refactor(tvix/glue): move builtins into separate directory r/6939

Change-Id: I25b7197458dbfbde8623545dc0a0286eb2744f10
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9911
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

4 files changed, 691 insertions, 0 deletions

diff --git a/tvix/glue/src/builtins/derivation.nix b/tvix/glue/src/builtins/derivation.nix
new file mode 100644
index 000000000000..9355cc3a96f0
--- /dev/null
+++ b/tvix/glue/src/builtins/derivation.nix
@@ -0,0 +1,36 @@
+# LGPL-2.1-or-later
+#
+# taken from: https://github.com/NixOS/nix/blob/master/src/libexpr/primops/derivation.nix
+#
+# TODO: rewrite in native Rust code
+
+/* This is the implementation of the ‘derivation’ builtin function.
+   It's actually a wrapper around the ‘derivationStrict’ primop. */
+
+drvAttrs @ { outputs ? [ "out" ], ... }:
+
+let
+
+  strict = derivationStrict drvAttrs;
+
+  commonAttrs = drvAttrs // (builtins.listToAttrs outputsList) //
+    {
+      all = map (x: x.value) outputsList;
+      inherit drvAttrs;
+    };
+
+  outputToAttrListElement = outputName:
+    {
+      name = outputName;
+      value = commonAttrs // {
+        outPath = builtins.getAttr outputName strict;
+        drvPath = strict.drvPath;
+        type = "derivation";
+        inherit outputName;
+      };
+    };
+
+  outputsList = map outputToAttrListElement outputs;
+
+in
+(builtins.head outputsList).value
diff --git a/tvix/glue/src/builtins/derivation.rs b/tvix/glue/src/builtins/derivation.rs
new file mode 100644
index 000000000000..4eb0141d544b
--- /dev/null
+++ b/tvix/glue/src/builtins/derivation.rs
@@ -0,0 +1,471 @@
+//! Implements `builtins.derivation`, the core of what makes Nix build packages.
+use crate::builtins::DerivationError;
+use crate::known_paths::{KnownPaths, PathKind, PathName};
+use nix_compat::derivation::{Derivation, Output};
+use nix_compat::nixhash;
+use std::cell::RefCell;
+use std::collections::{btree_map, BTreeSet};
+use std::rc::Rc;
+use tvix_eval::builtin_macros::builtins;
+use tvix_eval::generators::{self, emit_warning_kind, GenCo};
+use tvix_eval::{
+    AddContext, CatchableErrorKind, CoercionKind, ErrorKind, NixAttrs, NixList, Value, WarningKind,
+};
+
+// Constants used for strangely named fields in derivation inputs.
+const STRUCTURED_ATTRS: &str = "__structuredAttrs";
+const IGNORE_NULLS: &str = "__ignoreNulls";
+
+/// Helper function for populating the `drv.outputs` field from a
+/// manually specified set of outputs, instead of the default
+/// `outputs`.
+async fn populate_outputs(
+    co: &GenCo,
+    drv: &mut Derivation,
+    outputs: NixList,
+) -> Result<(), ErrorKind> {
+    // Remove the original default `out` output.
+    drv.outputs.clear();
+
+    for output in outputs {
+        let output_name = generators::request_force(co, output)
+            .await
+            .to_str()
+            .context("determining output name")?;
+
+        if drv
+            .outputs
+            .insert(output_name.as_str().into(), Default::default())
+            .is_some()
+        {
+            return Err(DerivationError::DuplicateOutput(output_name.as_str().into()).into());
+        }
+    }
+
+    Ok(())
+}
+
+/// Populate the inputs of a derivation from the build references
+/// found when scanning the derivation's parameters.
+fn populate_inputs<I: IntoIterator<Item = PathName>>(
+    drv: &mut Derivation,
+    known_paths: &KnownPaths,
+    references: I,
+) {
+    for reference in references.into_iter() {
+        let reference = &known_paths[&reference];
+        match &reference.kind {
+            PathKind::Plain => {
+                drv.input_sources.insert(reference.path.clone());
+            }
+
+            PathKind::Output { name, derivation } => {
+                match drv.input_derivations.entry(derivation.clone()) {
+                    btree_map::Entry::Vacant(entry) => {
+                        entry.insert(BTreeSet::from([name.clone()]));
+                    }
+
+                    btree_map::Entry::Occupied(mut entry) => {
+                        entry.get_mut().insert(name.clone());
+                    }
+                }
+            }
+
+            PathKind::Derivation { output_names } => {
+                match drv.input_derivations.entry(reference.path.clone()) {
+                    btree_map::Entry::Vacant(entry) => {
+                        entry.insert(output_names.clone());
+                    }
+
+                    btree_map::Entry::Occupied(mut entry) => {
+                        entry.get_mut().extend(output_names.clone().into_iter());
+                    }
+                }
+            }
+        }
+    }
+}
+
+/// Populate the output configuration of a derivation based on the
+/// parameters passed to the call, configuring a fixed-output derivation output
+/// if necessary.
