Add test for not extracting values for intermediate string concatenations

Author: owen-mc

go/ql/test/extractor-tests/no-intermediate-strings/tst.expected

@@ -0,0 +1,11 @@
+| tst.go:4:6:4:8 | "a" | a |
+| tst.go:4:6:4:14 | ...+... | <no string value stored> |
+| tst.go:4:6:4:20 | ...+... | <no string value stored> |
+| tst.go:4:6:4:26 | ...+... | <no string value stored> |
+| tst.go:4:6:4:32 | ...+... | <no string value stored> |
+| tst.go:4:6:4:38 | ...+... | abcdef |
+| tst.go:4:12:4:14 | "b" | b |
+| tst.go:4:18:4:20 | "c" | c |
+| tst.go:4:24:4:26 | "d" | d |
+| tst.go:4:30:4:32 | "e" | e |
+| tst.go:4:36:4:38 | "f" | f |

go/ql/test/extractor-tests/no-intermediate-strings/tst.go

@@ -0,0 +1,5 @@
+package main
+
+func main() {
+	_ = "a" + "b" + "c" + "d" + "e" + "f"
+}

go/ql/test/extractor-tests/no-intermediate-strings/tst.ql

@@ -0,0 +1,17 @@
+import go
+
+string checkStringValue(Expr e) {
+  result = e.getStringValue()
+  or
+  not exists(e.getStringValue()) and result = "<no string value stored>"
+}
+
+from Expr e
+where e.getType() instanceof StringType
+// We should get string values for `"a"`, `"b"`, `"c"` and `"a" + "b" + "c"
+// but not `"a" + "b"`. In the extractor we avoid storing the value of
+// intermediate strings in string concatenations because in pathological cases
+// this could lead to a quadratic blowup in the size of string values stored,
+// which then causes performance problems when we iterate through all string
+// values.
+select e, checkStringValue(e)