[platform] add Symbol (IDEA-190162)

platform/core-api/src/com/intellij/model/Symbol.java

@@ -0,0 +1,22 @@
+// Copyright 2000-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
+package com.intellij.model;
+
+import static org.jetbrains.annotations.ApiStatus.Experimental;
+
+/**
+ * An element in some model, e.g. language model or framework model.
+ * <p/>
+ * Symbol may be backed by a {@link com.intellij.psi.PsiElement PsiElement} but is not required to. <br/>
+ * Symbol may be bound to a {@link com.intellij.openapi.project.Project Project} but is not required to.
+ * <p/>
+ * Examples:
+ * <ul>
+ * <li>Java local variable is a symbol in Java language model, it's backed by some PsiElement</li>
+ * <li>Spring Bean is a symbol in Spring framework model, it's defined on-the-fly by framework support</li>
+ * <li>database column is a Symbol not backed by PsiElement (defined by data source) and not bound to a Project</li>
+ * </ul>
+ */
+@Experimental
+public interface Symbol {
+
+}