+///
+/// This function handles all possible combinations of the
+/// parameters, including invalid ones.
+///
+/// Due to the support for SRI hashes, and how these are passed along to
+/// builtins.derivation, outputHash and outputHashAlgo can have values which
+/// need to be further modified before constructing the Derivation struct.
+///
+/// If outputHashAlgo is an SRI hash, outputHashAlgo must either be an empty
+/// string, or the hash algorithm as specified in the (single) SRI (entry).
+/// SRI strings with multiple hash algorithms are not supported.
+///
+/// In case an SRI string was used, the (single) fixed output is populated
+/// with the hash algo name, and the hash digest is populated with the
+/// (lowercase) hex encoding of the digest.
+///
+/// These values are only rewritten for the outputs, not what's passed to env.
+fn handle_fixed_output(
+    drv: &mut Derivation,
+    hash_str: Option<String>,      // in nix: outputHash
+    hash_algo_str: Option<String>, // in nix: outputHashAlgo
+    hash_mode_str: Option<String>, // in nix: outputHashmode
+) -> Result<(), ErrorKind> {
+    // If outputHash is provided, ensure hash_algo_str is compatible.
+    // If outputHash is not provided, do nothing.
+    if let Some(hash_str) = hash_str {
+        // treat an empty algo as None
+        let hash_algo_str = match hash_algo_str {
+            Some(s) if s.is_empty() => None,
+            Some(s) => Some(s),
+            None => None,
+        };
+
+        // construct a NixHash.
+        let nixhash = nixhash::from_str(&hash_str, hash_algo_str.as_deref())
+            .map_err(DerivationError::InvalidOutputHash)?;
+
+        // construct the fixed output.
+        drv.outputs.insert(
+            "out".to_string(),
+            Output {
+                path: "".to_string(),
+                ca_hash: match hash_mode_str.as_deref() {
+                    None | Some("flat") => Some(nixhash::CAHash::Flat(nixhash)),
+                    Some("recursive") => Some(nixhash::CAHash::Nar(nixhash)),
+                    Some(other) => {
+                        return Err(DerivationError::InvalidOutputHashMode(other.to_string()))?
+                    }
+                },
+            },
+        );
+    }
+    Ok(())
+}
+
+/// Handles derivation parameters which are not just forwarded to
+/// the environment. The return value indicates whether the
+/// parameter should be included in the environment.
+async fn handle_derivation_parameters(
+    drv: &mut Derivation,
+    co: &GenCo,
+    name: &str,
+    value: &Value,
+    val_str: &str,
+) -> Result<Result<bool, CatchableErrorKind>, ErrorKind> {
+    match name {
+        IGNORE_NULLS => return Ok(Ok(false)),
+
+        // Command line arguments to the builder.
+        "args" => {
+            let args = value.to_list()?;
+            for arg in args {
+                match strong_coerce_to_string(co, arg).await? {
+                    Err(cek) => return Ok(Err(cek)),
+                    Ok(s) => drv.arguments.push(s),
+                }
+            }
+
+            // The arguments do not appear in the environment.
