Don't include <regex> in header files

This reduces compilation time by ~15 seconds (CPU time).

Issue #4045.

src/libexpr/eval.cc

@@ -356,6 +356,7 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
     , sEpsilon(symbols.create(""))
     , repair(NoRepair)
     , store(store)
+    , regexCache(makeRegexCache())
     , baseEnv(allocEnv(128))
     , staticBaseEnv(false, 0)
 {

src/libexpr/eval.hh

@@ -6,7 +6,6 @@
 #include "symbol-table.hh"
 #include "config.hh"
 
-#include <regex>
 #include <map>
 #include <optional>
 #include <unordered_map>
@@ -65,6 +64,11 @@ typedef std::list<SearchPathElem> SearchPath;
 void initGC();
 
 
+struct RegexCache;
+
+std::shared_ptr<RegexCache> makeRegexCache();
+
+
 class EvalState
 {
 public:
@@ -120,7 +124,7 @@ private:
     std::unordered_map<Path, Path> resolvedPaths;
 
     /* Cache used by prim_match(). */
-    std::unordered_map<std::string, std::regex> regexCache;
+    std::shared_ptr<RegexCache> regexCache;
 
 public:
 

src/libexpr/flake/flakeref.cc

@@ -1,6 +1,7 @@
 #include "flakeref.hh"
 #include "store-api.hh"
 #include "url.hh"
+#include "url-parts.hh"
 #include "fetchers.hh"
 #include "registry.hh"
 

src/libexpr/flake/lockfile.cc

@@ -1,5 +1,6 @@
 #include "lockfile.hh"
 #include "store-api.hh"
+#include "url-parts.hh"
 
 #include <nlohmann/json.hpp>
 

src/libexpr/primops.cc

@@ -3085,17 +3085,25 @@ static RegisterPrimOp primop_hashString({
     .fun = prim_hashString,
 });
 
-/* Match a regular expression against a string and return either
-   ‘null’ or a list containing substring matches. */
+struct RegexCache
+{
+    std::unordered_map<std::string, std::regex> cache;
+};
+
+std::shared_ptr<RegexCache> makeRegexCache()
+{
+    return std::make_shared<RegexCache>();
+}
+
 void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     auto re = state.forceStringNoCtx(*args[0], pos);
 
     try {
 
-        auto regex = state.regexCache.find(re);
-        if (regex == state.regexCache.end())
-            regex = state.regexCache.emplace(re, std::regex(re, std::regex::extended)).first;
+        auto regex = state.regexCache->cache.find(re);
+        if (regex == state.regexCache->cache.end())
+            regex = state.regexCache->cache.emplace(re, std::regex(re, std::regex::extended)).first;
 
         PathSet context;
         const std::string str = state.forceString(*args[1], context, pos);

src/libexpr/primops/fetchMercurial.cc

@@ -3,8 +3,7 @@
 #include "store-api.hh"
 #include "fetchers.hh"
 #include "url.hh"
-
-#include <regex>
+#include "url-parts.hh"
 
 namespace nix {