Add DocumentData to represent in-memory document with LSP info

crates/rust-analyzer/src/document.rs

@@ -0,0 +1,16 @@
+//! In-memory document information.
+
+/// Information about a document that the Language Client
+// knows about.
+// Its lifetime is driven by the textDocument/didOpen and textDocument/didClose
+// client notifications.
+#[derive(Debug, Clone)]
+pub(crate) struct DocumentData {
+    pub version: Option<i64>,
+}
+
+impl DocumentData {
+    pub fn new(version: i64) -> Self {
+        DocumentData { version: Some(version) }
+    }
+}