+            return Ok(Ok(false));
+        }
+
+        // Explicitly specified drv outputs (instead of default [ "out" ])
+        "outputs" => {
+            let outputs = value
+                .to_list()
+                .context("looking at the `outputs` parameter of the derivation")?;
+
+            populate_outputs(co, drv, outputs).await?;
+        }
+
+        "builder" => {
+            drv.builder = val_str.to_string();
+        }
+
+        "system" => {
+            drv.system = val_str.to_string();
+        }
+
+        _ => {}
+    }
+
+    Ok(Ok(true))
+}
+
+async fn strong_coerce_to_string(
+    co: &GenCo,
+    val: Value,
+) -> Result<Result<String, CatchableErrorKind>, ErrorKind> {
+    let val = generators::request_force(co, val).await;
+    match generators::request_string_coerce(co, val, CoercionKind::Strong).await {
+        Err(cek) => Ok(Err(cek)),
+        Ok(val_str) => Ok(Ok(val_str.as_str().to_string())),
+    }
+}
+
+#[builtins(state = "Rc<RefCell<KnownPaths>>")]
+pub(crate) mod derivation_builtins {
+    use super::*;
+    use nix_compat::store_path::hash_placeholder;
+    use tvix_eval::generators::Gen;
+
+    #[builtin("placeholder")]
+    async fn builtin_placeholder(co: GenCo, 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
+    /// use the higher-level `builtins.derivation` instead.
+    #[builtin("derivationStrict")]
+    async fn builtin_derivation_strict(
+        state: Rc<RefCell<KnownPaths>>,
+        co: GenCo,
+        input: Value,
+    ) -> Result<Value, ErrorKind> {
+        let input = input.to_attrs()?;
+        let name = generators::request_force(&co, input.select_required("name")?.clone())
+            .await
+            .to_str()
+            .context("determining derivation name")?;
+
+        if name.is_empty() {
+            return Err(ErrorKind::Abort("derivation has empty name".to_string()));
+        }
+
+        // Check whether attributes should be passed as a JSON file.
+        // TODO: the JSON serialisation has to happen here.
+        if let Some(sa) = input.select(STRUCTURED_ATTRS) {
+            if generators::request_force(&co, sa.clone()).await.as_bool()? {
+                return Err(ErrorKind::NotImplemented(STRUCTURED_ATTRS));
+            }
+        }
+
+        // Check whether null attributes should be ignored or passed through.
+        let ignore_nulls = match input.select(IGNORE_NULLS) {
+            Some(b) => generators::request_force(&co, b.clone()).await.as_bool()?,
+            None => false,
+        };
+
+        let mut drv = Derivation::default();
+        drv.outputs.insert("out".to_string(), Default::default());
+
+        async fn select_string(
+            co: &GenCo,
+            attrs: &NixAttrs,
+            key: &str,
+        ) -> Result<Result<Option<String>, CatchableErrorKind>, ErrorKind> {
+            if let Some(attr) = attrs.select(key) {
+                match strong_coerce_to_string(co, attr.clone()).await? {
+                    Err(cek) => return Ok(Err(cek)),
+                    Ok(str) => return Ok(Ok(Some(str))),
+                }
+            }
+
+            Ok(Ok(None))
+        }
+
+        for (name, value) in input.clone().into_iter_sorted() {
+            let value = generators::request_force(&co, value).await;
+            if ignore_nulls && matches!(value, Value::Null) {
+                continue;
+            }
+
+            match strong_coerce_to_string(&co, value.clone()).await? {
+                Err(cek) => return Ok(Value::Catchable(cek)),
+                Ok(val_str) => {
+                    // handle_derivation_parameters tells us whether the
+                    // argument should be added to the environment; continue
+                    // to the next one otherwise
+                    match handle_derivation_parameters(
+                        &mut drv,
+                        &co,
+                        name.as_str(),
+                        &value,
+                        &val_str,
+                    )
+                    .await?
+                    {
+                        Err(cek) => return Ok(Value::Catchable(cek)),
+                        Ok(false) => continue,
+                        _ => (),
+                    }
+
+                    // Most of these are also added to the builder's environment in "raw" form.
+                    if drv
+                        .environment
+                        .insert(name.as_str().to_string(), val_str.into())
+                        .is_some()
+                    {
+                        return Err(
+                            DerivationError::DuplicateEnvVar(name.as_str().to_string()).into()
+                        );
+                    }
+                }
+            }
+        }
+
+        // Configure fixed-output derivations if required.
+        {
+            let output_hash = match select_string(&co, &input, "outputHash")
+                .await
+                .context("evaluating the `outputHash` parameter")?
+            {
+                Err(cek) => return Ok(Value::Catchable(cek)),
+                Ok(s) => s,
+            };
+            let output_hash_algo = match select_string(&co, &input, "outputHashAlgo")
+                .await
+                .context("evaluating the `outputHashAlgo` parameter")?
