nix make-content-addressable: Add --json flag

Fixes #3274.

src/nix/make-content-addressable.cc

 #include "command.hh"
 #include "store-api.hh"
 #include "references.hh"
+#include "common-args.hh"
+#include "json.hh"
 
 using namespace nix;
 
-struct CmdMakeContentAddressable : StorePathsCommand
+struct CmdMakeContentAddressable : StorePathsCommand, MixJSON
 {
     CmdMakeContentAddressable()
     {
@@ -37,6 +39,9 @@ struct CmdMakeContentAddressable : StorePathsCommand
 
         std::map<StorePath, StorePath> remappings;
 
+        auto jsonRoot = json ? std::make_unique<JSONObject>(std::cout) : nullptr;
+        auto jsonRewrites = json ? std::make_unique<JSONObject>(jsonRoot->object("rewrites")) : nullptr;
+
         for (auto & path : paths) {
             auto pathS = store->printStorePath(path);
             auto oldInfo = store->queryPathInfo(path);
@@ -76,7 +81,8 @@ struct CmdMakeContentAddressable : StorePathsCommand
             info.narSize = sink.s->size();
             info.ca = makeFixedOutputCA(true, info.narHash);
 
-            printError("rewrote '%s' to '%s'", pathS, store->printStorePath(info.path));
+            if (!json)
+                printError("rewrote '%s' to '%s'", pathS, store->printStorePath(info.path));
 
             auto source = sinkToSource([&](Sink & nextSink) {
                 RewritingSink rsink2(oldHashPart, storePathToHash(store->printStorePath(info.path)), nextSink);
@@ -86,6 +92,9 @@ struct CmdMakeContentAddressable : StorePathsCommand
 
             store->addToStore(info, *source);
 
+            if (json)
+                jsonRewrites->attr(store->printStorePath(path), store->printStorePath(info.path));
+
             remappings.insert_or_assign(std::move(path), std::move(info.path));
         }
     }