Add test case for the need for fake_read callbacks

Author: eholk

src/test/ui/generator/drop-track-addassign-yield.rs

@@ -0,0 +1,41 @@
+// run-pass
+// compile-flags: -Zdrop-tracking
+
+// Based on addassign-yield.rs, but with drop tracking enabled. Originally we did not implement
+// the fake_read callback on ExprUseVisitor which caused this case to break.
+
+#![feature(generators)]
+
+fn foo() {
+    let _y = static || {
+        let x = &mut 0;
+        *{
+            yield;
+            x
+        } += match String::new() {
+            _ => 0,
+        };
+    };
+
+    // Please don't ever actually write something like this
+    let _z = static || {
+        let x = &mut 0;
+        *{
+            let inner = &mut 1;
+            *{
+                yield ();
+                inner
+            } += match String::new() {
+                _ => 1,
+            };
+            yield;
+            x
+        } += match String::new() {
+            _ => 2,
+        };
+    };
+}
+
+fn main() {
+    foo()
+}