feat(tvix/eval): implement 'throw' and 'abort' builtins

These do essentially the same, but return different error variants as
upstream Nix considers `throw` to be (sometimes) catchable.

Change-Id: I1a9ea84567d46fb37287dbf3f3f67052f9382cca
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6259
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -8,13 +8,26 @@ use std::{
     rc::Rc,
 };
 
-use crate::value::{Builtin, NixAttrs, NixString, Value};
+use crate::{
+    errors::ErrorKind,
+    value::{Builtin, NixAttrs, NixString, Value},
+};
 
 fn pure_builtins() -> Vec<Builtin> {
     vec![
+        Builtin::new("abort", 1, |mut args| {
+            return Err(
+                ErrorKind::Abort(args.pop().unwrap().to_string()?.as_str().to_owned()).into(),
+            );
+        }),
         Builtin::new("isNull", 1, |args| {
             Ok(Value::Bool(matches!(args[0], Value::Null)))
         }),
+        Builtin::new("throw", 1, |mut args| {
+            return Err(
+                ErrorKind::Throw(args.pop().unwrap().to_string()?.as_str().to_owned()).into(),
+            );
+        }),
         Builtin::new("toString", 1, |args| {
             // TODO: toString is actually not the same as Display
             Ok(Value::String(format!("{}", args[0]).into()))

tvix/eval/src/errors.rs

@@ -38,6 +38,10 @@ pub enum ErrorKind {
     ParseErrors(Vec<rnix::parser::ParseError>),
 
     AssertionFailed,
+
+    // These are user-generated errors through builtins.
+    Throw(String),
+    Abort(String),
 }
 
 #[derive(Clone, Debug)]