Add constant ‘nixPath’

It contains the Nix expression search path as a list of { prefix, path
} sets, e.g.

  [ { path = "/nix/var/nix/profiles/per-user/root/channels/nixos"; prefix = ""; }
    { path = "/etc/nixos/configuration.nix"; prefix = "nixos-config"; }
    { path = "/home/eelco/Dev/nix/inst/share/nix/corepkgs"; prefix = "nix"; }
  ]

src/libexpr/primops.cc

@@ -1352,6 +1352,17 @@ void EvalState::createBaseEnv()
     evalFile(path, v);
     addConstant("derivation", v);
 
+    /* Add a value containing the current Nix expression search path. */
+    mkList(v, searchPath.size());
+    int n = 0;
+    for (auto & i : searchPath) {
+        Value * v2 = v.list.elems[n++] = allocValue();
+        mkAttrs(*v2, 2);
+        mkString(*allocAttr(*v2, symbols.create("path")), i.second);
+        mkString(*allocAttr(*v2, symbols.create("prefix")), i.first);
+    }
+    addConstant("nixPath", v);
+
     /* Now that we've added all primops, sort the `builtins' set,
        because attribute lookups expect it to be sorted. */
     baseEnv.values[0]->attrs->sort();

tests/lang.sh

@@ -46,7 +46,7 @@ for i in lang/eval-okay-*.nix; do
         if test -e lang/$i.flags; then
             flags=$(cat lang/$i.flags)
         fi
-        if ! NIX_PATH=lang/dir3:lang/dir4_PATH nix-instantiate $flags --eval --strict lang/$i.nix > lang/$i.out; then
+        if ! NIX_PATH=lang/dir3:lang/dir4 nix-instantiate $flags --eval --strict lang/$i.nix > lang/$i.out; then
             echo "FAIL: $i should evaluate"
             fail=1
         elif ! diff lang/$i.out lang/$i.exp; then

tests/lang/eval-okay-search-path.nix

@@ -1,3 +1,10 @@
-assert builtins.pathExists <nix/buildenv.nix>;
+with import ./lib.nix;
+with builtins;
+
+assert pathExists <nix/buildenv.nix>;
+
+assert length nixPath == 3;
+assert length (filter (x: x.prefix == "nix") nixPath) == 1;
+assert length (filter (x: baseNameOf x.path == "dir4") nixPath) == 1;
 
 import <a.nix> + import <b.nix> + import <c.nix> + import <dir5/c.nix>