Add `repl-overlays`

Adds a `repl-overlays` option, which specifies files that can overlay
and modify the top-level bindings in `nix repl`. For example, with the
following contents in `~/.config/nix/repl.nix`:

    info: final: prev: let
      optionalAttrs = predicate: attrs:
        if predicate
        then attrs
        else {};
    in
      optionalAttrs (prev ? legacyPackages && prev.legacyPackages ? ${info.currentSystem})
      {
        pkgs = prev.legacyPackages.${info.currentSystem};
      }

We can run `nix repl` and use `pkgs` to refer to `legacyPackages.${currentSystem}`:

    $ nix repl --repl-overlays ~/.config/nix/repl.nix nixpkgs
    Lix 2.90.0
    Type :? for help.
    Loading installable 'flake:nixpkgs#'...
    Added 5 variables.
    Loading 'repl-overlays'...
    Added 6 variables.
    nix-repl> pkgs.bash
    «derivation /nix/store/g08b5vkwwh0j8ic9rkmd8mpj878rk62z-bash-5.2p26.drv»

Change-Id: Ic12e0f2f210b2f46e920c33088dfe1083f42391a

1 files changed, 5 insertions, 0 deletions

diff --git a/src/libcmd/meson.build b/src/libcmd/meson.build
index 867508926..b057daa37 100644
--- a/src/libcmd/meson.build
+++ b/src/libcmd/meson.build
@@ -32,8 +32,13 @@ libcmd_headers = files(
   'repl.hh',
 )
 
+libcmd_generated_headers = [
+  gen_header.process('repl-overlays.nix', preserve_path_from: meson.current_source_dir()),
+]
+
 libcmd = library(
   'nixcmd',
+  libcmd_generated_headers,
   libcmd_sources,
   dependencies : [
     liblixutil,