+            {
+                Err(cek) => return Ok(Value::Catchable(cek)),
+                Ok(s) => s,
+            };
+            let output_hash_mode = match select_string(&co, &input, "outputHashMode")
+                .await
+                .context("evaluating the `outputHashMode` parameter")?
+            {
+                Err(cek) => return Ok(Value::Catchable(cek)),
+                Ok(s) => s,
+            };
+            handle_fixed_output(&mut drv, output_hash, output_hash_algo, output_hash_mode)?;
+        }
+
+        // Scan references in relevant attributes to detect any build-references.
+        let references = {
+            let state = state.borrow();
+            if state.is_empty() {
+                // skip reference scanning, create an empty result
+                Default::default()
+            } else {
+                let mut refscan = state.reference_scanner();
+                drv.arguments.iter().for_each(|s| refscan.scan(s));
+                drv.environment.values().for_each(|s| refscan.scan(s));
+                refscan.scan(&drv.builder);
+                refscan.finalise()
+            }
+        };
+
+        // Each output name needs to exist in the environment, at this
+        // point initialised as an empty string because that is the
+        // way of Golang ;)
+        for output in drv.outputs.keys() {
+            if drv
+                .environment
+                .insert(output.to_string(), String::new().into())
+                .is_some()
+            {
+                emit_warning_kind(&co, WarningKind::ShadowedOutput(output.to_string())).await;
+            }
+        }
+
+        let mut known_paths = state.borrow_mut();
+        populate_inputs(&mut drv, &known_paths, references);
+
+        // At this point, derivation fields are fully populated from
+        // eval data structures.
+        drv.validate(false)
+            .map_err(DerivationError::InvalidDerivation)?;
+
+        // Calculate the derivation_or_fod_hash for the current derivation.
+        // This one is still intermediate (so not added to known_paths)
+        let derivation_or_fod_hash_tmp =
+            drv.derivation_or_fod_hash(|drv| known_paths.get_hash_derivation_modulo(drv));
+
+        // Mutate the Derivation struct and set output paths
+        drv.calculate_output_paths(&name, &derivation_or_fod_hash_tmp)
+            .map_err(DerivationError::InvalidDerivation)?;
+
+        let derivation_path = drv
+            .calculate_derivation_path(&name)
+            .map_err(DerivationError::InvalidDerivation)?;
+
+        // recompute the hash derivation modulo and add to known_paths
+        let derivation_or_fod_hash_final =
+            drv.derivation_or_fod_hash(|drv| known_paths.get_hash_derivation_modulo(drv));
+
+        known_paths.add_hash_derivation_modulo(
+            derivation_path.to_absolute_path(),
+            &derivation_or_fod_hash_final,
+        );
+
+        // mark all the new paths as known
+        let output_names: Vec<String> = drv.outputs.keys().map(Clone::clone).collect();
+        known_paths.drv(derivation_path.to_absolute_path(), &output_names);
+
+        for (output_name, output) in &drv.outputs {
+            known_paths.output(
+                &output.path,
+                output_name,
+                derivation_path.to_absolute_path(),
+            );
+        }
+
+        let mut new_attrs: Vec<(String, String)> = drv
+            .outputs
+            .into_iter()
+            .map(|(name, output)| (name, output.path))
+            .collect();
+
+        new_attrs.push(("drvPath".to_string(), derivation_path.to_absolute_path()));
+
+        Ok(Value::Attrs(Box::new(NixAttrs::from_iter(
+            new_attrs.into_iter(),
+        ))))
+    }
+
+    #[builtin("toFile")]
+    async fn builtin_to_file(
+        state: Rc<RefCell<KnownPaths>>,
+        co: GenCo,
+        name: Value,
+        content: Value,
+    ) -> Result<Value, ErrorKind> {
+        let name = name
+            .to_str()
+            .context("evaluating the `name` parameter of builtins.toFile")?;
+        let content = content
+            .to_str()
+            .context("evaluating the `content` parameter of builtins.toFile")?;
+
+        let mut refscan = state.borrow().reference_scanner();
+        refscan.scan(content.as_str());
+        let refs = {
+            let paths = state.borrow();
+            refscan
+                .finalise()
+                .into_iter()
+                .map(|path| paths[&path].path.to_string())
+                .collect::<Vec<_>>()
+        };
+
+        // TODO: fail on derivation references (only "plain" is allowed here)
+
+        let path = nix_compat::store_path::build_text_path(name.as_str(), content.as_str(), refs)
+            .map_err(|_e| {
+                nix_compat::derivation::DerivationError::InvalidOutputName(
+                    name.as_str().to_string(),
+                )
+            })
+            .map_err(DerivationError::InvalidDerivation)?
