Release 2.28.0 (2025-04-02)

This is an atypical release, and for almost all intents and purposes, it is just a continuation of 2.27; not a feature release.

We had originally set the goal of making 2.27 the Nixpkgs default for NixOS 25.05, but dependents that link to Nix need certain interface breaking changes in the C++ headers. This is not something we should do in a patch release, so this is why we branched 2.28 right off 2.27 instead of master.

This completes the infrastructure overhaul for the RFC 132 switchover to meson as our build system.

Major changes

  • Unstable C++ API reworked #12836 #12798 #12773

    Now the C++ interface confirms to common conventions much better than before:

    • All headers are expected to be included with the initial nix/, e.g. as #include "nix/....hh" (what Nix's headers now do) or #include <nix/....hh> (what downstream projects may choose to do). Likewise, the pkg-config files have -I${includedir} not -I${includedir}/nix or similar.

      Including without the nix/ like before sometimes worked because of how for #include C pre-process checks the directory containing the current file, not just the lookup path, but this was not reliable.

    • All configuration headers are included explicitly by the (regular) headers that need them. There is no more need to pass -include to force additional files to be included.

    • The public, installed configuration headers no longer contain implementation-specific details that are not relevant to the API. The vast majority of definitions that were previously in there are now moved to new headers that are not installed, but used during Nix's own compilation only. The remaining macro definitions are renamed to have NIX_ as a prefix.

    • The name of the Nix component the header comes from (e.g. util, store, expr, flake, etc.) is now part of the path to the header, coming after nix and before the header name (or rest of the header path, if it is already in a directory).

    Here is a contrived diff showing a few of these changes at once:

    @@ @@
-#include "derived-path.hh"
+#include "nix/store/derived-path.hh"
@@ @@
+// Would include for the variables used before. But when other headers
+// need these variables. those will include these config themselves.
+#include "nix/store/config.hh"
+#include "nix/expr/config.hh"
@@ @@
-#include "config.hh"
+// Additionally renamed to distinguish from components' config headers.
+#include "nix/util/configuration.hh"
@@ @@
-#if HAVE_ACL_SUPPORT
+#if NIX_SUPPORT_ACL
@@ @@
-#if HAVE_BOEHMGC
+#if NIX_USE_BOEHMGC
@@ @@
 #endif
 #endif
@@ @@
-const char *s = "hi from " SYSTEM;
+const char *s = "hi from " NIX_LOCAL_SYSTEM;

  • C API nix_flake_init_global removed #5638 #12759

    In order to improve the modularity of the code base, we are removing a use of global state, and therefore the nix_flake_init_global function.

    Instead, use nix_flake_settings_add_to_eval_state_builder. For example:

    -    nix_flake_init_global(ctx, settings);
-    HANDLE_ERROR(ctx);
-
     nix_eval_state_builder * builder = nix_eval_state_builder_new(ctx, store);
     HANDLE_ERROR(ctx);

+    nix_flake_settings_add_to_eval_state_builder(ctx, settings, builder);
+    HANDLE_ERROR(ctx);

    Although this change is not as critical, we figured it would be good to do this API change at the same time, also. Also note that we try to keep the C API compatible, but we decided to break this function because it was young and likely not in widespread use yet. This frees up time to make important progress on the rest of the C API.

Contributors

This earlier-than-usual release was made possible by the following 16 contributors: