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/serde/src/de.rs

@@ -3,8 +3,7 @@
 use bstr::ByteSlice;
 use serde::de::value::{MapDeserializer, SeqDeserializer};
 use serde::de::{self, EnumAccess, VariantAccess};
-pub use tvix_eval::Evaluation;
-use tvix_eval::{EvalIO, Value};
+use tvix_eval::{EvalIO, EvaluationBuilder, Value};
 
 use crate::error::Error;
 
@@ -36,7 +35,7 @@ pub fn from_str<'code, T>(src: &'code str) -> Result<T, Error>
 where
     T: serde::Deserialize<'code>,
 {
-    from_str_with_config(src, |_| /* no extra config */ ())
+    from_str_with_config(src, |b| /* no extra config */ b)
 }
 
 /// Evaluate the Nix code in `src`, with extra configuration for the
@@ -44,13 +43,12 @@ where
 pub fn from_str_with_config<'code, T, F>(src: &'code str, config: F) -> Result<T, Error>
 where
     T: serde::Deserialize<'code>,
-    F: FnOnce(&mut Evaluation<Box<dyn EvalIO>>),
+    F: for<'co, 'ro, 'env> FnOnce(
+        EvaluationBuilder<'co, 'ro, 'env, Box<dyn EvalIO>>,
+    ) -> EvaluationBuilder<'co, 'ro, 'env, Box<dyn EvalIO>>,
 {
     // First step is to evaluate the Nix code ...
-    let mut eval = Evaluation::new_pure();
-    config(&mut eval);
-
-    eval.strict = true;
+    let eval = config(EvaluationBuilder::new_pure().strict()).build();
     let result = eval.evaluate(src, None);
 
     if !result.errors.is_empty() {

tvix/serde/src/de_tests.rs

@@ -204,7 +204,7 @@ fn deserialize_enum_all() {
 fn deserialize_with_config() {
     let result: String = from_str_with_config("builtins.testWithConfig", |eval| {
         // Add a literal string builtin that just returns `"ok"`.
-        eval.src_builtins.push(("testWithConfig", "\"ok\""));
+        eval.add_src_builtin("testWithConfig", "\"ok\"")
     })
     .expect("should deserialize");
 
@@ -237,7 +237,7 @@ fn deserialize_with_extra_builtin() {
     let code = "builtins.prependHello \"world\"";
 
     let result: String = from_str_with_config(code, |eval| {
-        eval.builtins.append(&mut test_builtins::builtins());
+        eval.add_builtins(test_builtins::builtins())
     })
     .expect("should deserialize");