Lazily copy trees to the store

We now mount lazy accessors on top of /nix/store without materializing
them, and only materialize them to the real store if needed (e.g. in
the `derivation` primop).

src/libexpr/paths.cc

@@ -1,5 +1,7 @@
 #include "nix/store/store-api.hh"
 #include "nix/expr/eval.hh"
+#include "nix/util/mounted-source-accessor.hh"
+#include "nix/fetchers/fetch-to-store.hh"
 
 namespace nix {
 
@@ -18,4 +20,36 @@ SourcePath EvalState::storePath(const StorePath & path)
     return {rootFS, CanonPath{store->printStorePath(path)}};
 }
 
+StorePath EvalState::devirtualize(const StorePath & path, StringMap * rewrites)
+{
+    if (auto mount = storeFS->getMount(CanonPath(store->printStorePath(path)))) {
+        auto storePath = fetchToStore(
+            *store, SourcePath{ref(mount)}, settings.readOnlyMode ? FetchMode::DryRun : FetchMode::Copy, path.name());
+        assert(storePath.name() == path.name());
+        if (rewrites)
+            rewrites->emplace(path.hashPart(), storePath.hashPart());
+        return storePath;
+    } else
+        return path;
+}
+
+SingleDerivedPath EvalState::devirtualize(const SingleDerivedPath & path, StringMap * rewrites)
+{
+    if (auto o = std::get_if<SingleDerivedPath::Opaque>(&path.raw()))
+        return SingleDerivedPath::Opaque{devirtualize(o->path, rewrites)};
+    else
+        return path;
+}
+
+std::string EvalState::devirtualize(std::string_view s, const NixStringContext & context)
+{
+    StringMap rewrites;
+
+    for (auto & c : context)
+        if (auto o = std::get_if<NixStringContextElem::Opaque>(&c.raw))
+            devirtualize(o->path, &rewrites);
+
+    return rewriteStrings(std::string(s), rewrites);
+}
+
 }