builtins.nixVersion: return fixed fake version

This builtin is only going to cause us problems because we are not Nix,
so let's just falsify being in the 2.18 series, since that is the
closest target that has any meaning.

In future we might want to have a better feature detection mechanism,
for when we actually add stuff to some builtin's attr set argument. But
builtins.nixVersion is just going to be hopelessly broken and it should
be stubbed out.

Fixes https://git.lix.systems/lix-project/lix/issues/144

Change-Id: Id7390b32a29c6147f2977737d81846320de5d67e

src/libexpr/primops.cc

@@ -4299,25 +4299,16 @@ void EvalState::createBaseEnv()
         .impureOnly = true,
     });
 
-    v.mkString(nixVersion);
+    v.mkString("2.18.3-lix");
     addConstant("__nixVersion", v, {
         .type = nString,
         .doc = R"(
-          The version of Nix.
+          Legacy version of Nix. Always returns "2.18.3-lix" on Lix.
 
-          For example, where the command line returns the current Nix version,
-
-          ```shell-session
-          $ nix --version
-          nix (Nix) 2.16.0
-          ```
-
-          the Nix language evaluator returns the same value:
-
-          ```nix-repl
-          nix-repl> builtins.nixVersion
-          "2.16.0"
-          ```
+          To determine if features exist, Nix scripts should instead use direct
+          means of feature detection, such as checking for existence of
+          builtins they want to use. Doing so allows for much better compatibility
+          across implementations.
         )",
     });
 

tests/unit/libexpr/primops.cc

@@ -611,7 +611,7 @@ namespace nix {
 
     TEST_F(PrimOpTest, nixVersion) {
         auto v = eval("builtins.nixVersion");
-        ASSERT_THAT(v, IsStringEq(nixVersion));
+        ASSERT_THAT(v, IsStringEq("2.18.3-lix"));
     }
 
     TEST_F(PrimOpTest, currentSystem) {