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/eval/src/tests/nix_tests.rs

@@ -48,9 +48,10 @@ fn eval_test(code_path: PathBuf, expect_success: bool) {
 
     let code = std::fs::read_to_string(&code_path).expect("should be able to read test code");
 
-    let mut eval = crate::Evaluation::new_impure();
-    eval.strict = true;
-    eval.builtins.extend(mock_builtins::builtins());
+    let eval = crate::Evaluation::builder_impure()
+        .strict()
+        .add_builtins(mock_builtins::builtins())
+        .build();
 
     let result = eval.evaluate(code, Some(code_path.clone()));
     let failed = match result.value {
@@ -128,8 +129,10 @@ fn identity(#[files("src/tests/tvix_tests/identity-*.nix")] code_path: PathBuf)
 
     let code = std::fs::read_to_string(code_path).expect("should be able to read test code");
 
-    let mut eval = crate::Evaluation::new(Box::new(crate::StdIO) as Box<dyn EvalIO>, false);
-    eval.strict = true;
+    let eval = crate::Evaluation::builder(Box::new(crate::StdIO) as Box<dyn EvalIO>)
+        .disable_import()
+        .strict()
+        .build();
 
     let result = eval.evaluate(&code, None);
     assert!(

tvix/eval/src/tests/one_offs.rs

@@ -5,8 +5,9 @@ fn test_source_builtin() {
     // Test an evaluation with a source-only builtin. The test ensures
     // that the artificially constructed thunking is correct.
 
-    let mut eval = Evaluation::new_pure();
-    eval.src_builtins.push(("testSourceBuiltin", "42"));
+    let eval = Evaluation::builder_pure()
+        .add_src_builtin("testSourceBuiltin", "42")
+        .build();
 
     let result = eval.evaluate("builtins.testSourceBuiltin", None);
     assert!(
@@ -25,7 +26,9 @@ fn test_source_builtin() {
 
 #[test]
 fn skip_broken_bytecode() {
-    let result = Evaluation::new_pure().evaluate(/* code = */ "x", None);
+    let result = Evaluation::builder_pure()
+        .build()
+        .evaluate(/* code = */ "x", None);
 
     assert_eq!(result.errors.len(), 1);