Add fresh concatStringsSep without bug

The buggy version was previously renamed to
dropEmptyInitThenConcatStringsSep

src/libutil/meson.build

@@ -148,6 +148,7 @@ sources = files(
   'signature/signer.cc',
   'source-accessor.cc',
   'source-path.cc',
+  'strings.cc',
   'suggestions.cc',
   'tarfile.cc',
   'terminal.cc',
@@ -215,6 +216,8 @@ headers = [config_h] + files(
   'source-accessor.hh',
   'source-path.hh',
   'split.hh',
+  'strings.hh',
+  'strings-inline.hh',
   'suggestions.hh',
   'sync.hh',
   'tarfile.hh',

src/libutil/strings-inline.hh

@@ -0,0 +1,31 @@
+#pragma once
+
+#include "strings.hh"
+
+namespace nix {
+
+template<class C>
+std::string concatStringsSep(const std::string_view sep, const C & ss)
+{
+    size_t size = 0;
+    bool tail = false;
+    // need a cast to string_view since this is also called with Symbols
+    for (const auto & s : ss) {
+        if (tail)
+            size += sep.size();
+        size += std::string_view(s).size();
+        tail = true;
+    }
+    std::string s;
+    s.reserve(size);
+    tail = false;
+    for (auto & i : ss) {
+        if (tail)
+            s += sep;
+        s += i;
+        tail = true;
+    }
+    return s;
+}
+
+} // namespace nix

src/libutil/strings.cc

@@ -0,0 +1,12 @@
+#include <string>
+
+#include "strings-inline.hh"
+#include "util.hh"
+
+namespace nix {
+
+template std::string concatStringsSep(std::string_view, const Strings &);
+template std::string concatStringsSep(std::string_view, const StringSet &);
+template std::string concatStringsSep(std::string_view, const std::vector<std::string> &);
+
+} // namespace nix

src/libutil/strings.hh

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <list>
+#include <set>
+#include <string_view>
+#include <string>
+#include <vector>
+
+namespace nix {
+
+/**
+ * Concatenate the given strings with a separator between the elements.
+ */
+template<class C>
+std::string concatStringsSep(const std::string_view sep, const C & ss);
+
+extern template std::string concatStringsSep(std::string_view, const std::list<std::string> &);
+extern template std::string concatStringsSep(std::string_view, const std::set<std::string> &);
+extern template std::string concatStringsSep(std::string_view, const std::vector<std::string> &);
+
+}