Move call-flake.nix to nix-flake

As suggested by Ericson2314 in review
https://github.com/NixOS/nix/pull/12759#issuecomment-2755352343

src/libexpr/eval.cc

@@ -288,10 +288,6 @@ EvalState::EvalState(
         CanonPath("derivation-internal.nix"),
         #include "primops/derivation.nix.gen.hh"
     )}
-    , callFlakeInternal{internalFS->addFile(
-        CanonPath("call-flake.nix"),
-        #include "call-flake.nix.gen.hh"
-    )}
     , store(store)
     , buildStore(buildStore ? buildStore : store)
     , debugRepl(nullptr)

src/libexpr/eval.hh

@@ -274,14 +274,12 @@ public:
 
     /**
      * In-memory filesystem for internal, non-user-callable Nix
-     * expressions like call-flake.nix.
+     * expressions like `derivation.nix`.
      */
     const ref<MemorySourceAccessor> internalFS;
 
     const SourcePath derivationInternal;
 
-    const SourcePath callFlakeInternal;
-
     /**
      * Store used to materialise .drv files.
      */

src/libexpr/meson.build

@@ -126,7 +126,6 @@ generated_headers = []
 foreach header : [
   'imported-drv-to-derivation.nix',
   'fetchurl.nix',
-  'call-flake.nix',
 ]
   generated_headers += gen_header.process(header)
 endforeach

src/libflake/flake/flake.cc

@@ -16,6 +16,8 @@
 
 #include <nlohmann/json.hpp>
 
+#include "memory-source-accessor.hh"
+
 namespace nix {
 
 using namespace flake;
@@ -921,6 +923,25 @@ LockedFlake lockFlake(
     }
 }
 
+static ref<SourceAccessor> makeInternalFS() {
+    auto internalFS = make_ref<MemorySourceAccessor>(MemorySourceAccessor {});
+    internalFS->setPathDisplay("«flakes-internal»", "");
+    internalFS->addFile(
+        CanonPath("call-flake.nix"),
+        #include "call-flake.nix.gen.hh"
+    );
+    return internalFS;
+}
+
+static auto internalFS = makeInternalFS();
+
+static Value * requireInternalFile(EvalState & state, CanonPath path) {
+    SourcePath p {internalFS, path};
+    auto v = state.allocValue();
+    state.evalFile(p, *v); // has caching
+    return v;
+}
+
 void callFlake(EvalState & state,
     const LockedFlake & lockedFlake,
     Value & vRes)
@@ -960,8 +981,7 @@ void callFlake(EvalState & state,
 
     auto & vOverrides = state.allocValue()->mkAttrs(overrides);
 
-    auto vCallFlake = state.allocValue();
-    state.evalFile(state.callFlakeInternal, *vCallFlake);
+    Value * vCallFlake = requireInternalFile(state, CanonPath("call-flake.nix"));
 
     auto vLocks = state.allocValue();
     vLocks->mkString(lockFileStr);

src/libflake/meson.build

@@ -39,6 +39,15 @@ add_project_arguments(
 
 subdir('nix-meson-build-support/common')
 
+subdir('nix-meson-build-support/generate-header')
+
+generated_headers = []
+foreach header : [
+  'call-flake.nix',
+]
+  generated_headers += gen_header.process(header)
+endforeach
+
 sources = files(
   'flake/config.cc',
   'flake/flake.cc',
@@ -62,6 +71,7 @@ headers = files(
 this_library = library(
   'nixflake',
   sources,
+  generated_headers,
   dependencies : deps_public + deps_private + deps_other,
   prelink : true, # For C++ static initializers
   install : true,

src/libflake/package.nix

@@ -28,6 +28,7 @@ mkMesonLibrary (finalAttrs: {
     ../../.version
     ./.version
     ./meson.build
+    ./call-flake.nix
     (fileset.fileFilter (file: file.hasExt "cc") ./.)
     (fileset.fileFilter (file: file.hasExt "hh") ./.)
   ];