Make Graph::add_node private again

Exposing `add_node` as a public method creates a hazard as it allows callers to
add nodes to the graph without post-processing steps that other methods do (eg.
`add_op`). Make this method private again and add a more specific
`add_constant_node` alternative to handle the single use case for it outside the
graph module.

src/graph.rs

@@ -711,7 +711,11 @@ impl Graph {
         captures
     }
 
-    pub fn add_node(&mut self, node: Node) -> NodeId {
+    /// Add a node to the graph and return its ID.
+    ///
+    /// This contains the common logic for adding different types of node to
+    /// the graph.
+    fn add_node(&mut self, node: Node) -> NodeId {
         let node_id = NodeId::from_u32(self.nodes.len() as u32);
         self.nodes.push(node);
 
@@ -790,7 +794,7 @@ impl Graph {
         (op_node_id, op_out_id)
     }
 
-    /// Add a constant node to the graph.
+    /// Convert `value` to a constant node and add it to the graph.
     ///
     /// `name` is an identifier for this node that is used in debug messages etc.
     ///
@@ -800,12 +804,20 @@ impl Graph {
         V: Into<ConstantNodeData<T>>,
         ConstantNode<T>: Into<Constant>,
     {
-        let node = ConstantNode {
-            name: name.map(|s| s.to_owned()),
-            data: value.into(),
-        };
+        self.add_constant_node(
+            ConstantNode {
+                name: name.map(|s| s.to_owned()),
+                data: value.into(),
+            }
+            .into(),
+        )
+    }
 
-        self.add_node(Node::Constant(node.into()))
+    /// Add a constant node to the graph.
+    ///
+    /// Returns the ID of the added node.
+    pub fn add_constant_node(&mut self, node: Constant) -> NodeId {
+        self.add_node(Node::Constant(node))
     }
 
     /// Add a value node to the graph.

src/optimize.rs

@@ -78,6 +78,11 @@ impl GraphMutator {
         self.graph.add_constant(name, value)
     }
 
+    /// Add a new constant value to the graph.
+    fn add_constant_node(&mut self, const_node: Constant) -> NodeId {
+        self.graph.add_constant_node(const_node)
+    }
+
     /// Add a new operator to the graph with a single output node.
     ///
     /// `op_output_id` specifies the ID of the output node. If not specified,
@@ -105,11 +110,6 @@ impl GraphMutator {
         op_output_id
     }
 
-    /// Add a new node to the graph.
-    fn add_node(&mut self, node: Node) -> NodeId {
-        self.graph.add_node(node)
-    }
-
     /// Return a reference to the graph.
     ///
     /// Note there is no mutable variant of this method. All graph updates must
@@ -330,7 +330,7 @@ impl GraphOptimizer {
         let mut new_captures: FxHashSet<_> = graph.graph().captures().iter().copied().collect();
 
         for (capture_id, local_const) in captured_constants {
-            let const_id = graph.add_node(Node::Constant(local_const));
+            let const_id = graph.add_constant_node(local_const);
             new_captures.remove(&capture_id);
             graph.replace_value(capture_id, const_id);
         }