Improve checked json casting (#10087)

This introduces new utility functions to get elements from JSON — in an ergonomic way and with nice error messages if the expected type does not match.

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>

tests/unit/libfetchers/local.mk

@@ -0,0 +1,32 @@
+check: libfetchers-tests_RUN
+
+programs += libfetchers-tests
+
+libfetchers-tests_NAME = libnixfetchers-tests
+
+libfetchers-tests_ENV := _NIX_TEST_UNIT_DATA=$(d)/data
+
+libfetchers-tests_DIR := $(d)
+
+ifeq ($(INSTALL_UNIT_TESTS), yes)
+  libfetchers-tests_INSTALL_DIR := $(checkbindir)
+else
+  libfetchers-tests_INSTALL_DIR :=
+endif
+
+libfetchers-tests_SOURCES := $(wildcard $(d)/*.cc)
+
+libfetchers-tests_EXTRA_INCLUDES = \
+    -I tests/unit/libstore-support \
+    -I tests/unit/libutil-support \
+    $(INCLUDE_libfetchers) \
+    $(INCLUDE_libstore) \
+    $(INCLUDE_libutil)
+
+libfetchers-tests_CXXFLAGS += $(libfetchers-tests_EXTRA_INCLUDES)
+
+libfetchers-tests_LIBS = \
+    libstore-test-support libutil-test-support \
+    libfetchers libstore libutil
+
+libfetchers-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS)

tests/unit/libfetchers/public-key.cc

@@ -0,0 +1,18 @@
+#include <gtest/gtest.h>
+#include "fetchers.hh"
+#include "json-utils.hh"
+
+namespace nix {
+    TEST(PublicKey, jsonSerialization) {
+        auto json = nlohmann::json(fetchers::PublicKey { .key = "ABCDE" });
+
+        ASSERT_EQ(json, R"({ "key": "ABCDE", "type": "ssh-ed25519" })"_json);
+    }
+    TEST(PublicKey, jsonDeserialization) {
+        auto pubKeyJson = R"({ "key": "ABCDE", "type": "ssh-ed25519" })"_json;
+        fetchers::PublicKey pubKey = pubKeyJson;
+
+        ASSERT_EQ(pubKey.key, "ABCDE");
+        ASSERT_EQ(pubKey.type, "ssh-ed25519");
+    }
+}