Provide a structured JSON serialisation of hashes

src/libstore/unix/build/local-derivation-goal.cc

@@ -2659,8 +2659,8 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
                     act->result(resHashMismatch,
                         {
                             {"storePath", worker.store.printStorePath(drvPath)},
-                            {"wanted", wanted.to_string(HashFormat::SRI, true)},
+                            {"wanted", wanted},
-                            {"got", got.to_string(HashFormat::SRI, true)},
+                            {"got", got},
                         });
                 }
                 if (!newInfo0.references.empty()) {

src/libutil/hash.cc

@@ -14,6 +14,8 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 
+#include <nlohmann/json.hpp>
+
 #include <sodium.h>
 
 namespace nix {
@@ -456,4 +458,13 @@ std::string_view printHashAlgo(HashAlgorithm ha)
     }
 }
 
+void to_json(nlohmann::json & json, const Hash & hash)
+{
+    json = nlohmann::json::object(
+        {
+            {"algo", printHashAlgo(hash.algo)},
+            {"base16", hash.to_string(HashFormat::Base16, false)},
+        });
+}
+
 }

src/libutil/hash.hh

@@ -5,6 +5,8 @@
 #include "serialise.hh"
 #include "file-system.hh"
 
+#include <nlohmann/json_fwd.hpp>
+
 namespace nix {
 
 
@@ -209,6 +211,10 @@ std::optional<HashAlgorithm> parseHashAlgoOpt(std::string_view s);
  */
 std::string_view printHashAlgo(HashAlgorithm ha);
 
+/**
+ * Write a JSON serialisation of the format `{"algo":"<sha1|...>","base16":"<hex>"}`.
+ */
+void to_json(nlohmann::json & json, const Hash & hash);
 
 union Ctx;