input: I) -> T {
+        T::default()
+    }
+}
+impl<T> DefaultNode<T> {
+    pub const fn new() -> DefaultNode<T> {
+        DefaultNode(PhantomData)
+    }
+}
+
+pub struct DefaultRefNode<T>(ValueNode<T>);
+impl<T: 'static> Node for DefaultRefNode<T> {
+    type Input<'i> = () where T: 'i;
+    type Output<'o> = &'o T where T: 'o;
+    fn eval<'a, I: Borrow<Self::Input<'a>>>(&'a self, _input: I) -> &'a T {
+        self.0.exec()
+    }
+}
+#[cfg(feature = "const_default")]
+impl<T: ConstDefault> DefaultRefNode<T> {
+    pub const fn new() -> DefaultRefNode<T> {
+        DefaultRefNode(ValueNode::new(T::DEFAULT))
+    }
+}
+#[cfg(not(feature = "const_default"))]
+impl<T: Default> DefaultRefNode<T> {
+    pub fn new() -> DefaultRefNode<T> {
+        DefaultRefNode(ValueNode::new(T::default()))
+    }
+}