nix copy: Add --out-link

src/libcmd/command.cc

@@ -334,4 +334,29 @@ void MixEnvironment::setEnviron()
     }
 }
 
+void createOutLinks(
+    const std::filesystem::path & outLink,
+    const BuiltPaths & buildables,
+    LocalFSStore & store)
+{
+    for (const auto & [_i, buildable] : enumerate(buildables)) {
+        auto i = _i;
+        std::visit(overloaded {
+            [&](const BuiltPath::Opaque & bo) {
+                auto symlink = outLink;
+                if (i) symlink += fmt("-%d", i);
+                store.addPermRoot(bo.path, absPath(symlink.string()));
+            },
+            [&](const BuiltPath::Built & bfd) {
+                for (auto & output : bfd.outputs) {
+                    auto symlink = outLink;
+                    if (i) symlink += fmt("-%d", i);
+                    if (output.first != "out") symlink += fmt("-%s", output.first);
+                    store.addPermRoot(output.second, absPath(symlink.string()));
+                }
+            },
+        }, buildable.raw());
+    }
+}
+
 }

src/libcmd/command.hh

@@ -18,6 +18,7 @@ extern char * * savedArgv;
 class EvalState;
 struct Pos;
 class Store;
+class LocalFSStore;
 
 static constexpr Command::Category catHelp = -1;
 static constexpr Command::Category catSecondary = 100;
@@ -367,4 +368,13 @@ void printClosureDiff(
     const StorePath & afterPath,
     std::string_view indent);
 
+/**
+ * Create symlinks prefixed by `outLink` to the store paths in
+ * `buildables`.
+ */
+void createOutLinks(
+    const std::filesystem::path & outLink,
+    const BuiltPaths & buildables,
+    LocalFSStore & store);
+
 }

src/libcmd/installables.cc

@@ -917,4 +917,12 @@ void BuiltPathsCommand::applyDefaultInstallables(std::vector<std::string> & rawI
         rawInstallables.push_back(".");
 }
 
+BuiltPaths toBuiltPaths(const std::vector<BuiltPathWithResult> & builtPathsWithResult)
+{
+    BuiltPaths res;
+    for (auto & i : builtPathsWithResult)
+        res.push_back(i.path);
+    return res;
+}
+
 }

src/libcmd/installables.hh

@@ -86,6 +86,8 @@ struct BuiltPathWithResult
     std::optional<BuildResult> result;
 };
 
+BuiltPaths toBuiltPaths(const std::vector<BuiltPathWithResult> & builtPathsWithResult);
+
 /**
  * Shorthand, for less typing and helping us keep the choice of
  * collection in sync.