More property tests

Also put proper comparison methods on `DerivedPath` and
`NixStringContextElem`, which is needed for the tests but good in
general.

src/libexpr/tests/local.mk

@@ -14,6 +14,6 @@ libexpr-tests_SOURCES := \
 
 libexpr-tests_CXXFLAGS += -I src/libexpr -I src/libutil -I src/libstore -I src/libexpr/tests
 
-libexpr-tests_LIBS = libstore-tests libexpr libutil libstore libfetchers
+libexpr-tests_LIBS = libstore-tests libutils-tests libexpr libutil libstore libfetchers
 
 libexpr-tests_LDFLAGS := $(GTEST_LIBS) -lgmock

src/libexpr/tests/value/context.cc

@@ -1,6 +1,10 @@
-#include "value/context.hh"
+#include <nlohmann/json.hpp>
+#include <gtest/gtest.h>
+#include <rapidcheck/gtest.h>
 
+#include "tests/path.hh"
 #include "tests/libexpr.hh"
+#include "tests/value/context.hh"
 
 namespace nix {
 
@@ -70,3 +74,39 @@ TEST_F(NixStringContextElemTest, built) {
 }
 
 }
+
+namespace rc {
+using namespace nix;
+
+Gen<NixStringContextElem> Arbitrary<NixStringContextElem>::arbitrary()
+{
+    switch (*gen::inRange<uint8_t>(0, 2)) {
+    case 0:
+        return gen::just((NixStringContextElem) NixStringContextElem::Opaque {
+            .path = *gen::arbitrary<StorePath>(),
+        });
+    case 1:
+        return gen::just((NixStringContextElem) NixStringContextElem::DrvDeep {
+            .drvPath = *gen::arbitrary<StorePath>(),
+        });
+    default:
+        return gen::just((NixStringContextElem) NixStringContextElem::Built {
+            .drvPath = *gen::arbitrary<StorePath>(),
+            .output = (*gen::arbitrary<StorePathName>()).name,
+        });
+    }
+}
+
+}
+
+namespace nix {
+
+RC_GTEST_FIXTURE_PROP(
+    NixStringContextElemTest,
+    prop_round_rip,
+    (const NixStringContextElem & o))
+{
+    RC_ASSERT(o == NixStringContextElem::parse(store(), o.to_string(store())));
+}
+
+}

src/libexpr/tests/value/context.hh

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <rapidcheck/gen/Arbitrary.h>
+
+#include <value/context.hh>
+
+namespace rc {
+using namespace nix;
+
+template<>
+struct Arbitrary<NixStringContextElem> {
+    static Gen<NixStringContextElem> arbitrary();
+};
+
+}

src/libexpr/value/context.hh

@@ -1,9 +1,10 @@
 #pragma once
 
 #include "util.hh"
+#include "comparator.hh"
 #include "path.hh"
 
-#include <optional>
+#include <variant>
 
 #include <nlohmann/json_fwd.hpp>
 
@@ -31,7 +32,9 @@ class Store;
    Encoded as just the path: ‘<path>’.
 */
 struct NixStringContextElem_Opaque {
-   StorePath path;
+    StorePath path;
+
+    GENERATE_CMP(NixStringContextElem_Opaque, me->path);
 };
 
 /* Path to a derivation and its entire build closure.
@@ -43,7 +46,9 @@ struct NixStringContextElem_Opaque {
    Encoded in the form ‘=<drvPath>’.
 */
 struct NixStringContextElem_DrvDeep {
-   StorePath drvPath;
+    StorePath drvPath;
+
+    GENERATE_CMP(NixStringContextElem_DrvDeep, me->drvPath);
 };
 
 /* Derivation output.
@@ -51,8 +56,10 @@ struct NixStringContextElem_DrvDeep {
    Encoded in the form ‘!<output>!<drvPath>’.
 */
 struct NixStringContextElem_Built {
-   StorePath drvPath;
-   std::string output;
+    StorePath drvPath;
+    std::string output;
+
+    GENERATE_CMP(NixStringContextElem_Built, me->drvPath, me->output);
 };
 
 using _NixStringContextElem_Raw = std::variant<