+            .to_absolute_path();
+
+        state.borrow_mut().plain(&path);
+
+        // TODO: actually persist the file in the store at that path ...
+
+        Ok(Value::String(path.into()))
+    }
+}
+
+pub use derivation_builtins::builtins as derivation_builtins;
diff --git a/tvix/glue/src/builtins/derivation_error.rs b/tvix/glue/src/builtins/derivation_error.rs
new file mode 100644
index 000000000000..25471f0fdf48
--- /dev/null
+++ b/tvix/glue/src/builtins/derivation_error.rs
@@ -0,0 +1,27 @@
+//! Contains [crate::builtins::DerivationError].
+use nix_compat::{derivation::DerivationError, nixhash};
+use std::rc::Rc;
+use thiserror::Error;
+
+/// Errors related to derivation construction
+#[derive(Debug, Error)]
+pub enum Error {
+    #[error("an output with the name '{0}' is already defined")]
+    DuplicateOutput(String),
+    #[error("fixed-output derivations can only have the default `out`-output")]
+    ConflictingOutputTypes,
+    #[error("the environment variable '{0}' has already been set in this derivation")]
+    DuplicateEnvVar(String),
+    #[error("invalid derivation parameters: {0}")]
+    InvalidDerivation(DerivationError),
+    #[error("invalid output hash: {0}")]
+    InvalidOutputHash(nixhash::Error),
+    #[error("invalid output hash mode: '{0}', only 'recursive' and 'flat` are supported")]
+    InvalidOutputHashMode(String),
+}
+
+impl From<Error> for tvix_eval::ErrorKind {
+    fn from(err: Error) -> Self {
+        tvix_eval::ErrorKind::TvixError(Rc::new(err))
+    }
+}
diff --git a/tvix/glue/src/builtins/mod.rs b/tvix/glue/src/builtins/mod.rs
new file mode 100644
index 000000000000..6b39fd362ee3
--- /dev/null
+++ b/tvix/glue/src/builtins/mod.rs
@@ -0,0 +1,157 @@
+//! Contains builtins that deal with the store or builder.
+use std::{cell::RefCell, rc::Rc};
+
+use crate::known_paths::KnownPaths;
+
+mod derivation;
+mod derivation_error;
+
+pub use derivation_error::Error as DerivationError;
+
+/// Adds derivation-related builtins to the passed [tvix_eval::Evaluation].
+///
+/// These are `derivation` and `derivationStrict`.
+///
+/// As they need to interact with `known_paths`, we also need to pass in
+/// `known_paths`.
+pub fn add_derivation_builtins(
+    eval: &mut tvix_eval::Evaluation,
+    known_paths: Rc<RefCell<KnownPaths>>,
+) {
+    eval.builtins
+        .extend(derivation::derivation_builtins::builtins(known_paths));
+
+    // Add the actual `builtins.derivation` from compiled Nix code
+    eval.src_builtins
+        .push(("derivation", include_str!("derivation.nix")));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::add_derivation_builtins;
+    use crate::known_paths::KnownPaths;
+    use nix_compat::store_path::hash_placeholder;
+    use std::{cell::RefCell, rc::Rc};
+    use test_case::test_case;
+    use tvix_eval::EvaluationResult;
+
+    /// evaluates a given nix expression and returns the result.
+    /// Takes care of setting up the evaluator so it knows about the
+    // `derivation` builtin.
+    fn eval(str: &str) -> EvaluationResult {
+        let mut eval = tvix_eval::Evaluation::new_impure(str, None);
+
+        let known_paths: Rc<RefCell<KnownPaths>> = Default::default();
+
+        add_derivation_builtins(&mut eval, known_paths.clone());
+
+        // run the evaluation itself.
+        eval.evaluate()
+    }
+
+    #[test]
+    fn derivation() {
+        let result = eval(
+            r#"(derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux";}).outPath"#,
+        );
+
+        assert!(result.errors.is_empty(), "expect evaluation to succeed");
+        let value = result.value.expect("must be some");
+
+        match value {
+            tvix_eval::Value::String(s) => {
+                assert_eq!(
+                    "/nix/store/xpcvxsx5sw4rbq666blz6sxqlmsqphmr-foo",
+                    s.as_str()
+                );
+            }
+            _ => panic!("unexpected value type: {:?}", value),
+        }
+    }
+
+    /// a derivation with an empty name is an error.
