1 files changed, 40 insertions, 0 deletions
diff --git a/compiler/rustc_ast/src/node_id.rs b/compiler/rustc_ast/src/node_id.rs
new file mode 100644
index 000000000..7f928cb57
--- /dev/null
+++ b/compiler/rustc_ast/src/node_id.rs
@@ -0,0 +1,40 @@
+use rustc_span::LocalExpnId;
+use std::fmt;
+
+rustc_index::newtype_index! {
+    /// Identifies an AST node.
+    ///
+    /// This identifies top-level definitions, expressions, and everything in between.
+    /// This is later turned into [`DefId`] and `HirId` for the HIR.
+    ///
+    /// [`DefId`]: rustc_span::def_id::DefId
+    pub struct NodeId {
+        DEBUG_FORMAT = "NodeId({})"
+    }
+}
+
+rustc_data_structures::define_id_collections!(NodeMap, NodeSet, NodeId);
+
+/// The [`NodeId`] used to represent the root of the crate.
+pub const CRATE_NODE_ID: NodeId = NodeId::from_u32(0);
+
+/// When parsing and at the beginning of doing expansions, we initially give all AST nodes
+/// this dummy AST [`NodeId`]. Then, during a later phase of expansion, we renumber them
+/// to have small, positive IDs.
+pub const DUMMY_NODE_ID: NodeId = NodeId::MAX;
+
+impl NodeId {
+    pub fn placeholder_from_expn_id(expn_id: LocalExpnId) -> Self {
+        NodeId::from_u32(expn_id.as_u32())
+    }
+
+    pub fn placeholder_to_expn_id(self) -> LocalExpnId {
+        LocalExpnId::from_u32(self.as_u32())
+    }
+}
+
+impl fmt::Display for NodeId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(&self.as_u32(), f)
+    }
+}