feat(tvix/eval): add mechanism for placeholder builtins r/5181

These are builtins which can be basically implemented as the identity
function from the perspective of pure evaluation, and which help us
get closer to evaluating nixpkgs.

For now, builtins added here will be "usable" and just emit a warning
about not being implemented yet.

Change-Id: I0fce94677f01c98c0392aeefb7ab353c7dc7ec82
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7060
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

1 files changed, 19 insertions, 0 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 14eb673f78..073edf8ffb 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -9,6 +9,7 @@ use std::path::PathBuf;
 
 use regex::Regex;
 
+use crate::warnings::WarningKind;
 use crate::{
     errors::ErrorKind,
     value::{Builtin, CoercionKind, NixAttrs, NixList, NixString, Value},
@@ -636,6 +637,22 @@ fn pure_builtins() -> Vec<Builtin> {
     ]
 }
 
+/// Placeholder builtins that technically have a function which we do
+/// not yet implement, but which is also not easily observable from
+/// within a pure evaluation context.
+///
+/// These are used as a crutch to make progress on nixpkgs evaluation.
+fn placeholders() -> Vec<Builtin> {
+    vec![Builtin::new(
+        "addErrorContext",
+        &[false, false],
+        |mut args: Vec<Value>, vm: &mut VM| {
+            vm.emit_warning(WarningKind::NotImplemented("builtins.addErrorContext"));
+            Ok(args.pop().unwrap())
+        },
+    )]
+}
+
 fn builtins_set() -> NixAttrs {
     let mut map: BTreeMap<NixString, Value> = BTreeMap::new();
 
@@ -652,6 +669,8 @@ fn builtins_set() -> NixAttrs {
     };
 
     add_builtins(pure_builtins());
+    add_builtins(placeholders());
+
     #[cfg(feature = "impure")]
     {
         map.extend(impure::builtins());