feat(tvix/cli): implement builtins.placeholder

This doesn't require any other corresponding handling *yet*, as the
actual replacements happen in the builder logic (which we delegate to
cppnix at the moment).

Change-Id: I034147c933f05ae427c7a8794647132d108d0ede
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7972
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/cli/src/derivation.rs

@@ -2,6 +2,7 @@
 
 use data_encoding::BASE64;
 use nix_compat::derivation::{Derivation, Hash};
+use nix_compat::hash_placeholder;
 use std::cell::RefCell;
 use std::collections::{btree_map, BTreeSet};
 use std::rc::Rc;
@@ -250,6 +251,18 @@ fn strong_coerce_to_string(vm: &mut VM, val: &Value, ctx: &str) -> Result<String
 mod derivation_builtins {
     use super::*;
 
+    #[builtin("placeholder")]
+    fn builtin_placeholder(_: &mut VM, input: Value) -> Result<Value, ErrorKind> {
+        let placeholder = hash_placeholder(
+            input
+                .to_str()
+                .context("looking at output name in builtins.placeholder")?
+                .as_str(),
+        );
+
+        Ok(placeholder.into())
+    }
+
     /// Strictly construct a Nix derivation from the supplied arguments.
     ///
     /// This is considered an internal function, users usually want to
@@ -677,4 +690,17 @@ mod tests {
             vec!["--foo".to_string(), "42".to_string(), "--bar".to_string()]
         );
     }
+
+    #[test]
+    fn builtins_placeholder_hashes() {
+        assert_eq!(
+            hash_placeholder("out").as_str(),
+            "/1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9"
+        );
+
+        assert_eq!(
+            hash_placeholder("").as_str(),
+            "/171rf4jhx57xqz3p7swniwkig249cif71pa08p80mgaf0mqz5bmr"
+        );
+    }
 }