+    #[test]
+    fn derivation_empty_name_fail() {
+        let result = eval(
+            r#"(derivation { name = ""; builder = "/bin/sh"; system = "x86_64-linux";}).outPath"#,
+        );
+
+        assert!(!result.errors.is_empty(), "expect evaluation to fail");
+    }
+
+    /// construct some calls to builtins.derivation and compare produced output
+    /// paths.
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHashAlgo = "sha256"; outputHash = "sha256-Q3QXOoy+iN4VK2CflvRulYvPZXYgF0dO7FoF7CvWFTA="; }).outPath"#, "/nix/store/17wgs52s7kcamcyin4ja58njkf91ipq8-foo"; "r:sha256")]
+    #[test_case(r#"(builtins.derivation { name = "foo2"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHashAlgo = "sha256"; outputHash = "sha256-Q3QXOoy+iN4VK2CflvRulYvPZXYgF0dO7FoF7CvWFTA="; }).outPath"#, "/nix/store/gi0p8vd635vpk1nq029cz3aa3jkhar5k-foo2"; "r:sha256 other name")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHashAlgo = "sha1"; outputHash = "sha1-VUCRC+16gU5lcrLYHlPSUyx0Y/Q="; }).outPath"#, "/nix/store/p5sammmhpa84ama7ymkbgwwzrilva24x-foo"; "r:sha1")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHashAlgo = "md5"; outputHash = "md5-07BzhNET7exJ6qYjitX/AA=="; }).outPath"#, "/nix/store/gmmxgpy1jrzs86r5y05wy6wiy2m15xgi-foo"; "r:md5")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHashAlgo = "sha512"; outputHash = "sha512-DPkYCnZKuoY6Z7bXLwkYvBMcZ3JkLLLc5aNPCnAvlHDdwr8SXBIZixmVwjPDS0r9NGxUojNMNQqUilG26LTmtg=="; }).outPath"#, "/nix/store/lfi2bfyyap88y45mfdwi4j99gkaxaj19-foo"; "r:sha512")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHashAlgo = "sha256"; outputHash = "4374173a8cbe88de152b609f96f46e958bcf65762017474eec5a05ec2bd61530"; }).outPath"#, "/nix/store/17wgs52s7kcamcyin4ja58njkf91ipq8-foo"; "r:sha256 base16")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHashAlgo = "sha256"; outputHash = "0c0msqmyq1asxi74f5r0frjwz2wmdvs9d7v05caxx25yihx1fx23"; }).outPath"#, "/nix/store/17wgs52s7kcamcyin4ja58njkf91ipq8-foo"; "r:sha256 nixbase32")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHashAlgo = "sha256"; outputHash = "Q3QXOoy+iN4VK2CflvRulYvPZXYgF0dO7FoF7CvWFTA="; }).outPath"#, "/nix/store/17wgs52s7kcamcyin4ja58njkf91ipq8-foo"; "r:sha256 base64")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHashAlgo = "sha256"; outputHash = "sha256-fgIr3TyFGDAXP5+qoAaiMKDg/a1MlT6Fv/S/DaA24S8="; }).outPath"#, "/nix/store/xm1l9dx4zgycv9qdhcqqvji1z88z534b-foo"; "r:sha256 base64 nopad")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "flat"; outputHashAlgo = "sha256"; outputHash = "sha256-Q3QXOoy+iN4VK2CflvRulYvPZXYgF0dO7FoF7CvWFTA="; }).outPath"#, "/nix/store/q4pkwkxdib797fhk22p0k3g1q32jmxvf-foo"; "sha256")]
+    #[test_case(r#"(builtins.derivation { name = "foo2"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "flat"; outputHashAlgo = "sha256"; outputHash = "sha256-Q3QXOoy+iN4VK2CflvRulYvPZXYgF0dO7FoF7CvWFTA="; }).outPath"#, "/nix/store/znw17xlmx9r6gw8izjkqxkl6s28sza4l-foo2"; "sha256 other name")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "flat"; outputHashAlgo = "sha1"; outputHash = "sha1-VUCRC+16gU5lcrLYHlPSUyx0Y/Q="; }).outPath"#, "/nix/store/zgpnjjmga53d8srp8chh3m9fn7nnbdv6-foo"; "sha1")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "flat"; outputHashAlgo = "md5"; outputHash = "md5-07BzhNET7exJ6qYjitX/AA=="; }).outPath"#, "/nix/store/jfhcwnq1852ccy9ad9nakybp2wadngnd-foo"; "md5")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "flat"; outputHashAlgo = "sha512"; outputHash = "sha512-DPkYCnZKuoY6Z7bXLwkYvBMcZ3JkLLLc5aNPCnAvlHDdwr8SXBIZixmVwjPDS0r9NGxUojNMNQqUilG26LTmtg=="; }).outPath"#, "/nix/store/as736rr116ian9qzg457f96j52ki8bm3-foo"; "sha512")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHash = "sha256-Q3QXOoy+iN4VK2CflvRulYvPZXYgF0dO7FoF7CvWFTA="; }).outPath"#, "/nix/store/17wgs52s7kcamcyin4ja58njkf91ipq8-foo"; "r:sha256 outputHashAlgo omitted")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHash = "sha256-Q3QXOoy+iN4VK2CflvRulYvPZXYgF0dO7FoF7CvWFTA="; }).outPath"#, "/nix/store/q4pkwkxdib797fhk22p0k3g1q32jmxvf-foo"; "r:sha256 outputHashAlgo and outputHashMode omitted")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; }).outPath"#, "/nix/store/xpcvxsx5sw4rbq666blz6sxqlmsqphmr-foo"; "outputHash* omitted")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; outputs = ["foo" "bar"]; system = "x86_64-linux"; }).outPath"#, "/nix/store/hkwdinvz2jpzgnjy9lv34d2zxvclj4s3-foo-foo"; "multiple outputs")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; args = ["--foo" "42" "--bar"]; system = "x86_64-linux"; }).outPath"#, "/nix/store/365gi78n2z7vwc1bvgb98k0a9cqfp6as-foo"; "args")]
+    #[test_case(r#"
+                   let
+                     bar = builtins.derivation {
+                       name = "bar";
+                       builder = ":";
+                       system = ":";
+                       outputHash = "08813cbee9903c62be4c5027726a418a300da4500b2d369d3af9286f4815ceba";
+                       outputHashAlgo = "sha256";
+                       outputHashMode = "recursive";
+                     };
+                   in
+                   (builtins.derivation {
+                     name = "foo";
+                     builder = ":";
+                     system = ":";
+                     inherit bar;
+                   }).outPath
+        "#, "/nix/store/5vyvcwah9l9kf07d52rcgdk70g2f4y13-foo"; "full")]
+    fn test_outpath(code: &str, expected_path: &str) {
+        let value = eval(code).value.expect("must succeed");
+
+        match value {
+            tvix_eval::Value::String(s) => {
+                assert_eq!(expected_path, s.as_str());
+            }
+            _ => panic!("unexpected value type: {:?}", value),
+        }
+    }
+
+    /// construct some calls to builtins.derivation that should be rejected
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHashAlgo = "sha256"; outputHash = "sha256-00"; }).outPath"#; "invalid outputhash")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; system = "x86_64-linux"; outputHashMode = "recursive"; outputHashAlgo = "sha1"; outputHash = "sha256-Q3QXOoy+iN4VK2CflvRulYvPZXYgF0dO7FoF7CvWFTA="; }).outPath"#; "sha1 and sha256")]
+    #[test_case(r#"(builtins.derivation { name = "foo"; builder = "/bin/sh"; outputs = ["foo" "foo"]; system = "x86_64-linux"; }).outPath"#; "duplicate output names")]
+    fn test_outpath_invalid(code: &str) {
+        let resp = eval(code);
+        assert!(resp.value.is_none(), "Value should be None");
+        assert!(
+            !resp.errors.is_empty(),
+            "There should have been some errors"
+        );
+    }
+
+    #[test]
+    fn builtins_placeholder_hashes() {
+        assert_eq!(
+            hash_placeholder("out").as_str(),
+            "/1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9"
+        );
+
+        assert_eq!(
+            hash_placeholder("").as_str(),
+            "/171rf4jhx57xqz3p7swniwkig249cif71pa08p80mgaf0mqz5bmr"
+        );
+    }
+}