feat(tvix/eval): Support builtins.{add,sub} r/4738

First pass at supporting `builtins` for tvix. The following tests appear to be
WAI:

```shell
$ cd tvix/eval
$ cargo build
$ cargo test
```

Change-Id: I27cce23d503b17a886d1109e285e8b4be4264977
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6405
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 49f047543994..a0ada0233c59 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -13,8 +13,15 @@ use crate::{
     value::{Builtin, NixAttrs, NixList, NixString, Value},
 };
 
+use crate::arithmetic_op;
+
 fn pure_builtins() -> Vec<Builtin> {
     vec![
+        Builtin::new("add", 2, |mut args| {
+            let b = args.pop().unwrap();
+            let a = args.pop().unwrap();
+            Ok(arithmetic_op!(a, b, +))
+        }),
         Builtin::new("abort", 1, |mut args| {
             return Err(
                 ErrorKind::Abort(args.pop().unwrap().to_string()?.as_str().to_owned()).into(),
@@ -63,6 +70,11 @@ fn pure_builtins() -> Vec<Builtin> {
         Builtin::new("isString", 1, |args| {
             Ok(Value::Bool(matches!(args[0], Value::String(_))))
         }),
+        Builtin::new("sub", 2, |mut args| {
+            let b = args.pop().unwrap();
+            let a = args.pop().unwrap();
+            Ok(arithmetic_op!(a, b, -))
+        }),
         Builtin::new("throw", 1, |mut args| {
             return Err(
                 ErrorKind::Throw(args.pop().unwrap().to_string()?.as_str().to_owned()).into(),