refactor(tvix/eval): Builderize Evaluation

Make constructing of a new Evaluation use the builder pattern rather
than setting public mutable fields. This is currently a pure
refactor (no functionality has changed) but has a few advantages:

- We've encapsulated the internals of the fields in Evaluation, meaning
  we can change them without too much breakage of clients
- We have type safety that prevents us from ever changing the fields of
  an Evaluation after it's built (which matters more in a world where we
  reuse Evaluations).

More importantly, this paves the road for doing different things with
the construction of an Evaluation - notably, sharing certain things like
the GlobalsMap across subsequent evaluations in eg the REPL.

Fixes: b/262
Change-Id: I4a27116faac14cdd144fc7c992d14ae095a1aca4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11956
Tested-by: BuildkiteCI
Autosubmit: aspen <root@gws.fyi>
Reviewed-by: flokli <flokli@flokli.de>

tvix/glue/src/lib.rs

@@ -14,12 +14,14 @@ mod tests;
 /// Tell the Evaluator to resolve `<nix>` to the path `/__corepkgs__`,
 /// which has special handling in [tvix_io::TvixIO].
 /// This is used in nixpkgs to import `fetchurl.nix` from `<nix>`.
-pub fn configure_nix_path<IO>(
-    eval: &mut tvix_eval::Evaluation<IO>,
+pub fn configure_nix_path<'co, 'ro, 'env, IO>(
+    eval_builder: tvix_eval::EvaluationBuilder<'co, 'ro, 'env, IO>,
     nix_search_path: &Option<String>,
-) {
-    eval.nix_path = nix_search_path
-        .as_ref()
-        .map(|p| format!("nix=/__corepkgs__:{}", p))
-        .or_else(|| Some("nix=/__corepkgs__".to_string()));
+) -> tvix_eval::EvaluationBuilder<'co, 'ro, 'env, IO> {
+    eval_builder.nix_path(
+        nix_search_path
+            .as_ref()
+            .map(|p| format!("nix=/__corepkgs__:{}", p))
+            .or_else(|| Some("nix=/__corepkgs__".to_string())),
+    )
 }