feat(schema): add /thunk edges and clean up ref/writes (#5848)

* feat(schema): add /thunk edges and clean up ref/writes

* feat(schema): add java thunk example

kythe/docs/schema/schema.txt

@@ -1216,6 +1216,57 @@ struct S { static int foo(); };
 C<S> cs;
 --------------------------------------------------------------------------------
 
+[[refcallthunk]]
+ref/call/thunk
+~~~~~~~~~~~~~~
+
+Brief description::
+  A *ref/call/thunk* F if A is an anchor that may allow F to be called in the
+  future.
+Points from::
+  anchors
+Points toward::
+  <<function,functions>>
+Ordinals are used::
+  never
+See also::
+  <<refcall,[ref/call]>>
+Notes::
+  This is an experimental definition and is expected to undergo refinement.
+
+This edge is meant to represent delayed calls to functions, such as when a
+thread procedure is passed to a threading library.
+
+[kythe,C++,"In some contexts, uses of functions as values are thunks."]
+--------------------------------------------------------------------------------
+//- @f defines/binding FnF
+void f();
+//- @g defines/binding FnG
+void g(void (*h)());
+void i() {
+    //- @f ref FnF
+    //- @"g(f)" ref/call FnG
+    //- // @f ref/call/thunk FnF -- currently unsupported
+    //- !{ @f ref/call FnF }
+    g(f);
+    //- @f ref FnF
+    //- // @f ref/call/thunk FnF -- currently unsupported
+    auto j = f;
+}
+--------------------------------------------------------------------------------
+
+[kythe,Java,"Java method references make thunks."]
+--------------------------------------------------------------------------------
+import java.util.function.Consumer;
+public class E {
+  //- @f defines/binding FnF
+  void f(Object v) { }
+  //- @f ref FnF
+  //- // @"this::f" ref/call/thunk FnF -- currently unsupported
+  Consumer<Object> myF = this::f;
+}
+--------------------------------------------------------------------------------
+
 [[refdoc]]
 ref/doc
 ~~~~~~~
@@ -1496,8 +1547,6 @@ Points toward::
   semantic nodes
 Ordinals are used::
   never
-Notes::
-  This is an experimental definition and is expected to undergo refinement.
 
 [kythe,C++,"Various assignment expressions are writes"]
 --------------------------------------------------------------------------------
@@ -1516,6 +1565,44 @@ void f() {
 }
 --------------------------------------------------------------------------------
 
+[[refwritesthunk]]
+ref/writes/thunk
+~~~~~~~~~~~~~~~~
+
+Brief description::
+  A *ref/writes/thunk* B if A refers to B in an expression that is likely to
+  cause a later update the value of B.
+Commonly arises from::
+  value uses of setter functions, alias passing
+Points from::
+  anchors
+Points toward::
+  semantic nodes
+Ordinals are used::
+  never
+See also::
+  <<refwrites,[ref/writes]>>
+Notes::
+  This is an experimental definition and is expected to undergo refinement.
+
+[kythe,C++,"Various assignment expressions are thunkful writes"]
+--------------------------------------------------------------------------------
+void f(int* x_out, int& y_out, const int& z);
+void g() {
+  //- @x defines/binding VarX
+  //- @y defines/binding VarY
+  //- @z defines/binding VarZ
+  int x, y, z;
+  //- @x ref VarX
+  //- @y ref VarY
+  //- @z ref VarZ
+  //- // @x ref/writes/thunk VarX -- currently unsupported
+  //- // @y ref/writes/thunk VarY -- currently unsupported
+  //- !{ @z ref/writes/thunk VarZ }
+  f(&x, y, z);
+}
+--------------------------------------------------------------------------------
+
 [[satisfies]]
 satisfies
 ~~~~~~~~~