3 files changed, 68 insertions, 8 deletions
diff --git a/src/other_tools/just_mr/cli.hpp b/src/other_tools/just_mr/cli.hpp
index 4e363460..1d6580da 100644
--- a/src/other_tools/just_mr/cli.hpp
+++ b/src/other_tools/just_mr/cli.hpp
@@ -44,6 +44,7 @@ struct MultiRepoCommonArguments {
     std::optional<std::filesystem::path> git_path{std::nullopt};
     bool norc{false};
     std::size_t jobs{std::max(1U, std::thread::hardware_concurrency())};
+    std::vector<std::string> defines{};
 };
 
 struct MultiRepoLogArguments {
@@ -158,6 +159,14 @@ static inline void SetupMultiRepoCommonArguments(
                     clargs->jobs,
                     "Number of jobs to run (Default: Number of cores).")
         ->type_name("NUM");
+    app->add_option_function<std::string>(
+           "-D,--defines",
+           [clargs](auto const& d) { clargs->defines.emplace_back(d); },
+           "Define overlay configuration to be forwarded to the invocation of"
+           " just, in case the subcommand supports it; otherwise ignored.")
+        ->type_name("JSON")
+        ->trigger_on_parse();  // run callback on all instances while parsing,
+                               // not after all parsing is done
 }
 
 static inline auto SetupMultiRepoLogArguments(
diff --git a/src/other_tools/just_mr/main.cpp b/src/other_tools/just_mr/main.cpp
index 72dd7b79..6c172e6a 100644
--- a/src/other_tools/just_mr/main.cpp
+++ b/src/other_tools/just_mr/main.cpp
@@ -1259,6 +1259,7 @@ void DefaultReachableRepositories(
     bool use_config{false};
     bool use_build_root{false};
     bool use_launcher{false};
+    bool supports_defines{false};
     std::optional<std::filesystem::path> mr_config_path{std::nullopt};
 
     if (subcommand and kKnownJustSubcommands.contains(*subcommand)) {
@@ -1275,6 +1276,7 @@ void DefaultReachableRepositories(
         }
         use_build_root = kKnownJustSubcommands.at(*subcommand).build_root;
         use_launcher = kKnownJustSubcommands.at(*subcommand).launch;
+        supports_defines = kKnownJustSubcommands.at(*subcommand).defines;
     }
     // build just command
     std::vector<std::string> cmd = {arguments.common.just_path->string()};
@@ -1313,6 +1315,31 @@ void DefaultReachableRepositories(
     if (arguments.log.plain_log) {
         cmd.emplace_back("--plain-log");
     }
+    if (supports_defines) {
+        auto overlay_config = Configuration();
+        for (auto const& s : arguments.common.defines) {
+            try {
+                auto map = Expression::FromJson(nlohmann::json::parse(s));
+                if (not map->IsMap()) {
+                    Logger::Log(LogLevel::Error,
+                                "Defines entry {} does not contain a map.",
+                                nlohmann::json(s).dump());
+                    std::exit(kExitClargsError);
+                }
+                overlay_config = overlay_config.Update(map);
+            } catch (std::exception const& e) {
+                Logger::Log(LogLevel::Error,
+                            "Parsing defines entry {} failed with error:\n{}",
+                            nlohmann::json(s).dump(),
+                            e.what());
+                std::exit(kExitClargsError);
+            }
+        }
+        if (not overlay_config.Expr()->Map().empty()) {
+            cmd.emplace_back("-D");
+            cmd.emplace_back(overlay_config.ToString());
+        }
+    }
     // add args read from just-mrrc
     if (subcommand and arguments.just_cmd.just_args.contains(*subcommand)) {
         for (auto const& subcmd_arg :
diff --git a/src/other_tools/just_mr/utils.hpp b/src/other_tools/just_mr/utils.hpp
index a7d43415..d992a895 100644
--- a/src/other_tools/just_mr/utils.hpp
+++ b/src/other_tools/just_mr/utils.hpp
@@ -49,18 +49,42 @@ struct JustSubCmdFlags {
     bool config;
     bool build_root;
     bool launch;
+    bool defines;
 };
 
 // ordered, so that we have replicability
 std::map<std::string, JustSubCmdFlags> const kKnownJustSubcommands{
-    {"version", {false /*config*/, false /*build_root*/, false /*launch*/}},
-    {"describe", {true /*config*/, false /*build_root*/, false /*launch*/}},
-    {"analyse", {true /*config*/, true /*build_root*/, false /*launch*/}},
-    {"build", {true /*config*/, true /*build_root*/, true /*launch*/}},
-    {"install", {true /*config*/, true /*build_root*/, true /*launch*/}},
-    {"rebuild", {true /*config*/, true /*build_root*/, true /*launch*/}},
-    {"install-cas", {false /*config*/, true /*build_root*/, false /*launch*/}},
-    {"gc", {false /*config*/, true /*build_root*/, false /*launch*/}}};
+    {"version",
+     {false /*config*/,
+      false /*build_root*/,
+      false /*launch*/,
+      false /*defines*/}},
+    {"describe",
+     {true /*config*/,
+      false /*build_root*/,
+      false /*launch*/,
+      true /*defines*/}},
+    {"analyse",
+     {true /*config*/,
+      true /*build_root*/,
+      false /*launch*/,
+      true /*defines*/}},
+    {"build",
+     {true /*config*/, true /*build_root*/, true /*launch*/, true /*defines*/}},
+    {"install",
+     {true /*config*/, true /*build_root*/, true /*launch*/, true /*defines*/}},
+    {"rebuild",
+     {true /*config*/, true /*build_root*/, true /*launch*/, true /*defines*/}},
+    {"install-cas",
+     {false /*config*/,
+      true /*build_root*/,
+      false /*launch*/,
+      false /*defines*/}},
+    {"gc",
+     {false /*config*/,
+      true /*build_root*/,
+      false /*launch*/,
+      false /*defines*/}}};
 
 nlohmann::json const kDefaultConfigLocations = nlohmann::json::array(
     {{{"root", "workspace"}, {"path", "repos.json"}},